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Preface

The publication of this volume has been delayed owing to war conditions, but the delay is the less to be regretted in that it has allowed the inclusion of recent work on the subject. Much of the subject-matter is of common knowledge to lichenologists, but in the co-ordination and arrangement of the facts the original papers are cited throughout. The method has somewhat burdened the pages with citations, but it is hoped that, as a book of reference, its value has been enhanced thereby. The Glossary includes terms used in lichenology, or those with a special lichenological meaning. The Bibliography refers only to works consulted in the preparation of this volume. To save space, etc., the titles of books and papers quoted in the text are generally translated and curtailed: full citations will be found in the Bibliography. Subject-matter has been omitted from the index: references of importance will be found in the Table of Contents or in the Glossary.

I would record my thanks to those who have generously helped me during the preparation of the volume: to Lady Muriel Percy for taking notes of spore production, and to Dr Cavers for the loan of reprints. Prof. Potter and Dr Somerville Hastings placed at my disposal their photographs of the living plants. Free use has been made of published text-figures which are duly acknowledged.

I have throughout had the inestimable advantage of being able to consult freely the library and herbarium of the British Museum, and have thus been able to verify references to plants as well as to literature. A special debt of gratitude is due to my colleagues Mr Gepp and Mr Ramsbottom for their unfailing assistance and advice.

A. L. S.

LONDON, February, 1920

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I. HISTORY OF LICHENOLOGY

A. INTRODUCTORY 1

B. PERIOD I. PREVIOUS TO 1694 2

C. PERIOD II. 1694-1729 5

D. PERIOD III. 1729-1780 6

E. PERIOD IV. 1780-1803 9

F. PERIOD V. 1803-1846 10

G. PERIOD VI. 1846-1867 15

H. PERIOD VII. 1867 AND AFTER 18

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II. CONSTITUENTS OF THE LICHEN THALLUS

I. LICHEN GONIDIA

1. GONIDIA IN RELATION TO THE THALLUS

A. HISTORICAL ACCOUNT OF LICHEN GONIDIA 21

B. GONIDIA CONTRASTED WITH ALGAE 22

C. CULTURE EXPERIMENTS WITH THE LICHEN THALLUS 24

D. THEORIES AS TO THE ORIGIN OF GONIDIA 25

E. MICROGONIDIA 26

F. COMPOSITE NATURE OF THALLUS 27

G. SYNTHETIC CULTURES 27

H. HYMENIAL GONIDIA 30

I. NATURE OF ASSOCIATION BETWEEN ALGA AND FUNGUS 31 a. Consortium and symbiosis b. Different forms of association

J. RECENT VIEWS ON SYMBIOSIS AND PARASITISM 36

2. PHYSIOLOGY OF THE SYMBIONTS

A. NUTRITION OF LICHEN ALGAE 39 a. Character of algal cells b. Supply of nitrogen c. Effect on the alga d. Supply of carbon e. Nutrition within the symbiotic plant f. Affinities of lichen gonidia

B. NUTRITION OF LICHEN FUNGI 44

C. SYMBIOSIS OF OTHER PLANTS 45

II. LICHEN HYPHAE

A. ORIGIN OF HYPHAE 46

B. DEVELOPMENT OF LICHENOID HYPHAE 47

C. CULTURE OF HYPHAE WITHOUT GONIDIA 49

D. CONTINUITY OF PROTOPLASM IN HYPHAL CELLS 51

III. LICHEN ALGAE

A. TYPES OF ALGAE 51 a. Myxophyceae associated with Phycolichens b. Chlorophyceae associated with Archilichens

B. CHANGES INDUCED IN THE ALGA 60 a. Myxophyceae b. Chlorophyceae

C. CONSTANCY OF ALGAL CONSTITUENTS 63

D. DISPLACEMENT OF ALGAE WITHIN THE THALLUS 64 a. Normal displacement b. Local displacement

E. NON-GONIDIAL ORGANISMS ASSOCIATED WITH LICHEN HYPHAE 65

F. PARASITISM OF ALGAE ON LICHENS 65

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III. MORPHOLOGY

I. GENERAL ACCOUNT OF LICHEN STRUCTURE

ORIGIN OF LICHEN STRUCTURES

A. FORMS OF CELL-STRUCTURE 67

B. TYPES OF THALLUS 68 a. Endogenous thallus b. Exogenous thallus

II. STRATOSE THALLUS

1. CRUSTACEOUS LICHENS

A. GENERAL STRUCTURE 70

B. SAXICOLOUS LICHENS 70 a. Epilithic lichens aa. Hypothallus or protothallus bb. Formation of crustaceous tissues cc. Formation of areolae b. Endolithic lichens c. Chemical nature of the substratum

C. CORTICOLOUS LICHENS 77 a. Epiphloeodal lichens b. Hypophloeodal lichens

2. SQUAMULOSE LICHENS

A. DEVELOPMENT OF THE SQUAMULE 79

B. TISSUES OF SQUAMULOSE THALLUS 81

3. FOLIOSE LICHENS

A. DEVELOPMENT OF FOLIOSE THALLUS 82

B. CORTICAL TISSUES 82 a. Types of cortical structure b. Origin of variation in cortical structure c. Loss and renewal of cortex d. Cortical hairs

C. GONIDIAL TISSUES 87

D. MEDULLA AND LOWER CORTEX 88 a. Medulla b. Lower cortex c. Hypothallic structures

E. STRUCTURES FOR PROTECTION AND ATTACHMENT 91 a. Cilia b. Rhizinae c. Haptera

F. STRENGTHENING TISSUES OF STRATOSE LICHENS 95 a. Produced by development of cortex b. Produced by development of veins or nerves

III. RADIATE THALLUS

1. CHARACTERS OF RADIATE THALLUS

2. INTERMEDIATE TYPES OF THALLUS

3. FRUTICOSE AND FILAMENTOUS THALLUS

A. GENERAL STRUCTURE OF THALLUS 101

Cortical Structures a. The fastigiate cortex b. The fibrous cortex

B. SPECIAL STRENGTHENING STRUCTURES 103 a. Sclerotic strands b. Chondroid axis

C. SURVEY OF MECHANICAL TISSUES 105

D. RETICULATE FRONDS 106

E. ROOTING BASE IN FRUTICOSE LICHENS 108

IV. STRATOSE-RADIATE THALLUS

1. STRATOSE OR PRIMARY THALLUS

A. GENERAL CHARACTERISTICS 111

B. TISSUES OF PRIMARY THALLUS 112 a. Cortical tissue b. Gonidial tissue c. Medullary tissue d. Soredia

2. RADIATE OR SECONDARY THALLUS

A. ORIGIN OF THE PODETIUM 114

B. STRUCTURE OF THE PODETIUM 114 a. General structure b. Gonidial tissue c. Cortical tissue d. Soredia

C. DEVELOPMENT OF THE SCYPHUS 117 a. From abortive apothecia b. From polytomous branching c. From arrested growth d. Gonidia of the scyphus e. Species without scyphi

D. BRANCHING OF THE PODETIUM 119

E. PERFORATIONS AND RETICULATION OF THE PODETIUM 120

F. ROOTING STRUCTURES OF CLADONIAE 121

G. HAPTERA 122

H. MORPHOLOGY OF THE PODETIUM 122

I. PILOPHORUS AND STEREOCAULON 125

V. STRUCTURES PECULIAR TO LICHENS

1. AERATION STRUCTURES

A. CYPHELLAE AND PSEUDOCYPHELLAE 126 a. Historical b. Development of cyphellae c. Pseudocyphellae d. Occurrence and distribution

B. BREATHING-PORES 129 a. Definite breathing-pores b. Other openings in the thallus

C. GENERAL AERATION OF THE THALLUS 132

2. CEPHALODIA

A. HISTORICAL AND DESCRIPTIVE 133

B. CLASSIFICATION 135 I. CEPHALODIA VERA II. PSEUDOCEPHALODIA

C. ALGAE THAT FORM CEPHALODIA 136

D. DEVELOPMENT OF CEPHALODIA 137 a. Ectotrophic b. Endotrophic c. Pseudocephalodia

E. AUTOSYMBIOTIC CEPHALODIA 140

3. SOREDIA

A. STRUCTURE AND ORIGIN OF SOREDIA 141 a. Scattered soredia b. Isidial soredia c. Soredia as buds

B. SORALIA 144 a. Form and occurrence of soralia b. Position of soraliferous lobes c. Deep-seated soralia

C. DISPERSAL AND GERMINATION OF SOREDIA 147

D. EVOLUTION OF SOREDIA 148

4. ISIDIA

A. FORM AND STRUCTURE OF ISIDIA 149

B. ORIGIN AND FUNCTION OF ISIDIA 151

VI. HYMENOLICHENS

A. AFFINITY WITH OTHER PLANTS 152

B. STRUCTURE OF THALLUS 153

C. SPORIFEROUS TISSUES 154

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IV. REPRODUCTION

I. REPRODUCTION BY ASCOSPORES

A. HISTORICAL SURVEY 155

B. FORMS OF REPRODUCTIVE ORGANS 156 a. Apothecia b. Perithecia

C. DEVELOPMENT OF REPRODUCTIVE ORGANS 159

1. DISCOLICHENS

a. Carpogonia of gelatinous lichens b. Carpogonia of non-gelatinous lichens c. General summary d. Hypothecium and paraphyses e. Variations in apothecial development aa. Parmeliae bb. Pertusariae cc. Graphideae dd. Cladoniae

2. PYRENOLICHENS

a. Development of the perithecium b. Formation of carpogonia

D. APOGAMOUS REPRODUCTION 174

E. DISCUSSION OF LICHEN REPRODUCTION 177 a. The Trichogyne b. The Ascogonium

F. FINAL STAGES OF APOTHECIAL DEVELOPMENT 181 a. Open or closed apothecia b. Emergence of ascocarp

G. LICHEN ASCI AND SPORES 184 a. Historical b. Development of the ascus c. Development of the spores d. Spore germination e. Multinucleate spores f. Polaribilocular spores

II. SECONDARY SPORES

A. REPRODUCTION BY OIDIA 189

B. REPRODUCTION BY CONIDIA 190 a. Rare instances of conidial formation b. Comparison with Hyphomycetes

C. CAMPYLIDIUM AND ORTHIDIUM 191

III. SPERMOGONIA OR PYCNIDIA

A. HISTORICAL ACCOUNT OF SPERMOGONIA 192

B. SPERMOGONIA AS MALE ORGANS 193

C. OCCURRENCE AND DISTRIBUTION 193 a. Relation to thallus and apothecia b. Form and size c. Colour

D. STRUCTURE 196 a. Origin and growth b. Form and types of spermatiophores c. Periphyses and sterile filaments

E. SPERMATIA OR PYCNIDIOSPORES 201 a. Origin and form b. Size and structure c. Germination d. Variation in pycnidia

F. PYCNIDIA WITH MACROSPORES 204

G. GENERAL SURVEY 205 a. Sexual or asexual b. Comparison with fungi c. Influence of symbiosis d. Value in diagnosis

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V. PHYSIOLOGY

I. CELLS AND CELL PRODUCTS

A. CELL-MEMBRANES 209 a. Chitin b. Lichenin and allied carbohydrates c. Cellulose

B. CONTENTS AND PRODUCTS OF THE FUNGAL CELLS 213 a. Cell-substances b. Calcium Oxalate c. Importance of calcium oxalate

C. OIL-CELLS 215 a. Oil-cells of endolithic lichens b. Oil-cells of epilithic lichens c. Significance of oil-formation

D. LICHEN-ACIDS 221 a. Historical b. Occurrence and examination of acids c. Character of acids d. Causes of variation in quantity and quality e. Distribution of acids

E. CHEMICAL GROUPING OF ACIDS 225 I. ACIDS OF THE FAT SERIES II. ACIDS OF THE BENZOLE SERIES SUBSERIES I. ORCINE DERIVATIVES SUBSERIES II. ANTHRACENE DERIVATIVES

F. CHEMICAL REAGENTS AS TESTS FOR LICHENS 228

G. CHEMICAL REACTIONS IN NATURE 229

II. GENERAL NUTRITION

A. ABSORPTION OF WATER 229 a. Gelatinous lichens b. Crustaceous lichens c. Foliose lichens d. Fruticose lichens

B. STORAGE OF WATER 232

C. SUPPLY OF INORGANIC FOOD 232 a. In foliose and fruticose lichens b. In crustaceous lichens

D. SUPPLY OF ORGANIC FOOD 235 a. From the substratum b. From other lichens c. From other vegetation

III. ASSIMILATION AND RESPIRATION

A. INFLUENCE OF TEMPERATURE 238 a. High temperature b. Low temperature

B. INFLUENCE OF MOISTURE 239 a. On vital functions b. On general development

IV. ILLUMINATION OF LICHENS

A. EFFECT OF LIGHT ON THE THALLUS 240 a. Sun lichens b. Colour-changes due to light c. Shade lichens d. Varying shade conditions

B. EFFECT OF LIGHT ON REPRODUCTIVE ORGANS 244 a. Position and orientation of fruits with regard to light b. Influence of light on colour of fruits

V. COLOUR OF LICHENS

A. ORIGIN OF LICHEN-COLOURING 245 a. Colour given by the algal constituent b. Colour due to lichen-acids c. Colour due to amorphous substances d. Enumeration of amorphous pigments e. Colour due to infiltration

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VI. BIONOMICS

A. GROWTH AND DURATION OF LICHENS 252

B. SEASON OF FRUIT FORMATION 255

C. DISPERSAL AND INCREASE 256 a. Dispersal of crustaceous lichens b. Dispersal of foliose lichens c. Dispersal of fruticose lichens

D. ERRATIC LICHENS 258

E. PARASITISM 260 a. General statement b. Antagonistic symbiosis c. Parasymbiosis d. Parasymbiosis of fungi e. Fungi parasitic on lichens f. Mycetozoa parasitic on lichens

F. DISEASES OF LICHENS 268 a. Caused by parasitism b. Caused by crowding c. Caused by adverse conditions

G. HARMFUL EFFECT OF LICHENS 269

H. GALL-FORMATION 270

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VII. PHYLOGENY

I. GENERAL STATEMENT

A. ORIGIN OF LICHENS 272

B. ALGAL ANCESTORS 273

C. FUNGAL ANCESTORS 273 a. Basidiolichens b. Ascolichens

II. THE REPRODUCTIVE ORGANS

A. THEORIES OF DESCENT IN ASCOLICHENS 273

B. RELATION OF LICHENS TO FUNGI 275 a. Pyrenocarpineae b. Coniocarpineae c. Graphidineae d. Cyclocarpineae

III. THE THALLUS

A. GENERAL OUTLINE OF DEVELOPMENT OF THALLUS 281 a. Preliminary considerations b. Course of evolution in Hymenolichens c. Course of evolution in Ascolichens

B. COMPARATIVE ANTIQUITY OF ALGAL SYMBIONTS 282

C. EVOLUTION OF PHYCOLICHENS 283 a. Gloeolichens b. Ephebaceae and Collemaceae c. Pyrenidiaceae d. Heppiaceae and Pannariaceae e. Peltigeraceae and Stictaceae

D. EVOLUTION OF ARCHILICHENS 287 a. Thallus of Pyrenocarpineae b. Thallus of Coniocarpineae c. Thallus of Graphidineae d. Thallus of Cyclocarpineae AA. LECIDEALES aa. Coenogoniaceae bb. Lecideaceae and Gyrophoraceae cc. Cladoniaceae 1. Origin of Cladonia 2. Evolution of the primary thallus 3. Evolution of the secondary thallus 4. Course of podetial development 5. Variation in Cladonia 6. Causes of variation 7. Podetial development and spore-dissemination 8. Pilophorus, Stereocaulon and Argopsis BB. LECANORALES aa. Course of Development bb. Lecanoraceae cc. Parmeliaceae dd. Usneaceae ee. Physciaceae

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VIII. SYSTEMATIC

I. CLASSIFICATION

A. WORK OF SUCCESSIVE SYSTEMATISTS 304 a. Dillenius and Linnaeus b. Acharius c. Schaerer d. Massalongo and Koerber e. Nylander f. Müller-Argau g. Reinke h. Zahlbruckner

B. FAMILIES AND GENERA OF ASCOLICHENS 311

C. HYMENOLICHENS 342

II. NUMBER AND DISTRIBUTION

1. ESTIMATES OF NUMBER

2. GEOGRAPHICAL DISTRIBUTION

A. GENERAL SURVEY 343

B. LICHENS OF POLAR REGIONS 345

C. LICHENS OF THE TEMPERATE ZONES 348

D. LICHENS OF TROPICAL REGIONS 352

III. FOSSIL LICHENS

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IX. ECOLOGY

A. GENERAL INTRODUCTION 356

B. EXTERNAL INFLUENCES 357 a. Temperature b. Humidity c. Wind d. Human Agency

C. LICHEN COMMUNITIES 362

1. ARBOREAL 363 a. Epiphyllous b. Corticolous c. Lignicolous

2. TERRICOLOUS 367 a. On calcareous soil b. On siliceous soil c. On bricks d. On humus e. On peaty soil f. On mosses g. On fungi

3. SAXICOLOUS 371 a. Characters of mineral substrata b. Colonization on rocks c. Calcicolous d. Silicicolous

4. OMNICOLOUS LICHENS 376

5. LOCALIZED COMMUNITIES 378 a. Maritime lichens b. Sand-dune lichens c. Mountain lichens d. Tundra lichens e. Desert lichens f. Aquatic lichens

D. LICHENS AS PIONEERS 392 a. Soil-formers b. Outposts of vegetation

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X. ECONOMIC AND TECHNICAL

A. LICHENS AS FOOD 395 a. Food for insects b. Insect mimicry of lichens c. Food for the higher animals d. Food for man

B. LICHENS AS MEDICINE 405 a. Ancient remedies b. Doctrine of “signatures” c. Cure for hydrophobia d. Popular remedies

C. LICHENS AS POISONS 410

D. LICHENS USED IN TANNING, BREWING AND DISTILLING 411

E. DYEING PROPERTIES OF LICHENS 411 a. Lichens as dye-plants b. The orchil lichen, Roccella c. Purple dyes: orchil, cudbear and litmus d. Other orchil lichens e. Preparation of orchil f. Brown and yellow dyes g. Collecting of dye-lichens h. Lichen colours and spectrum characters

F. LICHENS IN PERFUMERY 418 a. Lichens as perfumes b. Lichens as hair-powder

G. SOME MINOR USES OF LICHENS 420

APPENDIX 421

ADDENDUM 422

BIBLIOGRAPHY 423

INDEX 448

GLOSSARY

=Acrogenous=, borne at the tips of hyphae; see spermatium, 312.

=Allelositismus=, Norman’s term to describe the thallus of Moriolaceae (mutualism), 313.

=Amorphous cortex=, formed of indistinct hyphae with thickened walls; cf. decomposed cortex.

=Amphithecium=, thalline margin of the apothecium, 157.

=Antagonistic symbiosis=, hurtful parasitism of one lichen on another, 261 et seq.

=Apothecium=, open or disc-shaped fructification, 11, 156 et passim. Veiled apothecium, 169. Closed or open at first, 182.

=Archilichens=, lichens in which the gonidia are bright green (Chlorophyceae), 52, 55 et passim.

=Ardella=, the small spot-like apothecium of Arthoniaceae, 158.

=Areola= (areolate), small space marked out by lines or chinks on the surface of the thallus, 73 et passim.

=Arthrosterigma=, septate tissue-like sterigma (spermatiophore), 197.

=Ascogonium=, the cell or cells that produce ascogenous hyphae, 180 et seq.

=Ascolichens=, lichens in which the fungus is an Ascomycete, 159, 173 et passim.

=Ascus=, enlarged cell in which a definite number of spores (usually 8) are developed; cf. theca, 157, 184.

=Ascyphous=, podetia without scyphi, 119 et passim.

=Biatorine=, apothecia that are soft or waxy, and often brightly coloured, as in Biatora, 158.

=Blasteniospore=, see polarilocular spore.

=Byssoid=, slender, thread-like, as in the old genus Byssus.

=Campylidium=, supposed new type of fructification in lichens, 191.

=Capitulum=, the globose apical apothecium of Coniocarpineae; cf. mazaedium, 319.

=Carpogonium=, primordial stage of fructification, 160, 164 et passim.

=Cephalodium=, irregular outgrowth from the thallus enclosing mostly blue-green algae; or intruded packet of algae within the thallus, 11, 133 et passim.

=Chrondroid=, hard and tough like cartilage, a term applied to strengthening strands of hyphae, 104, 114.

=Chroolepoid=, like the genus Chroolepis (Trentepohlia).

=Chrysogonidia=, yellow algal cells (Trentepohlia).

=Cilium=, hair-like outgrowth from surface or margin of thallus, or margin of apothecium, 91.

=Consortium= (consortism), mutual association of fungus and alga (Reinke); also termed “mutualism,” 31, 313.

=Corticolous=, living on the bark of trees, 363.

=Crustaceous=, crust-like closely adhering thallus, 70-79.

=Cyphella=, minute cup-like depression on the under surface of the thallus (Sticta, etc.), 11, 126.

=Decomposed=, term applied to cortex formed of gelatinous indistinct hyphae (amorphous), 73-81 et passim, 357.

=Determinate=, thallus with a definite outline, 72.

=Dimidiate=, term applied to the perithecium, when the outer wall covers only the upper portion, 159.

=Discoid=, disc-like, an open rounded apothecium, 156.

=Discolichens=, in which the fructification is an apothecium, 160 et seq.

=Dual hypothesis=, the theory of two organisms present in the lichen thallus, 27 et seq.

=Effigurate=, having a distinct form or figure; cf. placodioid, 80, 201.

=Endobasidial=, Steiner’s term for sporophore with a secondary sporiferous branch, 200.

=Endogenous=, produced internally, as spores in an ascus, 179; see also under thallus.

=Endolithic=, embedded in the rock, 75.

=Endosaprophytism=, term used by Elenkin for destruction of the algal contents by enzymes of the fungus, 36.

=Entire=, term applied to the perithecium when completely surrounded by an outer wall, 159.

=Epilithic=, growing on the rock surface, 70.

=Epiphloeodal=, thallus growing on the surface of the bark, 77.

=Epiphyllous=, growing on leaves, 363.

=Epithecium=, upper layer of thecium (hymenium), 158.

=Erratic lichens=, unattached and drifting, 259.

=Exobasidial=, Steiner’s term for sporophore without a secondary sporiferous branch, 200.

=Exogenous=, produced externally, as spores on tips of hyphae; see also under thallus.

=Fastigiate cortex=, formed of clustered parallel hyphal branches vertical to long axis of thallus, 82.

=Fat-cells=, specialized hyphal cells containing fat or oil, 75, 215 et passim.

=Fibrous cortex=, formed of hyphae parallel with long axis of thallus, 82.

=Filamentous=, slender thallus with radiate structure, 101 et seq.

=Foliose=, lichens with a leafy form and stratose in structure, 82-97.

=Foveolae=, =Foveolate=, pitted, 373.

=Fruticose=, upright or pendulous thallus, with radiate structure, 101 et seq.

=Fulcrum=, term used by Steiner for sporophore, 200.

=Gloeolichens=, lichens in which the gonidia are Gloeocapsa or Chroococcus, 284, 373, 389.

=Gonidium=, the algal constituent of the lichen thallus, 20-45 et passim.

=Gonimium=, blue-green algal cell (Myxophyceae), constituent of the lichen thallus, 52.

=Goniocysts=, nests of gonidia in Moriolaceae, 313.

=Gyrose=, curved backward and forward, furrowed fruit of Gyrophora, 184.

=Hapteron=, aerial organ of attachment, 94, 122.

=Haustorium=, outgrowth or branch of a hypha serving as an organ of suction, 32.

=Helotism=, state of servitude, term used to denote the relation of alga to fungus in lichen organization, 38, 40.

=Heteromerous=, fungal and algal constituents of the thallus in definite strata, 13, 68, 305 et passim.

=Hold-fast=, rooting organ of thallus, 109, 122 et passim.

=Homobium=, interdependent association of fungus and alga, 31.

=Homoiomerous=, fungal and algal constituents more or less mixed in the thallus, 13, 68, 305 et passim.

=Hymenial gonidia=, algal cells in the hymenium, 30, 314, 315, 327.

=Hymenium=, apothecial tissue consisting of asci and paraphyses; cf. thecium, 157.

=Hymenolichens=, lichens of which the fungal constituent is a Hymenomycete, 152-154, 342.

=Hypophloeodal=, thallus growing within the bark, 78, 364.

=Hypothallus=, first growth of hyphae (proto- or pro-thallus) persisting as hyphal growth at base or margin of the thallus, 70, 257 et passim.

=Hypothecium=, layer below the thecium (hymenium), 157.

=Intricate cortex=, composed of hyphae densely interwoven but not coalescent, 83.

=Isidium=, coral-like outgrowth on the lichen thallus, 149-151.

=Lecanorine=, apothecium with a thalline margin as in Lecanora, 158.

=Lecideine=, apothecium usually dark-coloured or carbonaceous and without a thalline margin, 158.

=Leprose=, mealy or scurfy, like the old form genera, Lepra, Lepraria, 191.

=Lichen-acids=, organic acids peculiar to lichens, 221 et seq.

=Lignicolous=, living on wood or trees, 366.

=Lirella=, long narrow apothecium of Graphideae, 158.

=Mazaedium=, fructification of Coniocarpineae, the spores lying as a powdery mass in the capitulum, 176.

=Medulla=, the loose hyphal layer in the interior of the thallus, 88 et passim.

=Meristematic=, term applied by Wainio to growing hyphae, 48.

=Microgonidia=, term applied by Minks to minute greenish bodies in lichen hyphae, 26.

=Multi-septate=, term applied to spores with numerous transverse septa, 316 et seq.

=Murali-divided=, =Muriform=, term applied to spores divided like the masonry of a wall, 187.

=Oidium=, reproductive cell formed by the breaking up of the hyphae, 189.

=Oil-cell=, hyphal cell containing fat globules, 215.

=Orculiform=, see polarilocular.

=Orthidium=, supposed new type of fructification in lichens, 192.

=Palisade-cells=, the terminal cells of the hyphae forming the fastigiate cortex, 82, 83.

=Panniform=, having a felted or matted appearance, 260.

=Paraphysis=, sterile filament in the hymenium, 157.

=Parasymbiosis=, associated harmless but not mutually useful growth of two organisms, 263.

=Parathecium=, hyphal layer round the apothecium, 157.

=Peltate=, term applied to orbicular and horizontal apothecia in the form of a shield, 336.

=Perithecium=, roundish fructification usually with an apical opening (ostiole) containing ascospores, 158 et passim.

=Pervious=, referring to scyphi with an opening at the base (Perviae), 118.

=Phycolichens=, lichens in which the gonidia are blue-green (Myxophyceae), 52 et passim.

=Placodioid=, thallus with a squamulose determinate outline, generally orbicular; cf. effigurate, 80.

=Placodiomorph=, see polarilocular.

=Plectenchyma= (=Plectenchymatous=), pseudoparenchyma of fungi and lichens, 66 et passim.

=Pleurogenous=, borne laterally on hyphal cells; see spermatium, 312.

=Pluri-septate=, term applied to spores with several transverse septa, 321 et seq.

=Podetium=, stalk-like secondary thallus of Cladoniaceae, 114, 293 et seq.

=Polarilocular=, =Polaribilocular=, two-celled spores with thick median wall traversed by a connecting tube, 188, 340-341.

=Polytomous=, arising of several branches of the podetium from one level, 118.

=Proper margin=, the hyphal margin surrounding the apothecium, 157.

=Prothallus=, =Protothallus=, first stages of hyphal growth; cf. hypothallus, 71.

=Pycnidiospores=, stylospores borne in pycnidia, 198 et passim.

=Pycnidium=, roundish fructification, usually with an opening at the apex, containing sporophores and stylospores; cf. spermogonium, 192 et seq.

=Pyrenolichens=, in which the fructification is a closed perithecium, 173 et passim.

=Radiate thallus=, the tissues radiate from a centre, 98 et seq.

=Rhagadiose=, deeply chinked, 74; cf. rimose.

=Rhizina=, attaching “rootlet,” 92-94.

=Rimose=, =Rimulose=, cleft or chinked into areolae, 73.

=Rimose-diffract=, widely cracked or chinked, 74.

=Scutellate=, shaped like a platter, 156.

=Scyphus=, cup-like dilatation of the podetium, 111, 117.

=Signature=, a term in ancient medicine to signify the resemblance of a plant to any part of the human body, 406, 409.

=Soralium=, group of soredia surrounded by a definite margin, 144.

=Soredium=, minute separable particle arising from the gonidial tissue of the thallus, and consisting of algae and hyphae, 141.

=Spermatium=, spore-like body borne in the spermogonium, regarded as a non-motile male cell or as a pycnidiospore, 201.

=Spermogonium=, roundish closed receptacle containing spermatia, 192.

=Sphaeroid-cell=, swollen hyphal cell, containing fat globules, 215.

=Squamule=, a small thalline lobe or scale, 74 et passim.

=Sterigma=, Nylander’s term for the spermatiophore, 197.

=Stratose thallus=, where the tissues are in horizontal layers, 70.

=Stratum=, a layer of tissue in the thallus, 70.

=Symbiont=, one of two dissimilar organisms living together, 32.

=Symbiosis=, a living together of dissimilar organisms, also termed commensalism, 31, 32 et seq.

=Tegulicolous=, living on tiles, 369.

=Terebrator=, boring apparatus, term used by Lindau for the lichen “trichogyne,” 179.

=Thalline margin=, an apothecial margin formed of and usually coloured like the thallus; cf. amphithecium.

=Thallus=, vegetative body or soma of the lichen plant, 11, 421. Endogenous thallus in which the alga predominates, 68. Exogenous thallus in which the fungus predominates, 69.

=Theca=, enlarged cell containing spores; cf. ascus.

=Thecium=, layer of tissue in the apothecium consisting of asci and paraphyses; cf. hymenium, 157.

=Trichogyne=, prolongation of the egg-cell in Florideae which acts as a receptive tube; septate hypha in lichens arising from the ascogonium, 160, 177-181, 273.

=Woronin’s hypha=, a coiled hypha occurring in the centre of the fruit primordium, 159, 163.

ERRATA

p. 24. For Baranetsky read Baranetzky. p. 277. For Ascolium read Acolium. p. 318. For Lepolichen coccophora read coccophorus.

Transcriber’s Note: The errata have been corrected.

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Introduction

Lichens are, with few exceptions, perennial aerial plants of somewhat lowly organization. In the form of spreading encrustations, horizontal leafy expansions, of upright strap-shaped fronds or of pendulous filaments, they take possession of the tree-trunks, palings, walls, rocks or even soil that afford them a suitable and stable foothold. The vegetative body, or thallus, which may be extremely long-lived, is of varying colour, white, yellow, brown, grey or black. The great majority of lichens are Ascolichens and reproduction is by ascospores produced in open or closed fruits (apothecia or perithecia) which often differ in colour from the thallus. There are a few Hymenolichens which form basidiospores. Vegetative reproduction by soredia is frequent.

Lichens abound everywhere, from the sea-shore to the tops of high mountains, where indeed the covering of perpetual snow is the only barrier to their advance; but owing to their slow growth and long duration, they are more seriously affected than are the higher plants by chemical or other atmospheric impurities and they are killed out by the smoke of large towns: only a few species are able to persist in somewhat depauperate form in or near the great centres of population or of industry.

The distinguishing feature of lichens is their composite nature: they consist of two distinct and dissimilar organisms, a fungus and an alga, which, in the lichen thallus, are associated in some kind of symbiotic union, each symbiont contributing in varying degree to the common support: it is a more or less unique and not unsuccessful venture in plant-life. The algae—Chlorophyceae or Myxophyceae—that become lichen symbionts or “gonidia” are of simple structure, and, in a free condition, are generally to be found in or near localities that are also the customary habitats of lichens. The fungus is the predominant partner in the alliance as it forms the fruiting bodies. It belongs to the Ascomycetes[1], except in a few tropical lichens (Hymenolichens), in which the fungus is a Basidiomycete. These two types of plants (algae and fungi) belonging severally to many different genera and species have developed in their associated life this new lichen organism, different from themselves as well as from all other plants, not only morphologically but physiologically. Thus there has arisen a distinct class, with families, genera and species, which through all their varying forms retain the characteristics peculiar to lichens.

In the absence of any “visible” seed, there was much speculation in early days as to the genesis of all the lower plants and many opinions were hazarded as to their origin. Luyken[2], for instance, thought that lichens were compounded of air and moisture. Hornschuch[3] traced their origin to a vegetable infusorium, Monas Lens, which became transformed to green matter and was further developed by the continued action of light and air, not only to lichens, but to algae and mosses, the type of plant finally evolved being determined by the varying atmospheric influences along with the chemical nature of the substratum. An account[4] is published of Nees von Esenbeck, on a botanical excursion, pointing out to his students the green substance, Lepraria botryoides, which covered the lower reaches of walls and rocks, while higher up it assumed the grey lichen hue. This afforded him sufficient proof that the green matter in that dry situation changed to lichens, just as in water it changed to algae. An adverse criticism by Dillenius[5] on a description of a lichen fructification is not inappropriate to those early theorists: “Ex quo apparet, quantum videre possint homines, si imaginatione polleant.”

A constant subject of speculation and of controversy was the origin of the green cells, so dissimilar to the general texture of the thallus. It was thought finally to have been established beyond dispute that they were formed directly from the colourless hyphae and, as a corollary, Protococcus and other algal cells living in the open were considered to be escaped gonidia or, as Wallroth[6] termed them, “unfortunate brood-cells,” his view being that they were the reproductive organs of the lichen plant that had failed to develop.

It was a step forward in the right direction when lichens were regarded as transformed algae, among others by Agardh[7], who believed that he had followed the change from Nostoc lichenoides to the lichen Collema limosum. Thenceforward their double resemblance, on the one hand to algae, on the other to fungi, was acknowledged, and influenced strongly the trend of study and investigation.

The announcement[8] by Schwendener[9] of the dual hypothesis solved the problem for most students, though the relation between the two symbionts is still a subject of controversy. The explanation given by Schwendener, and still held by some[10], that lichens were merely fungi parasitic on algae, was indeed a very inadequate conception of the lichen plant, and it was hotly contested by various lichenologists. Lauder Lindsay[11] dismissed the theory as “merely the most recent instance of German transcendentalism applied to the Lichens.” Earlier still, Nylander[12], in a paper dealing with cephalodia and their peculiar gonidia, had denounced it: “Locum sic suum dignum occupat algolichenomachia inter historias ridiculas, quae hodie haud paucae circa lichenes, majore imaginatione quam scientia, enarrantur.” He never changed his attitude and Crombie[13], wholly agreeing with his estimate of these “absurd tales,” translates a much later pronouncement by him[14]: “All these allegations belong to inept Schwendenerism and scarcely deserve even to be reviewed or castigated so puerile are they—the offspring of inexperience and of a light imagination. No true science there.” Crombie[15] himself in a first paper on this subject declared that “the new theory would necessitate their degradation from the position they have so long held as an independent class.” He scornfully rejected the whole subject as “a Romance of Lichenology, or the unnatural union between a captive Algal damsel and a tyrant Fungal master.” The nearest approach to any concession on the algal question occurs in a translation by Crombie[16] of one of Nylander’s papers. It is stated there that a saxicolous alga (Gongrosira Kütz.) had been found bearing the apothecia of Lecidea herbidula n. sp. Nylander adds: “This algological genus is one which readily passes into lichens.” At a later date, Crombie[17] was even more comprehensively contemptuous and wrote: “whether viewed anatomically or biologically, analytically or synthetically, it is instead of being true science, only the Romance of Lichenology.” These views were shared by many continental lichenologists and were indeed, as already stated, justified to a considerable extent: it was impossible to regard such a large and distinctive class of plants as merely fungi parasitic on the lower algae.

Controversy about lichens never dies down, and that view of their parasitic nature has been freshly promulgated among others by the American lichenologist Bruce Fink[18]. The genetic origin of the gonidia has also been restated by Elfving[19]: the various theories and views are discussed fully in the chapter on the lichen plant.

Much of the interest in lichens has centred round their symbiotic growth. No theory of simple parasitism can explain the association of the two plants: if one of the symbionts is withdrawn—either fungus or alga—the lichen as such ceases to exist. Together they form a healthy unit capable of development and change: a basis for progress along new lines. Permanent characters have been formed which are transmitted just as in other units of organic life.

A new view of the association has been advanced by F. and Mme Moreau[20]. They hold that the most characteristic lichen structures—more particularly the cortex—have been induced by the action of the alga on the fungus. The larger part of the thallus might therefore be regarded as equivalent to a gall: “it is a cecidium, an algal cecidium, a generalized biomorphogenesis.”

The morphological characters of lichens are of exceptional interest, conditioned as they are by the interaction of the two symbionts, and new structures have been evolved by the fungus which provides the general tissue system. Lichens are plants of physiological symbiotic origin, and that aspect of their life-history has been steadily kept in view in this work. There are many new requirements which have had to be met by the lichen hyphae, and the differences between them and the true fungal hyphae have been considered, as these are manifested in the internal economy of the compound plant, and in its reaction to external influences such as light, heat, moisture, etc.

The pioneers of botanical science were of necessity occupied almost exclusively with collecting and describing plants. As the number of known lichens gradually accumulated, affinities were recognized and more or less successful efforts were made to tabulate them in classes, orders, etc. It was a marvellous power of observation that enabled the early workers to arrange the first schemes of classification. Increasing knowledge aided by improved microscopes has necessitated changes, but the old fundamental “genus” Lichen is practically equivalent to the Class Lichenes.

The study of lichens has been a slow and gradual process, with a continual conflict of opinion as to the meaning of these puzzling plants—their structure, reproduction, manner of subsistence and classification as well as their relation to other plants. It has been found desirable to treat these different subjects from a historical aspect, as only thus can a true understanding be gained, or a true judgment formed as to the present condition of the science. It is the story of the evolution of lichenology as well as of lichens that has yielded so much of interest and importance.

The lichenologist may claim several advantages in the study of his subject: the abundant material almost everywhere to hand in country districts, the ease with which the plants are preserved, and, not least, the interest excited by the changes and variations induced by growth conditions; there are a whole series of problems and puzzles barely touched on as yet that are waiting to be solved.

In field work, it is important to note accurately and carefully the nature of the substratum as well as the locality. Crustaceous species should be gathered if possible along with part of the wood or rock to which they are attached; if they are scraped off, the pieces may be reassembled on gummed paper, but that is less satisfactory. The larger forms are more easily secured; they should be damped and then pressed before being laid away: the process flattens them, but it saves them from the risk of being crushed and broken, as when dry they are somewhat brittle. Moistening with water will largely restore their original form. All parts of the lichen, both thallus and fruit, can be examined with ease at any time as they do not sensibly alter in the herbarium, though they lose to some extent their colouring: the blue-grey forms, for instance, often become a uniform dingy brownish-grey.

Microscopic examination in the determination of species is necessary in many instances, but that disability—if it ranks as such—is shared by other cryptogams, and may possibly be considered an inducement rather than a deterrent to the study of lichens. For temporary examination of microscopic preparations, the normal condition is best observed by mounting them in water. If the plants are old and dry, the addition of a drop or two of potash—or ammonia—solution is often helpful in clearing the membranes of the cells and in restoring the shrivelled spores and paraphyses to their natural forms and dimensions.

If serial microtome sections are desired, more elaborate methods are required. For this purpose Peirce[21] has recommended that “when dealing with plants that are dry but still alive, the material should be thoroughly wetted and kept moist for two days, then killed and fixed in a saturated solution of corrosive sublimate in thirty-five per cent. alcohol.” The solution should be used hot: the usual methods of dehydrating and embedding in paraffin are then employed with extra precautions on account of the extremely brittle nature of lichens.

Another method that also gave good results has been proposed by French[22]: “first the lichen is put into 95 per cent. alcohol for 24 hours, then into thin celloidin and thick celloidin 24 hours each. After this the specimens are embedded in thick celloidin which is hardened in 70 per cent. alcohol for 24 hours and then cut.” French advises staining with borax carmine: it colours the fungal part pale carmine and the algal cells a greenish-red shade.

Modern research methods of work are generally described in full in the publications that are discussed in the following chapters. The student is referred to these original papers for information as to fixing, embedding, staining, etc.

Great use has been made of reagents in determining lichen species. They are extremely helpful and often give the clinching decision when morphological characters are obscure, especially if the plant has been much altered by the environment. It must be borne in mind, however, that a species is a morphological rather than a physiological unit, and it is not the structures but the cell-products that are affected by reagents. Those most commonly in use are saturated solutions of potash and of bleaching-powder (calcium hypochlorite). The former is cited in text-books as KOH or simply as K, the latter as CaCl or C. The C solution deteriorates quickly and must, therefore, be frequently renewed to produce the required reaction, i.e. some change of colour. These two reagents are used singly or, if conjointly, K followed by C. The significance of the colour changes has been considered in the discussion on lichen-acids.

Iodine is generally cited in connection with its staining effect on the hymenium of the fruit; the blue colour produced is, however, more general than was at one time supposed and is not peculiar to lichens; the asci of many fungi react similarly though to a less extent. The medullary hyphae in certain species also stain blue with iodine.

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I. HISTORY OF LICHENOLOGY

A. INTRODUCTORY

The term “lichen” is a word of Greek origin used by Theophrastus in his History of Plants to signify a superficial growth on the bark of olive-trees. The name was given in the early days of botanical study not to lichens, as we understand them, but to hepatics of the Marchantia type. Lichens themselves were generally described along with various other somewhat similar plants as “Muscus” (Moss) by the older writers, and more definitely as “Musco-fungus” by Morison[23]. In a botanical work published in 1700 by Tournefort[24] all the members of the vegetable kingdom then known were for the first time classified in genera, and the genus Lichen was reserved for the plants that have been so designated since that time, though Dillenius[25] in his works preferred the adjectival name Lichenoides.

A painstaking historical account of lichens up to the beginning of modern lichenology has been written by Krempelhuber[26], a German lichenologist. He has grouped the data compiled by him into a series of Periods, each one marked by some great advance in knowledge of the subject, though, as we shall see, the advance from period to period has been continuous and gradual. While following generally on the lines laid down by Krempelhuber, it will be possible to cite only the more prominent writers and it will be of much interest to British readers to note especially the work of our own botanists.

Krempelhuber’s periods are as follows:

I. From the earliest times to the end of the seventeenth century.

II. Dating from the arrangement of plants into classes called genera by Tournefort in 1694 to 1729.

III. From Micheli’s division of lichens into different orders in 1729 to 1780.

IV. The definite and reasoned establishment of lichen genera based on the structure of thallus and fruit by Weber in 1780 to 1803.

V. The arrangement of all known lichens under their respective genera by Acharius in 1803 to 1846.

VI. The recognition of spore characters in classification by De Notaris in 1846 to 1867.

A seventh period which includes modern lichenology, and which dates after the publication of Krempelhuber’s History, was ushered in by Schwendener’s announcement in 1867 of the hypothesis as to the dual nature of the lichen thallus. Schwendener’s theory gave a new impulse to the study of lichens and strongly influenced all succeeding investigations.

B. PERIOD I. PREVIOUS TO 1694

Our examination of lichen literature takes us back to Theophrastus, the disciple of Plato and Aristotle, who lived from 371 to 284 B.C., and who wrote a History of Plants, one of the earliest known treatises on Botany. Among the plants described by Theophrastus, there are evidently two lichens, one of which is either an Usnea or an Alectoria, and the other certainly Roccella tinctoria, the last-named an important economic plant likely to be well known for its valuable dyeing properties. The same or somewhat similar lichens are also probably alluded to by the Greek physician Dioscorides, in his work on Materia Medica, A.D. 68. About the same time Pliny the elder, who was a soldier and traveller as well as a voluminous writer, mentions them in his Natural History which was completed in 77 A.D.

During the centuries that followed, there was little study of Natural History, and, in any case, lichens were then and for a long time after considered to be of too little economic value to receive much attention.

In the sixteenth century there was a great awakening of scientific interest all over Europe, and, after the printing-press had come into general use, a number of books bearing on Botany were published. It will be necessary to chronicle only those that made distinct contributions to the knowledge of lichens.

The study of plants was at first entirely from a medical standpoint and one of the first works, and the first book on Natural History, printed in England, was the Grete Herball[27]. It was translated from a French work, Hortus sanitatis, and published by Peter Treveris in Southwark. One of the herbs recommended for various ailments is “Muscus arborum,” the tree-moss (Usnea). A somewhat crude figure accompanies the text.

Ruel[28] of Soissons in France, Dorstenius[29], Camerarius[30] and Tabernaemontanus[31] in Germany followed with works on medical or economic botany and they described, in addition to the tree-moss, several species of reputed value in the art of healing now known as Sticta (Lobaria) pulmonaria, Lobaria laetevirens, Cladonia pyxidata, Evernia prunastri and Cetraria islandica. Meanwhile L’Obel[32], a Fleming, who spent the latter part of his life in England and is said to have had charge of a physic garden at Hackney, was appointed botanist to James I. He published at Antwerp a large series of engravings of plants, and added a species of Ramalina to the growing list of recognized lichens. Dodoens[33], also a Fleming, records not only the Usnea of trees, but a smaller and more slender black form which is easily identifiable as Alectoria jubata. He also figures Lichen pulmonaria and gives the recipe for its use.

The best-known botanical book published at that time, however, is the Herball of John Gerard[34] of London, Master in Chirurgerie, who had a garden in Holborn. He recommends as medicinally valuable not only Usnea, but also Cladonia pyxidata, for which he coined the name “cuppe- or chalice-moss.” About the same time Schwenckfeld[35] recorded, among plants discovered by him in Silesia, lichens now familiar as Alectoria jubata, Cladonia rangiferina and a species of Peltigera.

Among the more important botanical writers of the seventeenth century may be cited Colonna[36] and Bauhin[37]. The former, an Italian, contributes, in his Ecphrasis, descriptions and figures of three additional species easily recognized as Physcia ciliaris, Xanthoria parietina and Ramalina calicaris. Kaspar Bauhin, a professor in Basle, who was one of the most advanced of the older botanists, was the first to use a binomial nomenclature for some of his plants. He gives a list in his Pinax of the lichens with which he was acquainted, one of them, Cladonia fimbriata, being a new plant.

John Parkinson’s[38] Herball is well known to English students; he adds one new species for England, Lobaria pulmonaria, already recorded on the Continent. Parkinson was an apothecary in London and held the office of the King’s Herbarist; his garden was situated in Long Acre. How’s[39] Phytographia is notable as being the first account of British plants compiled without reference to their healing properties. Five of the plants described by him are lichen species: “Lichen arborum sive pulmonaria” (Lobaria pulmonaria), “Lichen petraeus tinctorius” (Roccella), “Muscus arboreus” (Usnea), “Corallina montana” (Cladonia rangiferina) and “Muscus pixoides” (Cladonia). Several other British species were added by Merrett[40], who records in his Pinax, “Muscus arboreus umbilicatus” (Physcia ciliaris), “Muscus aureus tenuissimus” (Teloschistes flavicans), “Muscus caule rigido” (Alectoria) and “Lichen petraeus purpureus” (Parmelia omphalodes), the last-named, a rock lichen, being used, he tells us, for dyeing in Lancashire.

Merret or Merrett was librarian to the Royal College of Physicians. His Pinax was undertaken to replace How’s Phytographia published sixteen years previously and then already out of print. Merrett’s work was issued in 1666, but the first impression was destroyed in the great fire of London and most of the copies now extant are dated 1667. He arranged the species of plants in alphabetical order, but as the work was not critical it fell into disuse, being superseded by John Ray’s Catalogus and Synopsis. To Robert Plot[41] we owe the earliest record of Cladonia coccifera which had hitherto escaped notice; it was described and figured as a new and rare plant in the Natural History of Staffordshire[41]. Plot was the first Custos of Ashmole’s Museum in Oxford and he was also the first to prepare a County Natural History.

The greatest advance during this first period was made by Robert Morison[42], a Scotsman from Aberdeen. He studied medicine at Angers in France, superintended the Duke of Orleans’ garden at Blois, and finally, after his return to this country in 1669, became Keeper of the botanic garden at Oxford. In the third volume of his great work[42] on Oxford plants, which was not issued till after his death, the lichens are put in a separate group—“Musco-fungus”—and classified with some other plants under “Plantae Heteroclitae.” The publication of the volume projects into the next historical period.

Long before this date John Ray had begun to study and publish books on Botany. His Catalogue of English Plants[43] is considered to have commenced a new era in the study of the science. The Catalogue was followed by the History of Plants[44], and later by a Synopsis of British Plants[45], and in all of these books lichens find a place. Two editions of the Synopsis appeared during Ray’s lifetime, and to the second there is added an Appendix contributed by Samuel Doody which is entirely devoted to Cryptogamic plants, including not a few lichens—still called “Mosses”—discovered for the first time. Doody, himself an apothecary, took charge of the garden of the Apothecaries’ Society at Chelsea, but his chief interest was Cryptogamic Botany, a branch of the subject but little regarded before his day. Pulteney wrote of him as the “Dillenius of his time.”

Among Doody’s associates were the Rev. Adam Buddle, James Petiver and William Sherard. Buddle was primarily a collector and his herbarium is incorporated in the Sloane Herbarium at the British Museum. It contains lichens from all parts of the world, many of them contributed by Doody, Sherard and Petiver. Only a few of them bear British localities: several are from Hampstead where Buddle had a church.

The Society of Apothecaries had been founded in 1617 and the members acquired land on the river-front at Chelsea, which was extended later and made into a Physick Garden. James Petiver[46] was one of the first Demonstrators of Plants to the Society in connection with the garden, and one of his duties was to conduct the annual herborizing tours of the apprentices in search of plants. He thus collected a large herbarium on the annual excursions, as well as on shorter visits to the more immediate neighbourhood of London. He wrote many tracts on Natural History subjects, and in these some lichens are included. He was one of the best known of Ray’s correspondents, and owing to his connection with the Physic Garden received plants from naturalists in foreign countries.

Sherard, another of Doody’s friends, had studied abroad under Tournefort and was full of enthusiasm for Natural Science. It was he who brought Dillenius to England and finally nominated him for the position of the first Sherardian Professor of Botany at Oxford. Another well-known contemporary botanist was Leonard Plukenet[47] who had a botanical garden at Old Palace Yard, Westminster. He wrote several botanical works in which lichens are included.

Morison is the only one of all the botanists of the time who recognized lichens as a group distinct from mosses, algae or liverworts, and even he had very vague ideas as to their development. Malpighi[48] had noted the presence of soredia on the thallus of some species, and regarded them as seeds. Porta[49], a Neapolitan, has been quoted by Krempelhuber as probably the first to discover and place on record the direct growth of lichen fronds from green matter on the trunks of trees.

C. PERIOD II. 1694-1729

The second Period is ushered in with the publication of a French work, Les Élémens de Botanique by Tournefort[50], who was one of the greatest botanists of the time. His object was—“to facilitate the knowledge of plants and to disentangle a science which had been neglected because it was found to be full of confusion and obscurity.” Up to this date all plants were classified or listed as individual species. It was Tournefort who first arranged them in groups which he designated “genera” and he gave a careful diagnosis of each genus.

Les Élémens was successful enough to warrant the publication a few years later of a larger Latin edition entitled Institutiones[51] and thus fitted for a wider circulation. Under the genus Lichen, he included plants “lacking flowers but with a true cup-shaped shallow fruit, with very minute pollen or seed which appeared to be subrotund under the microscope.” Not only the description but the figures prove that he was dealing with ascospores and not merely soredia, though under Lichen along with true members of the “genus” he has placed a Marchantia, the moss Splachnum and a fern. A few lichens were placed by him in another genus Coralloides.

Tournefort’s system was of great service in promoting the study of Botany: his method of classification was at once adopted by the German writer Rupp[52] who published a Flora of plants from Jena. Among these plants are included twenty-five species of lichens, several of which he considered new discoveries, no fewer than five being some form of Lichen gelatinosus (Collema). Buxbaum[53], in his enumeration of plants from Halle, finds place for forty-nine lichen species, with, in addition, eleven species of Coralloides; and Vaillant[54] in listing the plants that grew in the neighbourhood of Paris gives thirty-three species for the genus Lichen of which a large number are figured, among them species of Ramalina, Parmelia, Cladonia, etc.

In England, however, Dillenius[55], who at this time brought out a third edition of Ray’s Synopsis and some years later his own Historia Muscorum, still described most of his lichens as “Lichenoides” or “Coralloides”; and no other work of note was published in our country until after the Linnaean system of classification and of nomenclature was introduced.

D. PERIOD III. 1729-1780

Lichens were henceforth regarded as a distinct genus or section of plants. Micheli[56], an Italian botanist, Keeper of the Grand Duke’s Gardens in Florence, realized the desirability of still further delimitation, and he broke up Tournefort’s large comprehensive genera into numerical Orders. In the genus Lichen, he found occasion for 38 of these Orders, determined mainly by the character of the thallus, and the position on it of apothecia and soredia. He enumerates the species, many of them new discoveries, though not all of them recognizable now. His great work on Plants is enriched by a series of beautiful figures. It was published in 1729 and marks the beginning of a new Period—a new outlook on botanical science. Micheli regarded the apothecia of lichens as “floral receptacles,” and the soredia as the seed, because he had himself followed the development of lichen fronds from soredia.

The next writer of distinction is the afore-mentioned Dillen or Dillenius. He was a native of Darmstadt and began his scientific career in the University of Giessen. His first published work[57] was an account of plants that were to be found near Giessen in the different months of the year. Mosses and lichens he has assigned to December and January. Sherard induced him to come to England in 1721, and at first engaged his services in arranging the large collections of plants which he, Sherard, had brought from Smyrna or acquired from other sources.

Three years after his arrival Dillenius had prepared the third edition of Ray’s Synopsis for the press, but without putting his name on the title-page[58]. Sherard explained, in a letter to Dr Richardson of Bierly in Yorkshire, that “our people can’t agree about an editor, they are unwilling a foreigner should put his name to it.” Dillenius, who was quite aware of the prejudice against aliens, himself writes also to Dr Richardson: “there being some apprehension (me being a foreigner) of making natives uneasy if I should publicate it in my name.” Lichens were already engaging his attention, and descriptions of 91 species were added to Ray’s work. So well did this edition meet the requirements of the age, that the Synopsis remained the text-book of British Botany until the publication of Hudson’s Flora Anglica in 1762.

William Sherard died in 1728. He left his books and plates to the University of Oxford with a sum of money to endow a Professorship of Botany. In his will he had nominated Dr Dillenius for the post. The great German botanist was accordingly appointed and became the first Sherardian Professor of Botany, though he did not remove to Oxford till 1734. The following years were devoted by him to the preparation of Historia Muscorum, which was finally published in 1741. It includes an account of the then known liverworts, mosses and lichens. The latter—still considered by Dillenius as belonging to mosses—were grouped under three genera, Usnea, Coralloides and Lichenoides. The descriptions and figures are excellent, and his notes on occasional lichen characteristics and on localities are full of interest. His lichen herbarium, which still exists at Oxford, mounted with the utmost care and neatness, has been critically examined by Nylander and Crombie[59] and many of the species identified.

Dillenius was ignorant of, or rejected, Micheli’s method of classification, adopting instead the form of the thallus as a guide to relationship. He also differed from him in his views as to propagation, regarding the soredia as the pollen of the lichen, and the apothecia as the seed-vessels, or even in certain cases as young plants.

Shortly after the publication of Dillenius’ Historia, appeared Haller’s[60] Systematic and Descriptive list of plants indigenous to Switzerland. The lichens are described as without visible leaves or stamens but with “corpuscula” instead of flowers and leaves. He arranged his lichen species, 160 in all, under seven different Orders: 1. “Lichenes Corniculati and Pyxidati”; 2. “L. Coralloidei”; 3. “L. Fruticosi”; 4. “L. Pulmonarii”; 5. “L. Crustacei” (with flower-shields); 6. “L. Scutellis” (with shields but with little or no thallus); and 7. “L. Crustacei” (without shields).

This period extends till near the end of the eighteenth century, and thus includes within its scope the foundation of the binomial system of naming plants established by Linnaeus[61]. The renowned Swedish botanist rather scorned lichens as “rustici pauperrimi,” happily translated by Schneider[62] as the “poor trash of vegetation,” but he named and listed about 80 species. He divided his solitary genus Lichen into sections: 1. “Leprosi tuberculati”; 2. “Leprosi scutellati”; 3. “Imbricati”; 4. “Foliacei”; 5. “Coriacei”; 6. “Scyphiferi”; 7. “Filamentosi.” By this ordered sequence Linnaeus showed his appreciation of development, beginning, as he does, with the leprose crustaceous thallus and continuing up to the most highly organized filamentous forms. He and his followers still included the genus Lichen among Algae.

A voluminous History of Plants had been published in 1751 by Sir John Hill[63], the first superintendent to be appointed to the Royal Gardens, Kew. In the History lichens are included under the Class “Mosses,” and are divided into several vaguely limited “genera”—Usnea, tree mosses, consisting of filaments only; Platysma, flat branched tree mosses, such as lung-wort; Cladonia, the orchil and coralline mosses, such as Cladonia furcata; Pyxidium, the cup-mosses; and Placodium, the crustaceous, friable or gelatinous forms. A number of plants are somewhat obscurely described under each genus. Not only were these new Lichen genera suggested by him, but among his plants are such binomials as Usnea compressa, Platysma corniculatum, Cladonia furcata and Cladonia tophacea; other lichens are trinomial or are indicated, in the way then customary, by a whole sentence. Hill’s studies embraced a wide variety of subjects; he had flashes of insight, but not enough concentration to make an effective application of his ideas. In his Flora Britannica[64], which was compiled after the publication of Linnaeus’s Species Plantarum, he abandoned his own arrangement in favour of the one introduced by Linnaeus and accepted again the single genus Lichen.

Sir William Watson[65], a London apothecary and physician of scientific repute at this period, proposed a rearrangement and some alteration of Linnaeus’s sections. He had failed to grasp the principle of development, but he gives a good general account of the various groups. Watson was the progenitor of those who decry the makers and multipliers of species. So in regard to Micheli, who had increased the number to “298,” he writes: “it is to be regretted, that so indefatigable an author, one whose genius particularly led him to scrutinize the minuter subjects of the science, should have been so solicitous to increase the number of species under all his genera: an error this, which tends to great confusion and embarrassment, and must retard the progress and real improvement of the botanic science.” Linnaeus however in redressing the balance earned his full approbation: “He has so far retrenched the genus (Lichen) that in his general enumeration of plants he recounts only 80 species belonging to it.”

Linnaeus’s binomial system was almost at once adopted by the whole botanical world and the discovery and tabulation of lichens as well as of other plants proceeded apace. Scopoli’s[66] Flora Carniolica, for instance, published in 1760, still adhered to the old descriptive method of nomenclature, but a second edition, issued twelve years later, is based on the new system: it includes 54 lichen species.

About this time Adanson[67] proposed a new classification of plants, dividing them into families, and these again into sections and genera. He transferred the lichens to the Family “Fungi,” and one of his sections contains a number of lichen genera, the names of these being culled from previous workers, Dillenius, Hill, etc. A few new ones are added by himself, and one of them, Graphis, still ranks as a good genus.

In England, Hudson[68], who was an apothecary and became sub-librarian of the British Museum, followed Linnaeus both in the first and later editions of the Flora Anglica. He records 102 lichen species. Withering[69] was also engaged, about this time, in compiling his Arrangement of Plants. He translated Linnaeus’s term “Algae” into the English word “Thongs,” the lichens being designated as “Cupthongs.” In later editions, he simply classifies lichens as such. Lightfoot[70], whose descriptive and economic notes are full of interest, records 103 lichens in the Flora Scotica, and Dickson[71] shortly after published a number of species from Scotland, some of them hitherto undescribed. Dickson was a nurseryman who settled in London, and his avocations kept him in touch with plant-lovers and with travellers in many lands.

E. PERIOD IV. 1780-1803

The inevitable next advance was made by Weber[72] who at the time was a Professor at Kiel. In a first work dealing with lichens he had followed Linnaeus; then he published a new method of classification in which the lichens are considered as an independent Order of Cryptogamia, and that Order, called “Aspidoferae,” he subdivided into genera. His ideas had been partly anticipated by Hill and by Adanson, but the work of Weber indicates a more correct view of the nature of lichens. He established eight fairly well-marked genera, viz. Verrucaria, Tubercularia, Sphaerocephalum and Placodium, which were based on fruit-characters, the thallus being crustaceous and rather insignificant, and a second group Lichen, Collema, Cladonia and Usnea, in which the thallus ranked first in importance. Though Weber’s scheme was published in 1780, it did not at first secure much attention. The great authority of Linnaeus dominated so strongly the botany of the period that for a long time no change was welcomed or even tolerated.

In our own country Relhan at Cambridge and Sibthorp[73] at Oxford were making extensive studies of plants. The latter was content to follow Linnaeus in his treatment of lichens. Relhan[74] also grouped his lichens under one genus though, in a second edition of his Flora, he broke away from the Linnaean tradition and adopted the classification of Acharius.

Extensive contributions to the knowledge of English plants generally were made by Sir James Edward Smith[75] who, in 1788, founded the Linnean Society of London of which he was President until his death in 1828. He began his great work, English Botany, in 1790 with James Sowerby as artist. Smith’s and Sowerby’s part of the work came to an end in 1814; but a supplement was begun in 1831 by Hooker who had the assistance of Sowerby’s sons in preparing the drawings. Nearly all the lichens recorded by Smith are published simply as Lichen, and his Botany thus belongs to the period under discussion, though in time it stretches far beyond.

Continental lichenologists had been more receptive to new ideas, and other genera were gradually added to Weber’s list, notably by Hoffmann[76] and Persoon[77].

For a long time little was known of the lichens of other than European countries. Buxbaum[78] in the East, Petiver[79] and Hans Sloane[80] in the West made the first exotic records. The latter notes how frequently lichens grew on the imported Jesuit’s bark, and he quaintly suggests in regard to some of these species that they may be identical with the “hyssop that springeth out of the wall.” It was not however till towards the end of the eighteenth century that much attention was given to foreign lichens, when Swartz[81] in the West Indies and Desfontaines[82] in N. Africa collected and recorded a fair number. Swartz describes about twenty species collected on his journey through the West Indian Islands (1783-87).

Interest was also growing in other aspects of lichenology. Georgi[83], a Russian Professor, was the first to make a chemical analysis of lichens. He experimented on some of the larger forms and extracted and examined the mucilaginous contents of Ramalina farinacea, Platysma glaucum, Lobaria pulmonaria, etc., which he collected from birch and pine trees. About this time also the French scientists Willomet[84], Amoreux and Hoffmann jointly published theses setting forth the economic value of such lichens as were used in the arts, as food, or as medicine.

F. PERIOD V. 1803-1846

The fine constructive work of Acharius appropriately begins a new era in the history of lichenology. Previous writers had indeed included lichens in their survey of plants, but always as a somewhat side issue. Acharius made them a subject of special study, and by his scientific system of classification raised them to the rank of the other great classes of plants.

Acharius was a country doctor at Wadstena on Lake Mälar in Sweden, as he himself calls it, “the country of lichens.” He was attracted to the study of them by their singular mode of growth and organization, both of thallus and reproductive organs, for which reason he finally judged that lichens should be considered as a distinct Order of Cryptogamia.

In his first tentative work[85] he had followed his great compatriot Linnaeus, classifying all the species known to him under the one genus Lichen, though he had progressed so far as to divide the unwieldy Genus into Families and these again into Tribes, these latter having each a tribal designation such as Verrucaria, Opegrapha, etc. He established in all twenty-eight tribes which, at a later stage, he transformed into genera after the example of Weber.

Acharius, from the beginning of his work, had allowed great importance to the structure of the apothecia as a diagnostic character though scarcely recognizing them as true fruits. He gave expression to his more mature views first in the Methodus Lichenum[86], then subsequently in the larger Lichenographia Universalia[87]. In the latter work there are forty-one genera arranged under different divisions; the species are given short and succinct descriptions, with habitat, locality and synonymy. No material alteration was made in the Synopsis Lichenum[88], a more condensed work which he published a few years later.

The Cryptogamia are divided by Acharius into six “Families,” one of which, “Lichenes,” is distinguished, he finds, by two methods of propagation: by propagula (soredia) and by spores produced in apothecia. He divides the family into classes characterized solely by fruit characters, and these again into orders, genera and species, of which diagnoses are given. With fuller knowledge many changes and rearrangements have been found necessary in the application and extension of the system, but that in no way detracts from the value of the work as a whole.

In addition to founding a scientific classification, Acharius invented a terminology for the structures peculiar to lichens. We owe to him the names and descriptions of “thallus,” “podetium,” “apothecium,” “perithecium,” “soredium,” “cyphella” and “cephalodium,” the last word however with a different meaning from the one now given to it. He proposed several others, some of which are redundant or have fallen into disuse, but many of his terms as we see have stood the test of time and have been found of service in allied branches of botany.

Lichens were studied with great zest by the men of that day. Hue[89] recalls a rather startling incident in this connection: Wahlberg, it is said, had informed Dufour that he had sent a large collection of lichens from Spain to Acharius who was so excited on receiving them, that he fell ill and died in a few days (Aug. 14th, 1819). Dufour, however, had added the comment that the illness and death might after all be merely a coincidence.

Among contemporary botanists, we find that De Candolle[90] in the volume he contributed to Lamarck’s French Flora, quotes only from the earlier work of Acharius. He had probably not then seen the Methodus, as he uses none of the new terms; the lichens of the volume are arranged under genera which are based more or less on the position of the apothecia on the thallus. Flörke[91], the next writer of consequence, frankly accepts the terminology and the new view of classification, though differing on some minor points.

Two lists of lichens, neither of particular note, were published at this time in our country: one by Hugh Davies[92] for Wales, which adheres to the Linnaean system, and the other by Forster[93] of lichens round Tonbridge. Though Forster adopts the genera of Acharius, he includes lichens among algae. A more important publication was S. F. Gray’s[94] Natural Arrangement of British Plants. Gray, who was a druggist in Walsall and afterwards a lecturer on botany in London, was only nominally[95] the author, as it was mainly the work of his son John Edward Gray[96], sometime Keeper of Zoology in the British Museum. Gray was the first to apply the principles of the Natural System of classification to British plants, but the work was opposed by British botanists of his day. The years following the French Revolution and the Napoleonic wars were full of bitter feeling and of prejudice, and anything emanating, as did the Natural System, from France was rejected as unworthy of consideration.

In the Natural Arrangement, Gray followed Acharius in his treatment of lichens; but whereas Acharius, though here and there confusing fungus species with lichens, had been clear-sighted enough to avoid all intermixture of fungus genera, with the exception of one only, the sterile genus Rhizomorpha, Gray had allowed the interpolation of several, such as Hysterium, Xylaria, Hypoxylon, etc. He had also raised many of Acharius’s subgenera and divisions to the rank of genera, thus largely increasing their number. This oversplitting of well-defined genera has somewhat weakened Gray’s work and he has not received from later writers the attention he deserves.

The lichens of Hooker’s[97] Flora Scotica, which is synchronous with Gray’s work, number 195 species, an increase of about 90 for Scotland since the publication of Lightfoot’s Flora more than 40 years before. Hooker also followed Acharius in his classification of lichens both in the Flora Scotica and in the Supplement to English Botany[98], which was undertaken by the younger Sowerbys and himself. To that work Borrer (1781-1862), a keen lichenologist, supplied many new and rare lichens collected mostly in Sussex.

It is a matter of regret that Greville should have so entirely ignored lichens in his great work on Scottish Cryptogams[99]. The two species of Lichina are the only ones he figured, and these he took to be algae. He[100] was well acquainted with lichens, for in the Flora Edinensis he lists 128 species for the Edinburgh district, arranging the genera under “Lichenes” with the exception of Opegrapha and Verrucaria which are placed with the fungus genus Poronia in “Hypoxyla.” Though he cites the publications of Acharius, he does not employ his scientific terms, possibly because he was writing his diagnoses in English. Two other British works of this time still remain to be chronicled: Hooker’s[101] contributions to Smith’s English Flora and Taylor’s[102] work on lichens in Mackay’s Flora Hibernica. Through these the knowledge of the subject was very largely extended in our country.

The classification of lichens and their place in the vegetable kingdom were now firmly established on the lines laid down by Acharius. Fries[103] in his important work Lichenographia Europaea more or less followed his distinguished countryman. The uncertainty as to the position and relationship of lichens had rendered the task of systematic arrangement one of peculiar difficulty and had unduly absorbed attention; but now that a satisfactory order had been established in the chaos of forms, the way was clear for other aspects of the study. Several writers expressed their views by suggesting somewhat different methods of classification, others wrote monographs of separate groups, or genera. Fée[104] published an Essay on the Cryptogams (mostly lichens) that grew on officinal exotic barks; Flörke[105] took up the difficult genus Cladonia; Wallroth[106] also wrote on Cladonia; Delise[107] on Sticta, and Chevalier[108] published a long and elaborate account of Graphideae.

Wallroth and Meyer at this time published, simultaneously, important studies on the general morphology and physiology of lichens. Wallroth[109] had contemplated an even larger work on the Natural History of Lichens, but only two of the volumes reached publication. In the first of these he devoted much attention to the “gonidia” or “brood-cells” and established the distinction between the heteromerous and homoiomerous distribution of green cells within the thallus; he also describes with great detail the “morphosis” and “metamorphosis” of the vegetative body. In the second volume he discusses their physiology—the contents and products of the thallus, colouring, nutrition, season of development, etc.—and finally the pathology of these organisms. He made no great use of the compound microscope, and his studies were confined to phenomena that could be observed with a single lens.

Meyer’s[110] work contains a still more exact study of the anatomy and physiology of lichens; he also devotes many passages to an account of their metamorphoses, pointing out that species alter so much in varying conditions, that the same one at different stages may be placed even in different genera; he however carries his theory of metamorphosis too far and unites together widely separated plants. Meyer was the first to describe the growth of the lichen from spores, though his description is somewhat confused. Possibly the honour of having first observed their germination should be given to a later botanist, Holle[111]. The works of both Wallroth and Meyer enjoyed a great and well-merited reputation: they were standard books of consultation for many years. Koerber[112], who devoted a long treatise to the study of gonidia, confirmed Wallroth’s theories: he considered at that time that the gonidia in the soredial condition were organs of propagation.

Mention should be made here of the many able and keen collectors who, in the latter half of the eighteenth century and the beginning of the nineteenth, did so much to further the knowledge of lichens in the British Isles. Among the earliest of these naturalists are Richard Pulteney (1730-1801), whose collection of plants, now in the herbarium of the British Museum, includes many lichens, and Hugh Davies (1739-1821), a clergyman whose Welsh plants also form part of the Museum collection. The Rev. John Harriman (1760-1831) sent many rare plants from Egglestone in Durham to the editors of English Botany and among them were not a few lichens. Edward Forster (1765-1849) lived in Essex and collected in that county, more especially in and near Epping Forest, and another East country botanist, Dawson Turner (1775-1858), though chiefly known as an algologist, gave considerable attention to lichens. In Scotland the two most active workers were Charles Lyell (1767-1849), of Kinnordy in Forfarshire, and George Don (1798-1856), also a Forfar man. Don was a gardener and became eventually a foreman at the Chelsea Physic Garden. Sir Thomas Gage of Hengrave Hall (1781-1823) botanized chiefly in his own county of Suffolk; but most of his lichens were collected in South Ireland and are incorporated in the herbarium of the British Museum. Miss Hutchins also collected in Ireland and sent her plants for inclusion in English Botany. But in later years, the principal lichenologist connected with that great undertaking was W. Borrer, who spent his life in Sussex: he not only supplied a large number of specimens to the authors, but he himself discovered and described many new lichens.

American lichenologists were also extremely active all through this period. The comparatively few lichens of Michaux’s[113] Flora grouped under “Lichenaceae” were collected in such widely separated regions as Carolina and Canada. A few years later Mühlenberg[114] included no fewer than 184 species in his Catalogue of North American Plants. Torrey[115] and Halsey[116] botanized over a limited area near New York, and the latter, who devoted himself more especially to lichens, succeeded in recording 176 different forms, old and new. These two botanists were both indebted for help in their work to Schweinitz, a Moravian brother, who moved from one country to another, working and publishing, now in America and now in Europe. His name is however chiefly associated with fungi. Later American lichenology is nobly represented by Tuckerman[117] who issued his first work on lichens in 1839, and who continued for many years to devote himself to the subject. He followed at first the classification and nomenclature that had been adopted by Fée, but as time went on he associated himself with all that was best and most enlightened in the growing science.

Travellers and explorers in those days of high adventure were constantly sending their specimens to European botanists for examination and determination, and the knowledge of exotic lichens as of other classes of plants grew with opportunity. Among the principal home workers in foreign material, at this time, may be cited Fée[118] who described a very large series on officinal barks (Cinchona, etc.) so largely coming into use as medicines; he also took charge of the lichens in Martius’s[119] Flora of Brazil. Montagne[120] named large collections, notably those of Leprieur collected in Guiana, and Hooker[121] and Walker Arnott determined the plants collected during Captain Beechey’s voyage, which included lichens from many different regions.

G. PERIOD VI. 1846-1867

The last work of importance, in which microscopic characters were ignored, was the Enumeratio critica Lichenum Europaeum by Schaerer[122], a veteran lichenologist, who rather sadly realized at the end the limitations of that work, as he asks the reader to accept it “such as it is.” Many years previously, Eschweiler[123] in his Systema and Fée[124] in his account of Cryptogams on Officinal Bark, had given particular attention to the internal structure as well as to the outward form of the lichen fructification. Fée, more especially, had described and figured a large number of spores; but neither writer had done more than suggest their value as a guide in the determination of genera and species.

It was an Italian botanist, Giuseppe de Notaris[125], a Professor in Florence, who took up the work where Fée had left it. His comparative studies of both vegetative and reproductive organs convinced him of the great importance of spore characters in classification, the spore being, as he rightly decided, the highest and ultimate product of the lichen plant. In his microscopic examination of the various recognized genera, he found that while, in some genera, the spores conformed to one distinct type, in others their diversities in form, septation or colour gave a decisive reason for the establishment of new genera, while minor differences in size, etc. of the spores proved to be of great value in distinguishing species. The spore standard thus marks a new departure in lichenology. De Notaris published the results of his researches in a fragment of a projected larger work that was never completed. Though his views were overlooked for a time, they were at length fully recognized and further elaborated by Massalongo[126] in Italy, by Norman[127] in Norway, by Koerber[128] in Germany and by Mudd[129] in our own country. Massalongo had drawn up the scheme of a great Scolia Lichenographica, but like de Notaris, he was only able to publish a part. After twelve years of ill-health, in which he struggled to continue his work, he died at the early age of 36.

Lindsay[130], Mudd and Leighton[131] were at this time devoting great attention to British lichens. Lauder Lindsay’s Popular History of British Lichens, with its coloured plates and its descriptive and economic account of these plants has enabled many to acquire a wide knowledge of the group. Mudd’s Manual, a more complete and extremely valuable contribution to the subject, followed entirely on the lines of Massalongo’s work. From his large experience in the examination of lichens he came to the conclusion that: “Of all organs furnished by a given group of plants, none offer so many real, constant and physiological characters as the spores of lichens, for the formation of a simple and natural classification.”

Meanwhile, a contemporary writer, William Nylander, was rising into fame. He was born at Uleaborg in Finland[132] in 1822 and became interested in lichens very early in his career. His first post was the professorship of botany at Helsingfors; but in 1863 he gave up his chair and removed to Paris where he remained, except for short absences, until his death. One of his excursions brought him to London in 1857 to examine Hooker’s herbarium. He devoted his whole life to the study of lichens, and from 1852, the date of his first lichen publication, which is an account of the lichens of Helsingfors, to the end of his life he poured out a constant succession of books or papers, most of them in Latin. One of his earliest works was an Essay on Classification[133] which he elaborated later, but which in its main features he never altered. He relied, in his system, on the structure and form of thallus, gonidia and fructifications, more especially on those of the spermogonia (pycnidia), but he rejected ascospore characters except so far as they were of use in the diagnosis of species. He failed by being too isolated and by his unwillingness to recognize results obtained by other workers. In 1866 he had discovered the staining reactions of potash and hypochlorite of lime on certain thalli, and though these are at times unreliable owing to growth conditions, etc., they have generally been of real service. Nylander, however, never admitted any criticism of his methods; his opinions once stated were never revised. He rejected absolutely the theory of the dual nature of lichens propounded by Schwendener without seriously examining the question, and regarded as personal enemies those who dared to differ from him. The last years of his life were passed in complete solitude. He died in March 1899.

Owing to the very inadequate powers of magnification at the service of scientific workers, the study of lichens as of other plants was for long restricted to the collecting, examining and classifying of specimens according to their macroscopic characters; the microscopic details observed were isolated and unreliable except to some extent for spore characters. Special interest is therefore attached to the various schemes of classification, as each new one proposed reflects to a large extent the condition of scientific knowledge of the time, and generally marks an advance. It was the improvement of the microscope from a scientific toy to an instrument of research that opened up new fields of observation and gave a new impetus to the study of a group of plants that had proved a puzzle from the earliest times.

Tulasne was one of the pioneers in microscopic botany. He made a methodical study of a large series of lichens[134] and traced their development, so far as he was able, from the spore onwards. He gave special attention to the form and function of spermogonia and spermatia, and his work is enriched by beautiful figures of microscopic detail. Lauder Lindsay[135] also published an elaborate treatise on spermogonia, on their occurrence in the lichen kingdom and on their form and structure. The paper embodies the results of wide microscopic research and is a mine of information regarding these bodies.

Much interesting work was contributed at this time by Itzigsohn[136], Speerschneider[137], Sachs[138], Thwaites[139], and others. They devoted their researches to some particular aspect of lichen development and their several contributions are discussed elsewhere in this work.

Schwendener[140] followed with a systematic study of the minute anatomy of many of the larger lichen genera. His work is extremely important in itself and still more so as it gradually revealed to him the composite character of the thallus.

Several important monographs date from this period: Leighton[141] reviewed all the British “Angiocarpous” lichens with special reference to their “sporidia” though without treating these as of generic value. He followed up this monograph by two others, on the Graphideae[142] and the Umbilicarieae[143], and Mudd[144] published a careful study of the British Cladoniae. On the Continent Th. Fries[145] issued a revision of Stereocaulon and Pilophoron and other writers contributed work on smaller groups.

H. PERIOD VII. 1867 AND AFTER

Modern lichenology begins with the enunciation of Schwendener’s[146] theory of the composite nature of the lichen plant. The puzzling resemblance of certain forms to algae, of others to fungi, had excited the interest of botanists from a very early date, and the similarity between the green cells in the thallus, and certain lower forms of algae had been again and again pointed out. Increasing observation concerning the life-histories of these algae and of the gonidia had eventually piled up so great a number of proofs of their identity that Schwendener’s announcement must have seemed to many an inevitable conclusion, though no one before had hazarded the astounding statement that two organisms of independent origin were combined in the lichen.

The dual hypothesis, as it was termed, was not however universally accepted. It was indeed bitterly and scornfully rejected by some of the most prominent lichenologists of the time, including Nylander[147], J. Müller and Crombie[148]. Schwendener held that the lichen was a fungus parasitic on an alga, and his opponents judged, indeed quite rightly, that such a view was wholly inadequate to explain the biology of lichens. It was not till a later date that the truer conception of the “consortium” or “symbiosis” was proposed. The researches undertaken to prove or disprove the new theories come under review in Chapter II.

Stahl’s work on the development of the carpogonium in lichens gave a new direction to study, and notable work has been done during the last forty years in that as in other branches of lichenology.

Exploration of old and new fields furnished the lichen-flora of the world with many new plants which have been described by various systematists—by Nylander, Babington, Arnold, Müller, Th. Fries, Stizenberger, Leighton, Crombie and many others, and their contributions are scattered through contemporary scientific journals. The number of recorded species is now somewhere about 40,000, though, in all probability, many of these will be found to be growth forms. Still, at the lowest computation, the number of different species is very large.

Systematic literature has been enriched by a series of important monographs, too numerous to mention here. While treating definite groups, they have helped to elucidate some of the peculiar biological problems of the symbiotic growth.

Morphology, since Schwendener’s time, has been well represented by Zukal, Reinke, Lindau, Fünfstück, Darbishire, Hue, and by an increasing number of modern writers whose work is duly acknowledged under each subject of study. Hesse and Zopf, and more recently Lettau, have been engaged in the examination of those unique products, the lichen acids, while other workers have investigated lichen derivatives such as fats. Ecology of lichens has also been receiving increased attention. Problems of physiology, symbiosis, etc., are not yet considered to be solved and are being attacked from various sides.

British lichenologists since 1867 have been mainly engaged on field work, with the exception of Lauder Lindsay who published after that date a second great paper on the spermogonia of crustaceous lichens. Leighton in his Lichen Flora and Crombie in numerous publications gave the lead in systematic work, and with them were associated a band of indefatigable collectors. Among these may be recalled Alexander Croall (1809-85), a parish schoolmaster in Scotland whose Plants of Braemar include many of the rarer mountain lichens. Henry Buchanan Holl (1820-86), a surgeon in London, collected in the Scottish Highlands as well as in England and Wales. William Joshua (1828-98) worked mostly in the Western counties of Somerset and Gloucestershire. Charles Du Bois Larbalestier, who died in 1911, was a keen observer and collector during many years; he discovered a number of new species in his native Jersey, in Cambridgeshire and also in Connemara; his plants were generally sent to Nylander to be determined and described. He issued two sets of lichens, one of Channel Island plants, the other of more general British distribution, and he had begun the issue of Cambridgeshire lichens. Isaac Carroll (1828-80), an Irish botanist, issued a first fascicle of Lichenes Hibernici containing 40 numbers. More recently Lett[149] has reported 80 species and varieties from the Mourne Mountains in Ireland. Other more extensive sets were issued by Mudd and by Leighton, and later by Crombie and by Johnson. All these have been of great service to the study of lichenology in our country. Other collectors of note are Curnow (Cornwall), Martindale (Westmoreland), and E. M. Holmes whose valuable herbarium has been secured by University College, Nottingham.

The publication of the volume dealing with Lichenes in Engler and Prantl’s Pflanzenfamilien has proved a boon to all who are interested in the study of lichens. Fünfstück[150] prepared the introduction, an admirable presentation of the morphological and physiological aspects of the subject, while Zahlbruckner[151], with equal success, took charge of the section dealing with classification.

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II. CONSTITUENTS OF THE LICHEN THALLUS

I. LICHEN GONIDIA

The thallus or vegetative body of lichens differs from that of other green plants in the sharp distinction both of form and colour between the assimilative cells and the colourless tissues, and in the relative positions these occupy within the thallus: in the greater number of lichen species the green chlorophyll cells are confined to a narrow zone or band some way beneath and parallel with the surface (Fig. 1); in a minority of genera they are distributed through the entire thallus (Fig. 2); but in all cases the tissues remain distinct. The green zone can be easily demonstrated in any of the larger lichens by scaling off the outer surface cells, or by making a vertical section through the thallus. The colourless cells penetrate to some extent among the green cells; they also form the whole of the cortical and medullary tissues.

These two different elements we now know to consist of two distinct organisms, a fungus and an alga. The green algal cells were at one time considered to be reproductive bodies, and were called “gonidia,” a term still in use though its significance has changed.

1. GONIDIA IN RELATION TO THE THALLUS

A. HISTORICAL ACCOUNT OF LICHEN GONIDIA

There have been few subjects of botanical investigation that have roused so much speculation and such prolonged controversy as the question of these constituents of the lichen plant. The green cells and the colourless filaments which together form the vegetative structure are so markedly dissimilar, that constant attempts have been made to explain the problem of their origin and function, and thereby to establish satisfactorily the relationship of lichens to other members of the Plant Kingdom.

In gelatinous lichens, represented by Collema, of which several species are common in damp places and grow on trees or walls or on the ground, the chains of green cells interspersed through the thallus have long been recognized as comparable with the filaments of Nostoc, a blue-green gelatinous alga, conspicuous in wet weather in the same localities as those inhabited by Collema. So among early systematists, we find Ventenat[152] classifying the few lichens with which he was acquainted under algae and hazarding the statement that a gelatinous lichen such as Collema was only a Nostoc changed in form. Some years later Cassini[153] in an account of Nostoc expressed a somewhat similar view, though with a difference: he suggested that Nostoc was but a monstrous form of Collema, his argument being that, as the latter bore the fruit, it was the normal and perfect condition of the plant. A few years later Agardh[154] claimed to have observed the metamorphosis of Nostoc up to the fertile stage of a lichen, Collema limosum. But long before this date, Scopoli[155] had demonstrated a green colouring substance in non-gelatinous lichens by rubbing a crustaceous or leprose thallus between the fingers; and Persoon[156] made use of this green colour characteristic of lichen crusts to differentiate these plants from fungi. Sprengel[157] went a step further in exactly describing the green tissue as forming a definite layer below the upper cortex of foliaceous lichens.

The first clear description and delimitation of the different elements composing the lichen thallus was, however, given by Wallroth[158]. He drew attention to the great similarity between the colourless filaments of the lichen and the hyphae of fungi. The green globose cells in the chlorophyllaceous lichens he interpreted as brood-cells or gonidia, regarding them as organs of reproduction collected into a “stratum gonimon.” To the same author we owe the terms “homoiomerous” and “heteromerous,” which he coined to describe the arrangement of these green cells in the tissue of the thallus. In the former case the gonidia are distributed equally through the structure; in the latter they are confined to a distinct zone.

Wallroth’s terminology and his views of the function of the gonidia were accepted as the true explanation for many years, the opinion that they were solely reproductive bodies being entirely in accordance with the well-known part played by soredia in the propagation of lichens—and soredia always include one or more green cells.

B. GONIDIA CONTRASTED WITH ALGAE

In describing the gonidia of the Graphideae Wallroth[159] had pointed out their affinity with the filaments of Chroolepus (Trentepohlia) umbrina. He considered these and other green algae when growing loose on the trunks of trees to be but “unfortunate brood-cells” which had become free and, though capable of growth and increase, were unable to form again a lichen plant.

Further observations on gonidia were made by E. Fries[160]: he found that the green cells escaped from the lichen matrix and produced new individuals; and also that the whole thallus in moist localities might become dissolved into the alga known as Protococcus viridis; but, he continues, “though these Protococcus cells multiplied exceedingly, they never could rise again to the perfect lichen.” Kützing[161], in a later account of Protococcus viridis, also recognized its affinity with lichens; he stated that he could testify from observation that, according to the amount of moisture present, it would develop, either in excessive moisture to a filamentous alga, or in drier conditions “to lichens such as Lecanora subfusca or Xanthoria parietina.”

A British botanist, G. H. K. Thwaites[162], at one time superintendent of the botanical garden at Peradeniya in Ceylon, published a notable paper on lichen gonidia in which he pointed out that as in Collema the green constituents of the thallus resembled the chains of Nostoc, so in the non-gelatinous lichens, the green globose cells were comparable or identical with Pleurococcus, and Thwaites further observed that they increased by division within the lichen thallus. He insisted too that in no instance were gonidia reproductive organs: they were essential component parts of the vegetative body and necessary to the life of the plant. In a further paper on Chroolepus ebeneus Ag., a plant consisting of slender dark-coloured felted filaments, he described these filaments as being composed of a central strand which closely resembled the alga Chroolepus, and of a surrounding sheath of dark-coloured cells (Fig. 3): “occasionally,” he writes, “the internal filament protrudes beyond the investing sheath, and may then be seen to consist of oblong cells containing the peculiar reddish oily-looking endochrome of Chroolepus.” Thwaites placed this puzzling plant in a new genus, Cystocoleus, at the same time pointing out its affinity with the lichen genus Coenogonium. The plant is now known as Coenogonium ebeneum. Thwaites was on the threshold of the discovery as to the true nature of the relationship between the central filament and the investing sheath, but he failed to take the next forward step.

Very shortly after, Von Flotow[163] published his views on some other lichen gonidia. He had come to the conclusion that the various species of the alga, Gloeocapsa, so frequently found in damp places, among mosses and lichens, were merely growth stages of the gonidia of Ephebe pubescens, and bore the same relation to Ephebe as did Lepra viridis (Protococcus) to Parmelia. The gonidium of Ephebe is the gelatinous filamentous blue-green alga Stigonema (Fig. 4), and the separate cells are not unlike those of Gloeocapsa. Flotow had also demonstrated that the same type of gonidium was enclosed in the cephalodia of Stereocaulon. Sachs[164], too, gave evidence as to the close connection between Nostoc and Collema. He had observed numerous small clumps of the alga growing in proximity to equally abundant thalli of Collema, with every stage of development represented from one to the other. He found cases where the gelatinous coils of Nostoc chains were penetrated by fine colourless filaments “as if invaded by a parasitic fungus.” Later these threads were seen to be attached to some cell of the Nostoc trichome. Sachs concluded, however, from very careful examination at the time, that the colourless filaments were produced by the green cells. As growth proceeded, the coloured Nostoc chains became massed towards the upper surface, while the colourless filaments tended to occupy the lower part of the thallus. He calculated that during the summer season the metamorphosis from Nostoc to a fertile Collema thallus took from three to four months. He judged that in favourable conditions the change would inevitably take place, though if there should be too great moisture no Collema would be formed. His study of Cladonia was less successful as he mistook some colonies of Gloeocapsa for a growth condition of Cladonia gonidia, an error corrected later by Itzigsohn[165].

But before this date Itzigsohn[166] had published a paper setting forth his views on thallus formation, which marked a distinct advance. He did not hazard any theory as to the origin of gonidia, but he had observed spermatia growing, much as did the cells of Oscillaria: by increase in length, and, by subsequent branching, filaments were formed which surrounded the green cells; these latter had meanwhile multiplied by repeated division till finally a complete thallus was built up, the filamentous tissue being derived from the spermatia, while the green layer came from the original gonidium. In contrasting the development with that of Collema, he represents Nostoc as a sterile product of a lichen and, like the gonidia of other lichens, only able to form a lichen thallus when it encounters the fructifying spermatia.

Braxton Hicks[167], a London doctor, some time later, made experiments with Chroococcus algae which grew in plenty on the bark of trees, and followed their development into a lichen thallus. He further claimed to have observed a Chlorococcus, which was associated with a Cladonia, divide and form a Palmella stage.

C. CULTURE EXPERIMENTS WITH THE LICHEN THALLUS

It had been repeatedly stated that the gonidia might become independent of the thallus, but absolute proof was wanting until Speerschneider[168], who had turned his attention to the subject, made direct culture experiments and was able to follow the liberation of the green cells. He took a thinnish section of the thallus of Hagenia (Physcia) ciliaris, and, by keeping it moist, he was able to observe that the gonidial cells increased by division; the moist condition at the same time caused the colourless filaments to die away. This method of investigation was to lead to further results. It was resorted to by Famintzin and Baranetzky[169] who made cultures of gonidia extracted from three different lichens, Physcia (Xanthoria) parietina, Evernia furfuracea and Cladonia sp. They were able to observe the growth and division of the green cells and, in addition, the formation of zoospores. They recognized the development as entirely identical with that of the unicellular green alga, Cystococcus humicola Naeg. Baranetzky[170] continued the experiments and made cultures of the blue-green gonidia of Peltigera canina and of Collema pulposum. In both instances he succeeded in isolating them from the thallus and in growing them in moist air as separate organisms. He adds that “many forms reckoned as algae, may be considered as vegetating lichen gonidia such as Cystococcus, Polycoccus, Nostoc, etc.” Meanwhile Itzigsohn[171] had further demonstrated by similar culture experiments that the gonidia of Peltigera canina corresponded with the algae known as Gloeocapsa monococca Kütz., and as Polycoccus punctiformis Kütz.

D. THEORIES AS TO THE ORIGIN OF GONIDIA

Though the relationship between the gonidia within the thallus and free-living algal organisms seemed to be proved beyond dispute, the manner in which gonidia first originated had not yet been discovered. Bayrhoffer[172] attacked this problem in a study of foliose and other lichens. According to his observations, certain colourless cells or filaments, belonging to the “gonimic” layer, grew in a downward direction and formed at their tips a faintly yellowish-green cell; it gradually enlarged and was at length thrown off as a free globose gonidium, which represented the female cell. Other filaments from the “lower fibrous layer” of the thallus at the same time grew upwards and from them were given off somewhat similar gonidia which functioned as male cells. His observations and deductions were fanciful, but it must be remembered that the attachment between hypha and alga in lichens is in many cases so close as to appear genetic, and also it often happens that as the gonidium multiplies it becomes free from the hypha.

In his Mémoire sur les Lichens, Tulasne[173] described the colourless filaments as being fungal in appearance. The green cells he recognized as organs of nutrition, and once and again in his paper he states that they arose directly by a sort of budding process from the medullary or cortical filaments, either laterally or at the apex. This apparently reasonable view of their origin was confirmed by other writers on the subject: by Speerschneider[174] in his account of the anatomy of Usnea barbata, by de Bary[175], and by Schwendener[176] in their earlier writings. But even while de Bary accepted the hyphal origin of the gonidia, he noted[177] that, accompanying Opegrapha atra and other Graphideae, on the bark were to be found free Chroolepus cells similar to the gonidia in the lichen thallus. He added that gonidia of certain other lichens in no way differed from Protococcus cells; and as for the gelatinous lichens he declared that “either they were the perfect fruiting form of Nostocaceae and Chroococcaceae—hitherto looked on as algae—or that these same Nostocaceae and Chroococcaceae are algae which take the form of Collema, Ephebe, etc., when attacked by an ascomycetous fungus.”

All these investigators, and other lichenologists such as Nylander[178], still regarded the free-living organisms identified by them as similar to the green cells of the thallus, as only lichen gonidia escaped from the matrix and vegetating in an independent condition.

The old controversy has in recent years been unexpectedly reopened by Elfving[179] who has sought again to prove the genetic origin of the green cells. His method has been to examine a large series of lichens by making sections of the growing areas, and he claims to have observed in every case the hyphal origin of the gonidia: not only of Cystococcus but also of Trentepohlia, Stigonema and Nostoc. In the case of Cystococcus, the gonidium, he says, arises by the swelling of the terminal cell of the hypha to a globose form, and by the gradual transformation of the contents to a chlorophyll-green colour, with power of assimilation. In the case of filamentous gonidia such as Trentepohlia, the hyphal cells destined to become gonidia are intercalary. In Peltigera the cells of the meristematic plectenchyma become transformed to blue-green Nostoc cells.

A study was also made by him of the formation of cephalodia[180], the gonidia of which differ from those of the “host” thallus. In Peltigera aphthosa he claims to have traced the development of these bodies to the branching and mingling of the external hairs which, in the end, form a ball of interwoven hyphae. The central cells of the ball are then gradually differentiated into Nostoc cells, which increase to form the familiar chains. Elfving allows that the gonidia mainly increase by division within the thallus, and that they also may escape and live as free organisms. His views are unsupported by direct culture experiments which are the real proof of the composite nature of the thallus.

E. MICROGONIDIA

Another attempt to establish a genetic origin for lichen gonidia was made by Minks[181]. He had found in his examination of Leptogium myochroum that the protoplasmic contents of the hyphae broke up into a regular series of globular corpuscles which had a greenish appearance. These minute bodies, called by him microgonidia, were, he states, at first few in number, but gradually they increased and were eventually set free by the mucilaginous degeneration of the cell wall. As free thalline gonidia, they increased in size and rapidly multiplied by division. Minks was at first enthusiastically supported by Müller[182] who had found from his own observations that microgonidia might be present in any of the lichen hyphae and in any part of the thallus, even in the germinating tube of the lichen spore, and was in that case most easily seen when the spores germinated within the ascus. He argued that as spores originated within the ascus, so microgonidia were developed within the hyphae. Minks’s theories were however not generally accepted and were at last wholly discredited by Zukal[183] who was able to prove that the greenish bodies were contracted portions of protoplasm in hyphae that suffered from a lowered supply of moisture, the green colour not being due to any colouring substance, but to light effect on the proteins—an outcome of special conditions in the vegetative life of the plant. Darbishire[184] criticized Minks’s whole work with great care and he has arrived at the conclusion that the microgonidium may be dismissed as a totally mistaken conception.

F. COMPOSITE NATURE OF THALLUS

Schwendener[185] meanwhile was engaged on his study of lichen anatomy. Though at first he adhered to the then accepted view of the genetic connection between hyphae and gonidia, his continued examination of the vegetative development led him to publish a short paper[186] in which he announced his opinion that the various blue-green and green gonidia were really algae and that the complete lichen in all cases represented a fungus living parasitically on an alga: in Ephebe, for example, the alga was a form of Stigonema, in the Collemaceae it was a species of Nostoc. In those lichens enclosing bright green cells, the gonidia were identical with Cystococcus humicola, while in Graphideae the brightly coloured filamentous cells were those of Chroolepus (Trentepohlia). This statement he repeated in an appendix to the larger work on lichens[187] and again in the following year[188] when he described more fully the different gonidial algae and the changes produced in their structure and habit by the action of the parasite: “though eventually the alga is destroyed,” he writes, “it is at first excited to more vigorous growth by contact with the fungus, and in the course of generations may become changed beyond recognition both in size and form.” In support of his theory of the composite constitution of the thallus, Schwendener pointed out the wide distribution and frequent occurrence in nature of the algae that become transformed to lichen gonidia. He claimed as further proof of the presence of two distinct organisms that, while the colourless filaments react in the same way as fungi on the application of iodine, the gonidia take the stain of algal membranes.

G. SYNTHETIC CULTURES

Schwendener’s “dual hypothesis,” as it was termed, excited great interest and no little controversy, the reasons for and against being debated with considerable heat. Rees[189] was the first who attempted to put the matter to the proof by making synthetic cultures. For this purpose he took spores from the apothecium of a Collema and sowed them on pure cultures of Nostoc, and as a result obtained the formation of a lichen thallus, though he did not succeed in producing any fructification. He observed further that the hyphal filaments from the germinating spore died off when no Nostoc was forthcoming.

Bornet[190] followed with his record of successful cultures. He selected for experiment the spores of Physcia (Xanthoria) parietina and was able to show that hyphae produced from the germinating spore adhered to the free-growing cells of Protococcus[191] viridis and formed the early stages of a lichen thallus. Woronin[192] contributed his observations on the gonidia of Parmelia (Physcia) pulverulenta which he isolated from the thallus and cultivated in pure water. He confirmed the occurrence of cell division in the gonidia and also the formation of zoospores, these again forming new colonies of algae identical in all respects with the thalline gonidia. He was able to see the germinating tube from a lichen spore attach itself to a gonidium, though he failed in his attempts to induce further growth. In our own country Archer[193] welcomed the new views on lichens, and attempted cultures but with very little success. Further synthetic cultures were made by Bornet[194], Treub[195] and Borzi[196] with a series of lichen spores. They also were able to observe the first stages of the thallus. Borzi observed spores of Physcia (Xanthoria) parietina scattered among Protococcus cells on the branch of a tree. The spores had germinated and the first branching hyphae had already begun to encircle the algae.

Additional evidence in favour of the theory of the independent origin of the colourless filaments and the green cells was furnished by Stahl’s[197] research on hymenial gonidia in Endocarpon (Fig. 5). By making synthetic cultures of the mature spores with these bodies, he was able to observe not only the germination of the spores and the attachment of the filaments to the gonidia (Fig. 6), but also the gradual building up of a complete lichen thallus to the formation of perithecia and spores.

Some years later Bonnier[198] made an interesting series of synthetic cultures between the spores of lichens germinated in carefully sterilized conditions, and algae taken from the open (Figs. 7 and 8). Separate control cultures of spores and algae were carried on at the same time, with the result that in one case lichen hyphae alone, in the other algae were produced. The various lichen spores with which he experimented were sown in association with the following algae:

(1) PROTOCOCCUS.

Pure synthetic cultures of Physcia (Xanthoria) parietina were begun in August 1884 on fragments of bark. In October 1886 the thallus was several centimetres in diameter, and some of the lobes were fruited.

Physcia stellaris was also grown on bark; in one case both thallus and apothecia were developed.

Parmelia acetabulum, another corticolous species, formed only a minute thallus about 5 mm. in diameter, but entirely identical with normally growing specimens.

(2) PLEUROCOCCUS.

Lecanora (Rinodina) sophodes, sown on rock in 1883, reached in 1886 a diameter of 13 mm. with fully developed apothecia.

Lecanora ferruginea and L. subfusca after three years’ culture formed sterile thalli only.

Lecanora coilocarpa in four years, and L. caesio-rufa in three years formed very small thalli without fructification.

(3) TRENTEPOHLIA (Chroolepus).

Opegrapha vulgata in two years had developed thallus and apothecia. The control culture of the spores formed, as in nature, a considerable felt of mycelium in the interstices of the bark, but no pycnidia or apothecia.

Graphis elegans. Only the beginning of a differentiated thallus was obtained with this species.

Verrucaria muralis (?)[199] gave in less than a year a completely developed thallus.

Bonnier also attempted cultures with species of Collema and Ephebe, but was unsuccessful in inducing the formation of a lichen plant.

H. HYMENIAL GONIDIA

Reference has already been made to the minute green cells which were originally described by Nylander[200] as occurring in the perithecia of a few Pyrenolichens as free gonidia, i.e. unentangled with lichen hyphae. Fuisting[201] found them in the perithecium of Polyblastia (Staurothele) catalepta at a very early stage of its development when the perithecial tissues were newly differentiated from those of the surrounding thallus. The gonidia enclosed in the perithecium differed in no wise from those of the thallus: they had become mechanically enclosed in the new tissue; and while those in the outer compact layers died off, those in the centre of the structure, where a hollow space arises, were subject to very active division, becoming smaller in the process and finally filling the cavity. Winter’s[202] researches on similar lichens confirmed Fuisting’s conclusions: he described them as similar to the thalline gonidia but lighter in colour and of smaller size, measuring frequently only 2·3 µ in diameter, though this size increased to about 7 µ when cultivated outside the perithecium.

Stahl[203] sufficiently demonstrated the importance of these gonidia in supplying the germinating spores with the necessary algae. They come to lie in vertical rows between the asci and, owing to pressure, assume an elongate form[204] (Figs. 5 and 6). They have been seen in very few lichens, in Endocarpon and Staurothele, both rather small genera of Pyrenolichens, and, so far as is known, in two Discolichens, Lecidea phylliscocarpa and L. phyllocaris, the latter recorded from Brazil by Wainio[205], and, on account of the inclusion of gonidia in the hymenium, placed by him in a section, Gonothecium.

I. NATURE OF ASSOCIATION BETWEEN ALGA AND FUNGUS

a. CONSORTIUM AND SYMBIOSIS. These cultures had established convincingly the composite nature of the lichen thallus, and Schwendener’s opinion, that the relationship between the two organisms was some varying degree of parasitism, was at first unhesitatingly accepted by most botanists. Reinke[206] was the first to point out the insufficiency of this view to explain the long continued healthy life of both constituents, a condition so different from all known instances of the disturbing or fatal parasitism of one individual on another. He recognized in the association a state of mutual growth and interdependence, that had resulted in the production of an entirely new type of plant, and he suggested Consortium as a truer description of the connection between the fungus and the alga. This term had originally been coined by his friend Grisebach in a paper[206] describing the presence of actively growing Nostoc algae in healthy Gunnera stems; and Reinke compared that apparently harmless association with the similar phenomenon in the lichen thallus. The comparison was emphasized by him in a later paper[207] on the same subject, in which he ascribes to each “consort” its function in the composite plant, and declares that if such a mutual life of Alga and Ascomycete is to be regarded as one of parasitism, it must be considered as reciprocal parasitism; and he insists that “much more appropriate for this form of organic life is the conception and title of Consortium.” In a special work on lichens, Reinke[208] further elaborated his theory of the physiological activity and mutual service of the two organisms forming the consortium.

Frank[209] suggested the term Homobium as appropriate, but it is faulty inasmuch as it expresses a relationship of complete interdependence, and it has been proved that the fungus partly, and the alga entirely, have the power of free growth.

A wider currency was given to this view of a mutually advantageous growth by de Bary[210]. He followed Reinke in refusing to accept as satisfactory the theory of simple parasitism, and adduced the evident healthy life of the algal cells—the alleged victims of the fungus—as incompatible with the parasitic condition. He proposed the happily descriptive designation of a Symbiosis or conjoint life which was mostly though not always, nor in equal degree, beneficial to each of the partners or symbionts.

b. DIFFERENT FORMS OF ASSOCIATION. The type of association between the two symbionts varies in different lichens. Bornet[211], in describing the development of the thallus in certain members of the Collemaceae, found that though as a rule the two elements of the thallus, as in some species of Collema itself, persisted intact side by side, there was in other members of the genus an occasional parasitism: short branches from the main hyphae applied themselves by their tips to some cell of the Nostoc chain (Fig. 9). The cell thus seized upon began to increase in size, and the plasma became granular and gathered at the side furthest away from the point of attachment. Finally the contents were used up, and nothing was left but an empty membrane adhering to the fungus hypha. In another species the hypha penetrated the cell. These instances of parasitism are most readily seen towards the edge of the thallus where growth is more active; towards the centre the attached cells have become absorbed, and only the shortened broken chains attest their disappearance. The other cells of the chains remain uninjured.

In Synalissa, a small shrubby gelatinous genus, the hypha, as described by Bornet and by Hedlund[212], pierces the outer wall of the gelatinous alga (Gloeocapsa) and swells inside to a somewhat globose haustorium which rests in a depression of the plasma (Fig. 10). The alga, though evidently undamaged, is excited to a division which takes place on a plane that passes through the haustorium; the two daughter-cells then separate, and in so doing free themselves from the hypha.

Hedlund followed the process of association between the two organisms in the lichens Micarea (Biatorina) prasina and M. denigrata (Biatorina synothea), crustaceous species which inhabit trunks of trees or palings. In these the alga, one of the Chlorophyceae, has assumed the character of a Gloeocapsa but on cultivation it was found to belong to the genus Gloeocystis. The cells are globose and rather small; they increase by the division of the contents into two or at most four portions which become rounded off and covered with a membrane before they become free from the mother-cell. The lichen hypha, on contact with any one of the green cells, bores through the outer membrane and swells within to a haustorium, as in the gonidia of Synalissa.

Penetrating haustoria were demonstrated by Peirce[213] in his study of the gonidia of Ramalina reticulata. In the first stage the tip of a hypha had pierced the outer wall of the alga, causing the protoplasm to contract away from the point of contact (Fig. 11). More advanced stages showed the extension of the haustorium into the centre of the cell, and, finally, the complete disappearance of the contents. In many cases it was found that penetration equally with clasping of the alga by the filament sets up an irritation which induces cell-division, and the alga, as in Synalissa, thus becomes free from the fungus. Hue[214] has recorded instances of penetration in an Antarctic species, Physcia puncticulata. It is easy, he says, to see the tips of the hyphae pierce the sheath of the gonidium and penetrate to the nucleus.

Lindau[215] has described the association between fungus and alga in Pertusaria and other crustaceous forms as one of contact only (Fig. 12). He found that the cell-membrane of the two adhering organisms was unbroken. Occasionally the algal cell showed a slight indentation, but was otherwise unchanged. The hyphal branch was somewhat swollen at the tip where it touched the alga, and the wall was slightly thinner. The attachment between the two cells was so close, however, that pressure on the cover-glass failed to separate them.

Generally the hypha simply surrounds the gonidium with clasping branches. Many algae also lie free in the gonidial zone, and Peirce[216] claims that these are larger, more deeply coloured and in every way healthier looking than those in the grasp of the fungus. He ignores, however, the case of the soredial algae which though very closely invested by the fungus are yet entirely healthy, since on their future increase depends in many cases the reproduction of new individual lichens.

In a recent study of a crustaceous sandstone lichen, “Caloplaca pyracea,” Claassen[217] has sought to prove a case of pure parasitism. The rock was at first covered with the green cells of Cystococcus sp. Later there appeared greyish-white patches on the green, representing the invasion of the lichen fungus. These patches increased centrifugally, leaving in time a bare patch in the centre of growth which was again colonized by the green alga. The lichen fruited abundantly, but wherever it encroached the green cells were more or less destroyed. The true explanation seems to be that the green cells were absorbed into the lichen thallus, though enough of them persisted to start new colonies on any bare piece of the stone. In the same way large patches of Trentepohlia aurea have been observed to be gradually invaded by the dark coloured hyphae of Coenogonium ebeneum. In time the whole of the alga is absorbed and nothing is to be seen but the dark felted lichen. The free alga as such disappears, but it is hardly correct to describe the process as one of destruction.

This algal genus Trentepohlia (Chroolepus) forms the gonidia of the Graphideae, Roccelleae, etc. It is a filamentous aerial alga which increases by apical growth. In the Graphideae, many of which grow on trees beneath the outer bark (hypophloeodal), the association between the two symbionts may be of the simplest character, but was considered by Frank[218] to be of an advanced type. According to his observations and to those of Lindau[219], the fungal hyphae penetrate first between the cells of the periderm. The alga, frequently Trentepohlia umbrina, tends to grow down into any cracks of the surface. It goes more deeply in when preceded by the hyphae. In some species both organisms maintain their independent growth, though each shows increased vigour when it comes into contact with the other. In some instances the cells of the alga are clasped by the fungus which causes the disintegration of the filament. The cells lose their bright yellow or reddish colour and are changed in appearance to greenish lichen gonidia; but no penetration by haustoria has ever been observed in Trentepohlia.

Bachmann’s[220] study of a similar gonidium in a calcicolous species of Opegrapha confirms Frank’s results. The algae had pierced not only between the looser lime granules but also through a crystal of calcium carbonate, and occupied nests scooped out in the rock by means of acid formed and excreted by their filaments. When association took place with the fungus, the algal cells were more restricted to a gonidial zone; but some of the cells, having been pushed aside by the hyphae, had started new centres of gonidia. On contact with the hyphae there was a tendency to bud out in a yeast-like growth.

In the thallus of the Roccelleae, the algal filament, also a Trentepohlia, is broken up into separate cells, but in the Coenogoniaceae, whether the gonidium be a Cladophora as in Racodium, or a Trentepohlia as in Coenogonium, the filaments remain intact and are invested more or less closely by the hyphae.

A somewhat different type of association takes place between alga and fungus in Strigula complanata, an epiphyllous lichen more or less common in tropical regions. Cunningham[221], who found it near Calcutta, described the algal constituent and placed it in a new genus, Mycoidea (Cephaleuros). It forms small plate-like expansions on the surface of the leaf, and also penetrates below the cuticle, burrowing between that and the epidermal cells; occasionally, as observed by Cunningham, rhizoid-like growths pierce deeper into the tissue—into and below the epidermal layer. Very frequently, in the wet season, a fungus takes possession of the alga and slender colourless hyphae creep along its surface by the side of the cell rows, sending out branches which grow downwards. Marshall Ward[222] described the same lichen from Ceylon. He states that the alga may be attacked at any stage, and if it is in a very young condition it is killed by the fungus; at a more advanced period of growth it continues to develop as an integral part of the lichen thallus, but with more frequently divided and smaller cells. Vaughan Jennings[223] observed Strigula complanata in New Zealand associated with a closely allied chroolepoid alga Phycopeltis expansa. He also noted the growth of the fungus over the alga breaking up the plates of tissue and separating the cells which, from yellow, change to a green colour and become rounded off (Fig. 13). The mature lichen, a white thallus dotted with black fruits, contrasts strikingly with the yellow membranous alga. Lichen formation usually begins near the edge of the leaf and the margin of the thallus itself is marked by a green zone showing where the fungus has recently come into contact with the alga.

More recently Hans Fitting[224] has described “Mycoidea parasitica” as it occurs on evergreen leaves in Java. The alga, a species of Cephaleuros, though at first an epiphyte, becomes partially parasitic at maturity. It penetrates below the cuticle to the outer epidermal cells and may even reach the tissue below. When it is joined by the lichen fungus, both constituents grow together to form the lichen. Fitting adds that the leaf is evidently but little injured. In this lichen the alga in the grip of the fungus loses its independence and may be killed off: it is an instance of something like intermittent parasitism.

J. RECENT VIEWS ON SYMBIOSIS AND PARASITISM

No hyphal penetration of the bright-green algal cell by means of haustoria had been observed by the earlier workers, Bornet[225], Bonnier[226] and others, though they followed Schwendener[227] in regarding the relationship as one of host and parasite. Lindau, also, after long study accepted parasitism as the only adequate explanation of the associated growth, though he never found the fungus actually preying on the alga.

In recent years interest in the subject has been revived by the researches of Elenkin[228], a Russian botanist who claims to have established a case for parasitism or rather “endosaprophytism.” He has demonstrated by means of staining reagents the presence in the thallus of large numbers of dead algal cells. A few empty membranes are to be found in the cortex and in the gonidial zone, but the larger proportion occur below the gonidial zone and partly in the medulla. He describes the lower layer as a “necral” or “hyponecral” zone, and he considers that the hyphae draw their nourishment chiefly from dead algal material. The fungus must therefore be regarded in this case as a saprophyte rather than a parasite. The algae, he considers, may have perished from want of sufficient light and air or they may have been destroyed by an enzyme produced by the fungus. The latter he thinks is the more probable, as dead cells are frequently present among the living algae of the gonidial zone. To the action of the enzyme he also attributes the angular deformed appearance of many gonidia and the paler colour and gradual disintegration of their contents which are finally used up as endosaprophytic nourishment by the fungus. Dead algal cells were more easily seen, he tells us, in crustaceous lichens associated with “Pleurococcus” or “Cystococcus”; they were much less frequent in the larger foliose or fruticose lichens. Dead cells of Trentepohlia were also difficult to find.

In a second paper Elenkin records one clear instance of a haustorium entering an algal cell, and says he found some evidence of hyphal branches penetrating otherwise uninjured gonidia, round holes being visible in their outer wall, but he holds that it is the cell-wall of the alga that is mainly dissolved by the ferment and then used as food by the hyphae.

No allowance has been made by Elenkin for the normal wasting common to all organic beings: the lichen fungus is continually being renewed, especially in the cortical structures, and the alga must also be subject to change. He[229] claims, nevertheless, that his observations have proved that the one symbiont is always preying on the other, either as a parasite or as a saprophyte. He has likened the conception of symbiosis to that of a balance between two organisms, “a moveable equilibrium of the symbionts.” If, he says, we could conceive a state where the conditions of life would be equally favourable for both partners there would be true mutualism, but in practice one only is favoured and gains the upper hand, using its advantage to prey on the other. Unless the balance is redressed, the complete destruction of the weaker is certain, and is followed in time by the death of the stronger. The fungus being the dominant partner, the balance, he considers, is tipped in its favour.

Elenkin’s conclusions are not borne out by the long continued and healthy life of the lichen. There is no record of either symbiont having succumbed to the other, and the alga, when set free, is unchanged and able to resume its normal development. Without the alga the fungus cannot form the ascigerous fruit. Is that because as a parasite within the lichen it has degenerated past recovery, or has it become so adapted to symbiosis that in saprophytic conditions it fails to develop?

Another Russian lichenologist, U. N. Danilov[230], records results which would seem to support the theory of parasitism. He found that from the clasping hyphae minute haustoria were produced, which penetrate the algal cell-wall, and branch when within the outer membrane, thus forming a delicate network over the plasma; secondary haustoria arising from this network protrude into the interior and rob the cell-contents. He observed gonidia filled with well-developed hyphae and these, after having exhausted one cell, travel onwards to others. Some gonidia under the influence of the fungus had become deformed and were finally killed. As a proof of this latter statement he adduces the presence in the thallus of some gonidia containing shrivelled protoplasm, of others entirely empty. He considers, as further evidence in favour of parasitism, the finding of empty membranes as well as of colourless gonidia filled with the hyphal network. This description hardly tallies with the usual healthy appearance of the gonidial zone in the normal thallus, and it has been suggested that where the fungus filled the algal cell, it was as a saprophyte preying on dead material.

The gradual perishing of algal cells in time by natural decay and their subsequent absorption by the fungus is undisputed. It is open to question whether the varying results recorded by these workers have any further significance.

These observations of Elenkin and Danilov have been proved to be erroneous by Paulson and Somerville Hastings[231]. They examined the thalli of several lichens (Xanthoria parietina, Cladonia sp., etc.) collected in early spring when vegetative growth in these plants was found to be at its highest activity. They found an abundant increase of gonidia within the thallus, which they regarded as sporulation of the algae, and the most careful methods of staining failed to reveal any case of penetration of the gonidia by the hyphae.

Nienburg[232] has published some recent observations on the association of the symbionts. In the wide cortex of a Pertusaria he found not only the densely compact hyphae, but also isolated gonidia. In front of these latter there was a small hollow cavity and, behind, parallel hyphae rich in contents. These gonidia had originated from the normal gonidial zone. They were moved upward by special hyphae called by Nienburg “push-hyphae.” After their transportation, the algae at once divide and the products of division pass to a resting stage and become the centre of a new thalline growth. A somewhat similar process was noted towards the apex of Evernia furfuracea. Radial hyphae pushed up the cortex, leaving a hollow space over the gonidial zone. Into the space isolated algae were thrust by “push-hyphae.” In this lichen he also observed the penetration of the algal cell by haustoria of the fungus. He considers that the alga reaps advantage but also suffers harm, and he proposes the term helotism to express the relationship.

An instructive case of the true parasitism of a fungus on an alga has been described by Zukal[233] in the case of Endomyces scytonemata which he calls a “half-lichen.” The mature fungus formed small swellings on the filaments of the Scytonema and, when examined, the hyphae were seen to have attacked the alga, penetrating the outer gelatinous sheath and then using up the contents of the green cells. It is only after the alga has been destroyed and absorbed, that asci are formed by the fungus. Zukal contrasts the development of this fungus with the symbiotic growth of the two constituents in Ephebe where both grow together for an indefinite time.

Mere associated growth however even between a fungus and an alga does not constitute a lichen. An instance of such growth is described by Sutherland[234] in an account of marine microfungi. One of these, a species of Mycosphaerella, was found on Pelvetia canaliculata, and Sutherland claims that as no apparent injury was done to the alga, it was a case of symbiosis and that there was formed a new type of lichen. The mycelium, always intercellular, pervaded the whole host-plant, and the fungal fruits were invariably formed on the algal receptacles close to the oogonia. Their position there is, of course, due to the greater food supply at that region. Both fungus and alga fruited freely. A closer analogy could have been found by the writer in the smut fungus which grows with the host-cereal until fruiting time; or with the mycorrhiza of Calluna which also pervades every part of the host-plant without causing any injury. In the true lichen, the alga, though constituting an important part of the vegetative body, takes no part in reproduction, except by division and increase of the vegetative cells within the thallus. The fruiting bodies are always of a modified fungal nature.

2. PHYSIOLOGY OF THE SYMBIONTS

The occurrence of isolated cases of parasitism—the fungus preying on the alga—in any case leaves the general problem unsolved. The whole question turns on the physiological activity and requirements of the two component elements of the thallus. From what sources do they each procure the materials essential to them as living organisms? It is chiefly a question of nutrition.

A. NUTRITION OF ALGAE

a. CHARACTER OF ALGAL CELLS. Gonidia are chlorophyll-containing bodies and assimilate carbon-dioxide from the atmosphere by photosynthesis as do the chlorophyll cells of other plants. They also require water and mineral salts which, in a free condition, they absorb from their immediate surroundings, but which, in the lichen thallus, they must obtain from the fungal hyphae. If the nutriment supplied to them in their inclosed position be greater or even equal to what the cells could procure as free-living algae, then they undoubtedly gain rather than lose by their association with the fungus, and are not to be considered merely as victims of parasitism.

b. SUPPLY OF NITROGEN. Important contributions on the subject of algal nutrition have been made by Beyerinck[235] and Artari[236]. The former conducted a series of culture experiments with green algae, including the gonidia of Physcia (Xanthoria) parietina. He successfully isolated the lichen gonidia and, at first, attempted to grow them on gelatine with an infusion of the Elm bark from which he had taken the lichen. Growth was very slow and very feeble until he added to the culture-medium a solution of malt-extract which contains peptones and sugar. Very soon he obtained an active development of the gonidia, and they multiplied rapidly by division[237] as in the lichen thallus. This proved to him conclusively the great advantage to the algae of an abundant supply of nitrogen.

Artari in his work has demonstrated that there are two different physiological races of green algae: (1) those that absorb peptones—which he designates peptone-algae—and (2) those that do not so absorb peptones. He tested the cells of Cystococcus humicola taken from the thallus of Physcia parietina, and found that they belonged to the peptone group and were therefore dependent on a sufficiency of nitrogenous material to attain their normal vigorous growth. It was also discovered by Artari that the one race can be made by cultivation to pass over to the other: that ordinary algae can be educated to live on peptones, and peptone-algae to do without.

We learn further from Beyerinck’s researches that Ascomycetes, the group of fungi from which the hyphae of most lichens are derived, are what he terms ammonia-sugar fungi; that is to say, the hyphae can abstract nitrogen from ammonia salts and, with the addition of sugar, can form peptones. The lichen peptone-algae are thus evidently, by their contact with such fungi, in a favourable position for securing the nitrogenous food supply most suited to their requirements. In their deep-seated layers, they are to a large extent deprived of light, but it has been proved by Artari[238] in a series of culture experiments extending over a long period, that the gonidia of Xanthoria parietina remain green in the dark under very varied conditions of nutriment, though the colour is distinctly fainter.

Recently Treboux[239] has revised the work done by Artari and Beyerinck in reference to Cystococcus humicola. He denies that two physiological races are represented in this alga, the lichen gonidia, in regard to the nitrogen that they absorb, behaving exactly as do the free-living forms of the species. He finds that the gonidium is not a peptone-carbohydrate organism in the sense that it requires nitrogen in the form of peptones, inorganic ammonia salts being a more acceptable food supply. Treboux concludes that his results favour the view that the gonidia are in an unfavourable situation for receiving the kind of nitrogenous compound most advantageous to them, that they are therefore in a sense “victims” of parasitism, though he qualifies the condition as being a lichen-parasitism or helotism. This view does not accord with Chodat’s[240] results: in his cultures of gonidia he observed that with glycocoll or peptone, which are nearly equivalent, they developed four times better than with potassium nitrate as their nitrogenous food, and he concluded that they assimilated nitrogen better from bodies allied to peptides.

c. EFFECT OF SYMBIOSIS ON THE ALGA. Treboux’s observations however convinced him that the alga leads but a meagre existence within the thallus. Cell-division—the expression of active vitality—was, he held, of rare occurrence in the slowly growing lichen-plant, and zoospore formation in entire abeyance. He contrasts this sluggish increase[241] with the rapid multiplication of the free-living algal cells which cover whole tree-trunks with their descendants in a comparatively short time. These latter cells, he finds, are indeed rather smaller, being generally the products of recent division, but mixed with them are numbers of larger resting cells, comparable in size with the lichen gonidia. He states further, that the gonidia are less brightly green and, as he judges, less healthy, though in soredial formation or in the open they at once regain both colour and power of division. Treboux had entirely failed to observe the sporulation which is so abundant at certain seasons.

Their quick recovery seems also a strong argument in favour of the absolutely normal condition of metabolism within the gonidial cell; and the paler appearance of the chlorophyll is doubtless associated with the acquisition of carbohydrates from other sources than by photosynthesis. There is a wide difference between any degree of unfavourable life-conditions and parasitism however slight, even though the balance of gain is on the side of the fungus. It is not too fanciful to conclude that the demand for nitrogen on the part of the alga has influenced its peculiar association with the fungus. In the thallus of hypophloeodal lichens it has been proved indeed that the alga Trentepohlia with apical growth is an active agent in the symbiotic union. Cystococcus and other green algal cells are stationary, but they are doubtless equally ready for—as many of them are equally benefited by—the association. Keeble[242] has pointed out in the case of Convoluta roscoffensis that nitrogen-hunger induces the green algae to combine forces with an animal organism, though the benefit to them is only temporary and though they are finally sacrificed. The lichen gonidia, on the contrary, persist for a long time, probably far beyond their normal period of existence as free algae.

Examples of algal association with other plants might be cited here: of Nostoc in the roots of Cycas and in the cells of Anthoceros, and of Anabæna in the leaf-cells of Azolla, but in these instances it is generally held that the alga secures only shelter. It was by comparing the lichen-association with the harmless invasion of Gunnera cells by Nostoc that Reinke[243] arrived at his conception of “consortism.”

d. SUPPLY OF CARBON. Carbon, the essential constituent of all organic life, is partly drawn from the carbon-dioxide of the air, and assimilated by the green cells; it is also partly contributed by the fungus as a product of its metabolism. A proof of this is afforded by Dufrenoy[244]: he found a Parmelia growing closely round pine needles and even sending suckers into the stomata. He covered the lichen with a black cloth and after seven weeks found that the gonidia had remained very green. That growth had not been checked was evidenced by an unusual development of soredia and of spermogonia. Dufrenoy describes the condition as a parasitism of the algae on the fungus which in turn was drawing nourishment from the pine needles.

Artari[245] has proved that lichen gonidia can obtain carbohydrates from the substratum as well as by photosynthesis. He cultivated the gonidia of Xanthoria parietina and Placodium murorum on media which contained organic substances as well as mineral salts, while depriving them of atmospheric carbon-dioxide and in some cases of light also. The gonidia not only grew well but, even in the dark, they remained normally green, a phenomenon coinciding with Etard and Bouilhac’s[246] experience in growing Nostoc in the dark: with suitable culture media the alga retained its colour. Nostoc also grows in the dark in the rhizome of Gunnera. Radais’[247] experiments with Chlorella vulgaris confirmed these results. On certain organic media growth and cell-division were as rapid in the dark as in the light, and chlorophyll was formed. The colour was at first yellowish and the full green arrived slowly, especially on sugar media, but in ten days it was uniform and normal.

When making further experiments with the alga, Stichococcus bacillaris, Artari[248] found that it also grew well on an organic medium and that grape sugar was the most valuable carbonaceous food supply. Chodat[249] also found that sugar or glucose was a desirable ingredient of culture media.

Treboux[250], in his work on organic acids, has also proved by experimental cultures with a large series of algae, including the gonidia of Peltigera, that these green plants in the absence of light and in pure cultures would grow and form carbohydrates if the culture medium contained a small percentage of organic acids. The acids he employed were combined with potassium and were thus rendered neutral or slightly alkaline; acetate of potash proved to be the most advantageous compound of any that was tested. Amino-acids and ammonia salts were added to provide the necessary nitrogen. Oxalic acid and other organic acids of varying composition are peculiarly abundant in lichen tissues and may be a source of carbon supply. Marshall Ward[251] has found calcium carbonate crystals in the lower air-containing tissues of Strigula complanata.

Treboux finally concluded from his researches that just as fungi can extract carbohydrates from many sources, so algae can secure their carbon supply in a variety of ways. He affirms that the metabolic activity of the alga in these cultural conditions is entirely normal, and the various cell-contents are formed as in the light. Whether, in this case, starch is formed directly from the acids or through a series of combinations has not been determined. Uhlir[252], with electric lighting, made successful cultures of Nostoc isolated from Collemaceae on silicic acid, proving thereby that these gonidia do not require a rich nutriment. A certain definite humidity was however essential, and bacteria were never eliminated as they are associated with the gelatinous membranes of Nostocaceae.

e. NUTRITION WITHIN THE SYMBIOTIC PLANT. Culture experiments bearing more directly on the nutrition of lichens as a whole were carried out by F. Tobler[253]. He proved that the gonidia had undoubtedly drawn on the calcium oxalate secreted by the hyphae for their supply of carbon. In a culture medium of poplar-bark gelatine he grew hyphae of Xanthoria parietina, and noted an abundant deposit of oxalate crystals on their cell-walls. A piece of the lichen thallus including both symbionts and grown on a similar medium formed no crystals, and microscopic examination showed that crystals were likewise absent from the hyphae of the thallus that had grown normally on the tree, the inference being that the gonidia used them up as quickly as they were deposited. It must be remembered in this connection, however, that Zopf[254] has stated that where lichen acids are freely formed as, for instance, in Xanthoria parietina, there is always less formation and deposit of calcium oxalate crystals, which may partly account for their absence in the normal thallus so rich in parietin.

Tobler next introduced lichen gonidia into a culture medium in which the isolated hyphal constituent of a thallus had been previously cultivated, and placed the culture in the dark. In these circumstances he found that the gonidia were able to thrive but formed no colour: they were obtaining their carbohydrates, he decided, not from photosynthesis, but from the excretory products such as calcium oxalate that had been deposited in the culture medium by the lichen hyphae. We may conclude with more or less certainty that the loss of carbohydrates, due to the partial deprivation of light and air suffered by the alga owing to its position in the lichen thallus, is more than compensated by a physiological symbiosis with the fungus[255]. It has indeed been proved that in the absence of free carbon-dioxide, algae may utilize the half-bound CO₂ of carbonates, chiefly those of calcium and magnesium, dissolved in water.

f. AFFINITIES OF LICHEN GONIDIA. Chodat[256] has, in recent years, made cultures of lichen gonidia with a view to discovering their relation to free-living algae and to testing at the same time their source of carbon supply. He has come to the conclusion that lichen gonidia are probably in no instance the normal Protococcus viridis: they differ from that alga in the possession of a pyrenoid and in their reproduction by zoospores when free.

Careful cultures were made of different Cladonia gonidia which were morphologically indistinguishable, and which varied in size from 10 to 16µ in diameter, though smaller ones were always present. He recognized them to be species of Cystococcus: they have a pyrenoid[257] in the centre and a disc-like chromatophore more or less starred at the edge. These gonidia grew well on agar, still better on agar-glucose, but best of all with an addition of peptone to the culture. There was invariably at first a slight difference in form and colour in the mass between the gonidia of one species and those of another, but as growth continued they became alike.

In testing for carbon supply, he found that gonidia grew slowly without sugar (glucose), and that, as sources of carbon, organic acids could not entirely replace glucose though, in the dark, the gonidia used them to some extent; the colony supplied with potassium nitrate, and grown in the dark, had reached a diameter of only 2 mm. in three months. With glucose, it measured 5 mm. in three weeks, while in three months it formed large culture patches.

A further experiment was made to test their absorption of peptones by artificial cultures carried out both in the light and the dark. The gonidia grew poorly in all combinations of organic nitrogen compounds. When combined with glucose, growth was at once more vigorous though only half as much in the dark as in the light, the difference in this respect being especially noticeable in the gonidia from Cladonia pyxidata. He concludes that as gonidia in these cultures are saprophytic, so in the lichen thallus also they are probably more or less saprophytic, obtaining not only their nitrogen in organic form but also, when possible, their carbon material as glucose or galactose from the hyphal symbiont which in turn is saprophytic on humus, etc.

B. NUTRITION OF FUNGI

Fungi being without chlorophyll are always indebted to other organisms for their supply of carbohydrates. There has never therefore been any question as to the advantage accruing to the hyphal constituent in the composite thallus. The gonidia, as various workers have proved, have also a marked preference for organized nourishment, and, in addition, they obtain carbon by photosynthesis. Chodat[258] considers that probably they are thus able to assimilate carbon-dioxide in excess, a distinct advantage to the hyphae. In some instances the living gonidium is invaded and the contents used up by the fungus and any dead gonidia are likewise utilized for food supply. It is also taken for granted that the fungus takes advantage of the presence of humus whether in the substratum or in aerial dust. In such slow growing organisms, there is not any large demand for nourishment on the part of the hyphae: for many lichens it seems to be mere subsistence with a minimum of growth from year to year.

C. SYMBIOSIS OF OTHER PLANTS

The conception of an advantageous symbiosis of fungi with other plants has become familiar to us in Orchids and in the mycorhizal formation on the roots of trees, shrubs, etc. Fungal hyphae are also frequent inhabitants of the rhizoids of hepatics though, according to Gargeaune[259], the benefit to the hepatic host-plant is doubtful.

An association of fungus and green plant of great interest and bearing directly on the question of mutual advantage has been described by Servettaz[260]. In his study of mosses, he was able to confirm Bonnier’s[261] account of lichen hyphae growing over such plants as Vaucheria and the protonema of mosses, which is undoubtedly hurtful; but he also found an association of a moss with one of the lower fungi, Streptothrix or Oospora, which was distinctly advantageous. In separate cultivation the fungus developed compact masses and grew well in peptone agar broth.

Cultures of the moss, Phascum cuspidatum, were also made from the spores on a glucose medium. The specimens in association with the fungus were fully grown in two months, while the control cultures, without any admixture of the fungus, had not developed beyond the protonema stage. Servettaz draws attention to the proved fact that, in certain instances, plants benefit when provided with substances similar to their own decay products, and he considers that the fungus, in addition to its normal gaseous products, has elaborated such substances, as acid products, from the glucose medium to the great advantage of the moss plant.

A symbiotic association of Nostoc with another alga, described by Wettstein[262], is also of interest. The blue-green cells were lodged in the pyriform outgrowths of the siphoneous alga, Botrydium pyriforme Kütz., which the author of the paper places in a new genus, Geosiphon. The sheltering Nostoc symbioticum fills all of the host left vacant by the plasma, and when the season of decay sets in, it forms resting spores which migrate into the rhizoids of the host, so that both plants regenerate together.

Wettstein has compared this symbiotic association with that of lichens, and finds the analogy all the more striking in that the membrane of his new alga had become chitinous, which he thinks may be due to organic nutrition.

II. LICHEN HYPHAE

A. ORIGIN OF HYPHAE

Lichen hyphae form the ground tissue of the thallus apart from the gonidia or algal cells. They are septate branched filaments of single cell rows and are colourless or may be tinged by pigments or lichen acids to some shade of yellow, brown or black. They are of fungal nature, and are produced by the mature lichen spore.

The germination of the spore was probably first observed by Meyer[263]. His account of the actual process is somewhat vague, and he misinterpreted the subsequent development into thallus and fruit entirely for want of the necessary magnification; but that he did succeed in germinating the spores is unquestionable. He cultivated them on a smooth surface and they grew into a “dendritic formation”—a true hypothallus. Many years later the development of hyphae from lichen spores was observed by Holle[264] who saw and figured the process unmistakably in Borrera (Physcia) ciliaris.

A series of spore cultures was undertaken by Tulasne[265] with the twofold object of discovering the exact origin of hyphae and gonidia and of their relationship to each other. The results of his classical experiment with the spores of Verrucaria muralis—as interpreted by him—were accepted by the lichenologists of that time as conclusive evidence of the genetic origin of the gonidia within the thallus.

The spores of the lichen in large numbers had been sown by Tulasne in early spring on the smooth polished surface of a piece of limestone, and were covered with a watch-glass to protect them from dust, etc. At irregular intervals they were moistened with water, and from time to time a few spores were abstracted from the culture and examined microscopically. Tulasne observed that the spore did not increase or change in volume in the process of germination, but that gradually the contents passed out into the growing hyphae, till finally a thin membrane only was left and still persisted after two months (Fig. 14). For a considerable time there was no septation; at length cross-divisions were formed, at first close to the spore, and then later in the branches. The hyphae meanwhile increased in dimension, the cells becoming rounder and somewhat wider, though always more slender than the spore which had given rise to them. In time a felted tissue was formed with here and there certain cells, filled with green colouring matter, similar to the gonidia of the lichen and thus the early stages at least of a new thallus were observed. The green cells, we now know, must have gained entrance to the culture from the air, or they may have been introduced with the water.

B. DEVELOPMENT OF LICHENOID HYPHAE

Lichen hyphae are usually thick-walled, thus differing from those of fungi generally, in which the membranes, as a rule, remain comparatively thin. This character was adduced by the so-called “autonomous” school as a proof of the fundamental distinction between the hyphal elements of the two groups of plants. It can, however, easily be observed that, in the early stages of germination, the lichen hyphae, as they issue from the spore, are thin-walled and exactly comparable with those of fungi. Growth is apical, and septation and branching arise exactly as in fungi, and, in certain circumstances, anastomosis takes place between converging filaments. But if algae are present in the culture the peculiar lichen characteristics very soon appear.

Bonnier[266], who made a large series of synthetic cultures, distinguishes three types of growth in lichenoid hyphae (Fig. 15):

1. Clasping filaments, repeatedly branched, which attach and surround the algae.

2. Filaments with rather short swollen cells which ultimately form the hyphal tissues of cortex and medulla.

3. Searching filaments which elongate towards the periphery and go to the encounter of new algae.

In five days after germination of the spores, the clasping hyphae had laid hold of the algae which meanwhile had increased by division; the swollen cells had begun to branch out and ten days later a differentiation of tissue was already apparent. The searching filaments had increased in number and length, and anastomosis between them had taken place when no further algae were encountered. The cell-walls of the swollen hyphae and their branches had begun to thicken and to become united to form a kind of cellular tissue or “paraplectenchyma[267].” At a later date, about a month after the sowing of the spores, there was a definite cellular cortex formed over the thallus. The hyphal cells are uninucleate, though in the medulla they may be 1-2-nucleate.

The hyphae in close contact with the gonidia remain thin-walled, and have been termed by Wainio[268] “meristematic.” They furnish the growing elements of the lichen either apical or intercalary. In most genera the organs of fructification take rise from them, or in their immediate neighbourhood, and isidia and soredia also originate from these gonidial hyphae.

As the filaments pass from the gonidial zone to other layers, the cell-walls become thicker with a consequent reduction of the cell-lumen, very noticeable in the pith, but carried to its furthest extent in the “decomposed” cortex where the cells in the degenerate tissue often become reduced to disconnected streaks indicating the cell-lumen, and the outer cortical layer is merely a continuous mass of mucilage.

All lichen tissues arise from the branching and septation of the hyphae, the septa always forming at right angles to the long axis of the filaments. There is no instance of longitudinal cell-division except in the spores of certain genera (Collema, Urceolaria, Polyblastia, etc.). The branching of the hypha is dichotomous or lateral, and very irregular. Frequent septation and coherent growth result in the formation of plectenchyma.

C. CULTURE OF HYPHAE WITHOUT GONIDIA

Artificial cultures had demonstrated the germination of lichen spores, with the formation of hyphae, and from synthetic cultures of fungus and alga complete lichen plants had been produced. To Möller[269] we owe the first cultures of a thalline body from the fungus alone, both from spermatia and from ascospores. The germination of the spermatia has a direct bearing on their function as spores or as sexual organs and is described in a later chapter.

The ascospores of Lecanora subfusca were caught in a drop of water on a slide as they were ejaculated from the ascus, and, on the following day, a very fine germinating tube was seen to have pierced the exospore. The hypha became slightly thicker, and branching began on the third day. If in water alone the culture soon died off, but in a nutrient solution growth slowly continued. The hyphae branched out in all directions from the spore as a centre and formed an orbicular expansion which in fourteen days had reached a size of ·1 mm. in diameter. After three weeks’ growth it was large enough to be visible without a lens; the mycelial threads were more crowded, and certain terminal hyphae had branched upwards in an aerial tuft, this development taking place from the centre outwards. Möller marked this stage as the transition from a mere protothallus to a thallus formation. In three months a diameter of 1·5-2 mm. was reached; a transverse section gave a thickness of ·86 mm. and from the under side loose hyphae branched downwards and attached the thallus, when it had been transferred to a solid substratum such as cork. Above these rhizoidal hyphae, a stratum of rather loose mycelium represented the medulla, and, surmounting that, a cortical layer in which the hyphae were very closely compacted. Delicate terminal branches rose into the air over the whole surface, very similar in character to hypothallic hyphae at the margin of the thallus.

Lecanora subfusca has a rather small simple spore; it emitted germinating tubes from each end, and a septum across the middle of the spore appeared after germination had taken place. Another experiment was with a much larger muriform spore measuring 80 µ in length and 20 µ in thickness. On germination about 20 tubes were formed, some of them rising into the air at once, the others encircling the spore, so that the thallus took form immediately; growth in this case also was centrifugal. In three months a diameter of 6 mm. was reached with a thickness of 1 to 2 mm. and showing a differentiation into medulla and cortex. The hyphae did not increase in width, but frequently globose or ovate swellings arose in or at the ends, a character which recurs in the natural growth of hyphae both of lichens and of Ascomycetes. These swellings depend on the nutrition.

Pertusaria communis possesses a very large simple spore, but it is multinucleate and germinates with about 100 tubes which reach their ultimate width of 3 to 4 µ before they emerge from the exospore. The hyphae encircle the spore, and an opaque thalline growth is quickly formed from which rise terminal hyphal branches. In ten weeks the differentiation into medulla and cortex was reached, and in five months the hyphal thallus measured 4 mm. in diameter and 1 to 2 mm. in thickness.

Möller instituted a comparison between the thalli he obtained from the spores and those from the spermatia of another crustaceous lichen, Buellia punctiformis (B. myriocarpa). After germination had taken place the hyphae from the spermatia grew at first more quickly than those from the ascospores, but as soon as thallus formation began the latter caught up and, in eight weeks, both thalli were of equal size.

Another comparative culture with the spermatia and ascospores of Opegrapha subsiderella gave similar results: the spores of that species are elongate-fusiform and 6-to 8-septate; germination took place from the end cells in two to three days after sowing. The germinating hyphae corresponded exactly with those from the spermatia and growth was equally slow in both. The middle cells of the spores may also produce germinating tubes, but never more than about five were observed from any one spore. A browning of the cortical layer was especially apparent in the hyphal culture from another lichen, Graphis scripta: a clear brown colour gradually changing to black appeared about the same period in all the cultures.

The hyphae from the spores of Arthonia developed quickest of all: the hyphae were very slender, but in three to four months the growth had reached a diameter of 8 mm. In this plant there was the usual outgrowth of delicate hyphae from the surface; no definite cortical layer appeared, but only a very narrow line of more closely interwoven somewhat darker hyphae. Frequently, from the surface of the original thallus, excrescences arose which were the beginnings of further thalli.

Tobler[270] experimenting with Xanthoria parietina gained very similar results. The spores were grown in malt extract for ten days, then transferred to gelatine. In three to five weeks there was formed an orbicular mycelial felt about 3 mm. in diameter and 2 mm. thick. The mycelium was frequently brownish even in healthy cultures, but the aerial hyphae which, at first, rose above the surface were always colourless. After these latter disappeared a distinct brownish tinge of the thallus was visible. In seven months it had increased in size to 15 mm. in length, 7 mm. in width and 3 mm. thick with a differentiation into three layers: a lower rather dense tissue representing the pith, above that a layer of loose hyphae where the gonidial zone would normally find place, and above that a second compact tissue, or outer cortex, from which arose the aerial hyphae. The culture could not be prolonged more than eight months.

D. CONTINUITY OF PROTOPLASM IN HYPHAL CELLS

Wahrlich[271] demonstrated that continuity of protoplasm was as constant between the cells of fungi as it has been proved to be between the cells of the higher plants. His researches included the hyphae of the lichens, Cladonia fimbriata and Physcia (Xanthoria) parietina.

Baur[272] and Darbishire[273] found independently that an open connection existed between the cells of the carpogonial structures in the lichens they examined. The subject as regards the thalline hyphae was again taken up by Kienitz-Gerloff[274] who obtained his best results in the hypothecial tissue of Peltigera canina and P. polydactyla. Most of the cross septa showed one central protoplasmic strand traversing the wall from cell to cell, but in some instances there were as many as four to six pits in the walls. The thickening of the cell-walls is uneven and projects variously into the cavity of the cell. Meyer’s[275] work was equally conclusive: all the cells of an individual hypha, he found, are in protoplasmic connection; and in plectenchymatous tissue the side walls are frequently perforated. Cell-fusions due to anastomosis are frequent in lichen hyphae, and the wall at or near the point of fusion is also traversed by a thread of protoplasm, though such connections are regarded as adventitious. Fusions with plasma connections are numerous in the matted hairs on the upper surface of Peltigera canina and they also occur between the hyphae forming the rhizoids of that lichen. The work of Salter[276] may also be noted. He claimed that his researches tended to show complete anatomical union between all the tissues of the lichen plant, not only between the hyphae of the various tissues but also between hyphae and gonidia.

III. LICHEN ALGAE

A. TYPES OF ALGAE

The algal constituents of the lichen thallus belong to the two classes, Myxophyceae, generally termed blue-green algae, and Chlorophyceae which are coloured bright-green or yellow-green. Most of them are land forms, and, in a free condition, they inhabit moist or shady situations, tree-trunks, walls, etc. They multiply by division or by sporulation within the thallus; zoospores are never formed except in open cultivation. The determination of the genera and species to which the lichen algae severally belong is often uncertain, but their distribution within the lichen kingdom is as follows:

a. MYXOPHYCEAE ASSOCIATED WITH PHYCOLICHENS. The blue-green algae are characterized by the colour of their pigments which persists in the gonidial condition giving various tints to the component lichens, and by the gelatinous sheath in which most of them are enclosed. This sheath, both in the lichen gonidia[277] and in free-living forms, imbibes and retains moisture to a remarkable extent and the thallus containing blue-green algae profits by its power of storing moisture. Myxophyceae form the gonidia of the gelatinous lichens as well as of some other non-gelatinous genera. Several families are represented[278]:

Fam. CHROOCOCCACEAE. This family includes unicellular algae with thick gelatinous sheaths. They increase normally by division, and colonies arise by the cohesion of the cells. Several genera form gonidia:

1. CHROOCOCCUS Naeg. Solitary or forming small colonies of 2-4-8 cells (Fig. 16) generally surrounded by firm gelatinous colourless sheaths in definite layers (lamellate). Chroococcus is considered by some lichenologists to form the gonidium of Cora, a genus of Hymenolichens.

2. MICROCYSTIS Kütz. Globose or subglobose cells forming large colonies surrounded by a common gelatinous layer (gonidia of Coriscium).

3. GLOEOCAPSA Kütz. (including Xanthocapsa). Globose cells with a lamellate gelatinous wall, forming colonies enclosed in a common sheath (Fig. 17); the inner integument is often coloured red or orange. These two genera form the gonidia in the family Pyrenopsidaceae. Gloeocapsa polydermatica Kütz. has been identified as a lichen gonidium.

Fam. NOSTOCACEAE. Filamentous algae unbranched and without base or apex.

NOSTOC Vauch. Composed of flexuous trichomes, with intercalary heterocysts (colourless cells) (Fig. 18). Dense gelatinous colonies of definite form are built up by cohesion. In some lichens the trichomes retain their chain-like appearance, in others they are more or less broken up and massed together, with disappearance of the gelatinous sheath (as in Peltigera); colour mostly dark blue-green.

Nostoc occurs in a few or all of the genera of Pyrenidiaceae, Collemaceae, Pannariaceae, Peltigeraceae and Stictaceae, and N. sphaericum Vauch. (N. lichenoides Kütz.) has been determined as the lichen gonidium. When the chains are broken up it has been wrongly classified as another alga, Polycoccus punctiformis.

Fam. SCYTONEMACEAE. Trichomes of single-cell rows, differentiated into base and apex. Pseudo-branching arises at right angles to the main filament.

SCYTONEMA Ag. Pseudo-branches piercing the sheath and passing out as twin filaments (Fig. 19); colour, golden-brown. This alga occurs in genera of Pyrenidiaceae, Ephebaceae, Pannariaceae, Heppiaceae, in Petractis a genus of Gyalectaceae, and in Dictyonema one of the Hymenolichens.

Fam. STIGONEMACEAE. Trichomes of several-cell rows with base and apex; colour, golden-brown.

STIGONEMA Ag. Stouter than Scytonema, with transverse and vertical division of the cells, and generally copious branching (Fig. 20). This alga occurs only in a few genera of Ephebaceae. S. panniforme Kirchn. (Sirosiphon pulvinatus Bréb.) has been determined as forming the gonidium.

Fam. RIVULARIACEAE. Trichomes with a heterocyst at the base and tapering upwards, enclosed in mucilage (Fig. 21).

RIVULARIA Thuret. In tufts fixed at the base and forming roundish gelatinous colonies; colour, blue-green. The gonidium of Lichinaceae has been identified as R. nitida Ag.

Algae belonging to one or other of these genera of Myxophyceae also combine with the hyphae of Archilichens to form cephalodia[279] and Krempelhuber[280] has recorded and figured a blue-green alga, probably Gloeocapsa, in Baeomyces paeminosus from the South Sea Islands. They also form the gonidia in a few species and genera of such families as Stictaceae and Peltigeraceae.

b. CHLOROPHYCEAE ASSOCIATED WITH ARCHILICHENS. The lichens of this group are by far the most numerous both in genera and species, though fewer algal families are represented.

Fam. PROTOCOCCACEAE. Consisting of globular single cells, aggregated in loose colonies, dividing variously.

1. PROTOCOCCUS VIRIDIS Ag. (Pleurococcus vulgaris Menegh., Cystococcushumicola Naeg.). Cells dividing into 2, 4 or 8 daughter-cells and not separating readily; in excessive moisture forming short filaments. The cells contain parietal chloroplasts, and, according to Chodat[281], are without a pyrenoid (Fig. 22). This alga, and allied species, forms the familiar green coating of tree-trunks, walls etc., and, in lichenological literature, are quoted as the gonidia of most of the crustaceous foliose and fruticose lichens. Chodat[281], who has recently made comparative artificial cultures of algae, throws doubt on the identity of many such gonidia. He lays great emphasis on the presence or absence of a pyrenoid in algal cells. West, on the contrary, considers the pyrenoid as an inconstant character. Chodat insists that the gonidia that contain pyrenoids belong to another genus, Cystococcus Chod. (non Naeg.), a pyrenoid-containing alga, which, in addition to multiplying by division of the cells, also forms spores and zoospores when cultivated. He further records the results of his cultures of gonidia, and finds that those taken from closely related lichens, such as different species of Cladonia, though they are alike morphologically, yet show constant variations in the culture colonies. These, he holds, are sufficient to indicate difference of race if not of species and he designates the algae, according to the lichen in which they occur, as Cystococcus Cladoniae pyxidatae, C. Cladoniae fimbriatae, etc.

Meanwhile Paulson and Somerville Hastings[282] by their careful research on the growing thallus have thrown considerable light on the identity of the Protococcaceous lichen gonidium. They selected such well-known lichens as Xanthoria parietina, Cladonia spp. and others, which they collected during the spring months, February to April, the period of most active growth. Many of the gonidia, they found, were in a stage of reproduction, that showed a simultaneous rounding off of the gonidium contents into globose bodies varying in number up to 32. Chodat had figured this method of “sporulation” in his cultures of the lichen gonidium both in Chlorella Beij. and in Cystococcus Chod. (Fig. 23). It has now been abundantly proved that this form of increase is of frequent occurrence in the thallus itself. Chlorella has been suggested as probably the alga forming these gonidia and recently West has signified his acquiescence in this view[283].

2. CHLORELLA Beij. Occurring frequently on damp ground, bark of trees, etc., dividing into numerous daughter-cells, probably reduced zoogonidia (Fig. 23).

Chodat distinguishes between Cystococcus and Chlorella in that Cystococcus may form zoospores (though rarely), Chlorella only aplanospores. He found three gonidial species, Chlorella lichina in Cladonia rangiferina, Ch. viscosa and Ch. Cladoniae in other Cladonia spp.

3. COCCOBOTRYS Chod. The cells of this new algal genus are smaller than those of Cystococcus or Protococcus and have no pyrenoid. They were isolated by Chodat from the thallus of Verrucaria nigrescens (Fig. 24), and, as they have thick membranes, they adhere in a continuous layer or thallus. Chodat also claims to have isolated a species of Coccobotrys from Dermatocarpon miniatum, a foliose Pyrenolichen.

4. COCCOMYXA Schmidle. Cells ellipsoid, also without a pyrenoid. Two species were obtained by Chodat from the thallus of Solorinae and are recorded as Coccomyxa Solorinae croceae and C. Solorinae saccatae.

Coccomyxa subellipsoidea is given[284] as the gonidium of the primitive lichen Botrydina vulgaris (Fig. 25). The cells are surrounded by a common gelatinous sheath.

5. DIPLOSPHAERA Bial.[285] D. Chodati was taken from the thallus of Lecanora tartarea and successfully cultivated. It resembles Protococcus, but has smaller cells and grows more rapidly; it is evidently closely allied to that genus, if not merely a form of it.

6. UROCOCCUS Kütz. Cells more or less globose, rather large, and coloured with a red-brown pigment, with the cell-wall thick and lamellate, forming elongate strands of cells (Fig. 26). Recorded by Hue[286] in the cephalodium of Lepolichen coccophorus, a Chilian lichen.

Fam. TETRASPORACEAE. Cells in groups of 2 or 4 surrounded by a gelatinous sheath.

1. PALMELLA Lyngb. Cells globose, oblong or ellipsoid, grouped without order in a formless mucilage (Fig. 27). Among lichens associated with Palmella are the Epigloeaceae and Chrysothricaceae.

2. GLOEOCYSTIS Naeg. Cells oblong or globose with a lamellate sheath forming small colonies; colour, red-brown (Fig. 28). This alga along with Urococcus was found by Hue in the cephalodia of Lepolichen coccophora, but whereas Gloeocystis frequently occupies the cephalodium alone, Urococcus is always accompanied by Scytonema, the normal gonidium of the cephalodium.

Fam. TRENTEPOHLIACEAE. Filamentous and branched, the filaments short and creeping or long and forming tufts and felts or cushions; colour, brownish-yellow or reddish-orange.

TRENTEPOHLIA Born. Branching alternate; cells filled with red or orange oil; no pyrenoids (Fig. 29). A large number of lichens are associated with this genus: Pyrenulaceae, Arthoniaceae, Graphidaceae, Roccellaceae, Thelotremaceae, Gyalectaceae and Coenogoniaceae, etc., in whole or in part. Two species have been determined, T. umbrina Born., the gonidium of the Graphidaceae, and T. aurea which is associated with the only European Coenogonium, C. ebeneum (Fig. 3). Deckenbach[287] claimed that he had proved by cultures that T. umbrina was a growth stage of T. aurea.

Fam. CLADOPHORACEAE. Filamentous, variously and copiously branched, the cells rather large and multinucleate.

CLADOPHORA Kütz. Filaments branching, of one-cell rows, attached at the base; colour, bright or dark green; mostly aquatic and marine (Fig. 30). Only one lichen, Racodium rupestre, a member of the Coenogoniaceae, is associated with Cladophora. It is a British lichen, and is always sterile.

Fam. MYCOIDEACEAE. Epiphytic algae consisting of thin discs which are composed of radiating filaments.

1. MYCOIDEA Cunningh. (Cephaleuros Kunze). In Mycoidea parasitica the filaments of the disc are partly erect and partly decumbent, reddish to green (Fig. 31). It forms the gonidium of the parasitic lichen, Strigula complanata, which was studied by Marshall Ward in Ceylon[288]. Zahlbruckner gives Phyllactidium as an alternative gonidium of Strigulaceae.

2. PHYCOPELTIS Millard. Disc a stratum one-cell thick, bearing seta, adnate to the lower surface of the leaf, yellow-green in colour. Phycopeltis (Fig. 32) has been identified as the gonidium of Strigula complanata in New Zealand and of Mazosia (Chiodectonaceae), a leaf lichen from tropical America.

There is some confusion as to the genera of algae that form the gonidia of these epiphyllous lichens. Phyllactidium given by Zahlbruckner as the gonidium of all the Strigulaceae (except Strigula in part) is classified by de Toni[289] as probably synonymous with Phycopeltis Millard, and as differing from Mycoidea parasitica in the mode of growth.

Fam. PRASIOLACEAE. Thallus filamentous, often expanded into broad sheets by the fusion of the filaments in one plane.

PRASIOLA Ag. Thallus filamentous, of one-to many-cell rows, or widely expanded (Fig. 33). The gonidium of Mastoidiaceae (Pyrenocarpeae).

B. CHANGES INDUCED IN THE ALGA

a. MYXOPHYCEAE. Though, as a general rule, the alga is less affected by its altered life-conditions than the fungus, yet in many instances it becomes considerably modified in appearance. In species of the genus Pyrenopsis—small gelatinous lichens—the alga is a Gloeocapsa very similar to G. magma. In the open it forms small colonies of blue-green cells surrounded by a gelatinous sheath which is coloured red with gloeocapsin. As a gonidium lying towards or on the outside of the granules composing the thallus, the red sheath of the cells is practically unchanged, so that the resemblance to Gloeocapsa is unmistakable. In the inner parts of the thallus, the colonies are somewhat broken up by the hyphae and the sheaths are not only less evident but much more faintly coloured. In Synalissa, a minute shrubby lichen which has the same algal constituent, the tissue of the thallus is more highly evolved, and in it the red colour can barely be seen and then only towards the outside; at the centre it disappears entirely. The long chaplets of Nostoc cells persist almost unchanged in the thallus of the Collemaceae, but in heteromerous genera such as Pannaria and Peltigera they are broken up, or they are coiled together and packed into restricted areas or zones. The altered alga has been frequently described as Polycoccus punctiformis. A similar modification occurs in many cephalodia, so that the true affinity of the alga, in most instances, can only be ascertained after free cultivation.

Bornet[290] has described in Coccocarpia molybdaea the change that the alga Scytonema undergoes as the thallus develops: in very young fronds the filaments of Scytonema are unchanged and are merely enclosed between layers of hyphae. At a later stage, with increase of the thallus in thickness, the algal filaments are broken up, their covering sheath disappears, and the cells become rounded and isolated. Petractis (Gyalecta) exanthematica has also a Scytonema as gonidium, and equally exact observations have been made by Fünfstück[291] on the way it is transformed by symbiosis: with the exception of a very thin superficial layer, the thallus is immersed in the rock and is permeated by the alga to its lowest limits, 3 to 4 mm. below the surface, Petractis being a homoiomerous lichen. The Scytonema trichomes embedded in the rock become narrower, and the sheath, which in the epilithic part of the thallus is 4µ wide, disappears almost entirely. The green colour of the cells fades and septation is less frequent and less regular. The filaments in that condition are very like oil-hyphae and can only be distinguished as algal by staining reagents such as alkanna. They never seem to be in contact with the fungal elements: there is no visible appearance of parasitism nor even of consortism.

b. CHLOROPHYCEAE. As a rule the green-celled gonidium such as Protococcus is not changed in form though the colour may be less vivid, but in certain lichens there do occur modifications in its appearance. In Micarea (Biatorina) prasina, Hedlund[292] noted that the gonidium was a minute alga possessing a gelatinous sheath similar to that of a Gloeocapsa. He isolated the alga, made artificial cultures and found that, in the altered conditions, it gradually increased in size, threw off the gelatinous sheath and developed into normal Protococcus cells, measuring 7 to 10µ in diameter. The gelatinous sheath was thus proved to be merely a biological variation, probably of value to the lichen owing to its capacity to imbibe and retain moisture. Zukal[293] also made cultures of this alga, but wrongly concluded it was a Gloeocystis.

Moebius[294] has described the transformation from algae to lichen gonidia in a species epiphytic on Orchids in Porto Rico. He had observed that most of the leaves were inhabited by a membranaceous alga, Phyllactidium, and that constantly associated with it were small scraps of a lichen thallus containing isolated globose gonidia. The cells of the alga, under the influence of the invading fungus, were, in this case, formed into isolated round bodies which divided into four, each daughter-cell becoming surrounded by a membrane and being capable, in turn, of further division.

Frank[295] followed the change from a free alga to a gonidium in Chroolepus (Trentepohlia) umbrinum, as shown in the hypophloeodal thalli of the Graphideae. The alga itself is frequent on beech bark, where it forms wide-spreading brownish-red incrustations consisting of short chains occasionally branched. The individual cells have thick laminated membranes and vary in width from 20 µ to 37 µ. The free alga constantly tends to penetrate below the cortical layers of the tree on which it grows, and the immersed cells become not only longer and of a thinner texture, but the characteristic red colour so entirely disappears, that the growing penetrating apical cell may be light green or almost colourless. As a lichen gonidium the alga undergoes even more drastic changes: the red oily granules gradually vanish and the cells become chlorophyll-green or, if any retain a bright colour, they are orange or yellow. The branching of the chains is more regular, the cells more elongate and narrower; usually they are about 13 to 21 µ long and 8 µ wide, or even less. Deeper down in the periderm, the chains become disintegrated into separate units. Another notable alteration takes place in the cell-membrane which becomes thin and delicate. It has, however, been observed that if these algal cells reach the surface, owing to peeling of the bark, etc., they resume the appearance of a normal Trentepohlia.

In certain cases where two kinds of algae were supposed to be present in some lichens, it has been proved that one species only is represented, the difference in their form being caused by mechanical pressure of the surrounding hyphae, as in Endocarpon and Staurothele where the hymenial gonidia are cylindrical in form and much smaller than those of the thallus. They were on this account classified by Stahl[296] under a separate algal genus, Stichococcus, but they are now known to be growth forms of Protococcus, the alga that is normally present in the thallus. Similar variations were found by Neubner[297] in the gonidia of the Caliciaceae, but, by culture experiments with the gonidia apart from the hyphae, he succeeded in demonstrating transition forms in all stages between the “Pleurococcus” cells and those of “Stichococcus,” though the characters acquired by the latter are transmitted to following generations. The transformation from spherical to cylindrical algal cells had been also noted by Krabbe[298] in the young podetia of some species of Cladonia, the change in form being due to the continued pressure in one direction of the parallel hyphae.

Isolated algal cells have been observed within the cortex of various lichens. They are carried thither by the hyphae from the gonidial zone in the process of cortical formation, but they soon die off as in that position they are deprived of a sufficiency of air and of moisture. Forssell[299] found Xanthocapsa cells embedded in the hymenium of Omphalaria Heppii. They were similar to those of the thallus, but they were not associated with hyphae and had undergone less change than the thalline algae.

C. CONSTANCY OF ALGAL CONSTITUENTS

Lichen hyphae of one family or genus, as a rule, combine with the same species of alga, and the continuity of genera and species is maintained. There are, however, related lichens that differ chiefly or only in the characters of the gonidia. Among such closely allied genera or sections of genera may be cited Sticta with bright-green algae and the section Stictina with blue-green; Peltidea similarly related to Peltigera and Nephroma to Nephromium. In the genus Solorina, some of the species possess bright-green, others blue-green algae, while in one, S. crocea[300], there is an upper layer of small bright-green gonidia that project in irregular pyramids into the upper cortex; while below these there stretches a more or less interrupted band of blue-green Nostoc cells. The two layers are usually separated by strands of hyphae, but occasionally they come into close contact, and the hyphal filaments pass from one zone to the other. In this genus cephalodia containing blue-green Nostoc are characteristic of all the “bright-green” species. Harmand[301] has recorded the presence of two different types of gonidia in Lecanora atra f. subgrumosa; one of them, the normal Protococcus alga of the species, the other, pale-blue-green cells of Nostoc affinity.

Forssell[302] states that in Lecanora (Psoroma) hypnorum, the normal bright-green gonidia of some of the squamules may be replaced by Nostoc. In that case they are regarded as cephalodia, though in structure they exactly resemble the squamules of Pannaria pezizoides, and Forssell considers that there is sufficient evidence of the identity of the hyphal constituent in these two lichens, the alga alone being different.

It may be that in Archilichens with a marked capacity to form a second symbiotic union with blue-green algae, a tendency to revert to a primitive condition is evident—a condition which has persisted wholly in Peltigera with its Nostoc zone, but is manifested only by cephalodia formation in the Peltidea section of the genus. In this connection, however, we must bear in mind Forssell’s view that it is the Archilichens that are the more primitive[303].

The alien blue-green algae with their gelatinous sheaths are adapted to the absorption and retention of moisture, and, in this way, they doubtless render important service to the lichens that harbour them in cephalodia.

D. DISPLACEMENT OF ALGAE WITHIN THE THALLUS

a. NORMAL DISPLACEMENT. Lindau[304] has contrasted the advancing apical growth of the creeping alga Trentepohlia with the stationary condition of the unicellular species that multiply by repeated division or by sporulation, and thus form more or less dense zones and groups of gonidia in most lichens. The fungus in the latter case pushes its way among the algae and breaks up the compact masses by a shoving movement, thus letting in light and air. The growing hypha usually applies itself closely round an algal cell, and secondary branches arise which in time encircle it in a network of short cells. In the thallus of Variolaria[305] the hyphae from the lower tissues, termed push-hyphae by Nienburg[306], push their way into the algal groups and filaments composed of short cells come to lie closely round the individual gonidia. Continued growth is centrifugal, and the algae are carried outward with the extension of the hyphae (Fig. 12). Cell-division is more active at the periphery, that being the area of vigorous growth, and the algal cells are, in consequence, generally smaller in that region than those further back, the latter having entered more or less into a resting condition, or, as is more probable, these smaller cells are aplanospores not fully mature.

b. LOCAL DISPLACEMENT. Specimens of Parmelia physodes were found several times by Bitter, the grey-green surface of which was marbled with whitish lines, caused by the absence of gonidia under these lighter-coloured areas. The thallus was otherwise healthy as was manifested by the freely fruiting condition: no explanation of the phenomenon was forthcoming. Bitter compared the condition with the appearance of lighter areas on the thallus of Parmelia obscurata.

Something of the same nature was observed on the thallus of a Peltigera collected by F. T. Brooks near Cambridge. The marking took the form of a series of concentric circles, starting from several centres. The darker lines were found on examination to contain the normal blue-green algal zone, while the colour had faded from the lighter parts. The cause of the difference in colouration was not apparent.

E. NON-GONIDIAL ORGANISMS ASSOCIATED WITH LICHEN HYPHAE

Bonnier[307] made a series of cultures with lichen spores and green cells other than those that form lichen gonidia. In one instance he substituted _Protococcus botryoide_s for the normal gonidia of Parmelia (Xanthoria) parietina; in another of his cultures he replaced Protococcus viridis by the filamentous alga Trentepohlia abietina. In both cases the hyphae attached themselves to the green cells and a certain stage of thallus formation was reached, though growth ceased fairly early. Another experiment made with the large filaments of Vaucheria sessilis met with the same amount of success (Fig. 34). The germinating hyphae attached themselves to the alga and grew all round it, but there was no advance to tissue formation.

Cultures were also made with the protonema of mosses. Either spores of mosses and lichens were germinated together, or lichen spores were sown in close proximity to fully formed protonemata. The developing hyphae seized on the moss cells and formed a network of branching anastomosing filaments along the whole length of the protonema without, however, penetrating the cells. If suitable algae were encountered, proper thallus formation commenced, and Bonnier considers that the hyphae receive stimulus and nourishment from the protonema sufficient to tide them over a considerable period, perhaps until the algal symbiont is met. An interesting variation was noted in connection with the cultures of Mnium hornum[308]. If the protonema were of the usual vigorous type, the whole length was encased by the hyphal network; but if it were delicate and slender, the protoplasm collected in the cell that was touched by hyphae and formed a sort of swollen thick-walled bud (Fig. 35). This new body persisted when the rest of the filament and the hyphae had disappeared, and, in favourable conditions, grew again to form a moss plant.

F. PARASITISM OF ALGAE ON LICHENS

A curious instance of undoubted parasitism by an alga, not as in Strigula on one of the higher plants, but on a lichen thallus, is recorded by Forssell[309]. A group of Protococcus-like cells established on the thallus of Peltigera had found their way into the tissue, the underlying cortical cells having degenerated. The blue-green cells of the normal gonidial layer had died off before their advance but no zone was formed by the invading algae; they simply withdrew nourishment and gave seemingly no return. The phenomenon is somewhat isolated and accidental but illustrates the capacity of the alga to absorb food supply from lichen hyphae.

An instance of epiphytic growth has also been recorded by Zahlbruckner[310]. He found an alga, Trentepohlia abietina, covering the thallus of a Brazilian lichen, Parmelia isidiophora, and growing so profusely as to obscure the isidiose character towards the centre of the thallus. There was no genetic connection of the alga with the lichen as the former was not that of the lichen gonidium. Lichen thalli are indeed very frequently the habitat of green algae, though their occurrence may be and probably is accidental.

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