An introduction to vegetable physiology

Front matter

H. K. LEWIS’S

MEDICAL AND SCIENTIFIC

Circulating Cibrarp.

Preface

Although during recent years considerable additions have been made to our elementary botanical textbooks, not one has appeared which deals solely, or at any length, with the subject of vegetable physiology. This has been eitherpresented to the reader as a particular section in a comprehensive work, or treated of incidentally in connection with anatomical detail. This is the more strange, as an adequate and intelligent appreciation of the forms and structure of vegetable organisms can only be gained by a consideration of the work they have to carry out. It must be evident to the student of Nature that the peculiarities of external and internal form, of which any particular plant has become possessed, have arisen necessarily in connection with the need of mechanisms to do certain work, to overcome particular disadvantages, and generally to bring the organism into a satisfactory relationship with the surroundings among which it finds itself.

I have been led by these considerations to endeavour to fill this gap by writing an introduction to the subject, which, while putting physiology into its proper prominence among the branches of botanical study, shall serve to pave the way of the student and of the general reader to the more complete discussion of the subject which may be met with in the advanced textbooks of Sachs, Vines, and Pfeffer.

VI

With this view I have endeavoured to present the plant as a living organism, endowed with particular properties and powers, realising certain needs, and meeting definite dangers. I have attempted to show it to be properly equipped to encounter such adverse conditions, and to avail itself of all the advantages presented to it by its environment.

I have also set before myself another purpose, which, however, is naturally subordinate to the one just mentioned. When we consider the origin of the different organisms which we find around us, we are led irresistibly to the conclusion that the classification of living beings into animals and plants has been too strongly insisted upon in the past, and that while much has been made of their differences, their points of resemblance have been minimised, The fact that organisms exist, which it is difficult or impossible to refer with certainty to either kingdom, points to a fundamental unity of living substance. Protoplasm in short is the same material, whether we call it animal or vegetable. This being the case, its conditions of life and its immediate necessities must be practically the same, whatever its degree of differentiation in either direction. I have tried to bring out this identity of living substance throughout the book, and to indicate that apparent differences of behaviour and structural arrangement are to be traced rather to differences of environment and habit of life than to those of constitution. The correspondence of the processes of respiration in animals and plants has long been recognised ; many points of similarity in those of nutrition have been observed. The idea is, however, still prevalent that plants live" upon inorganic materials absorbed from the air and from the soil. This seems to indicate a fundamental difference between the modes of nutrition of animal and vegetable protoplasm. I have

PREFACE vii'

endeavoured to show that this view is erroneous and that both are nourished similarly.

I have also tried to show that the sensitiveness of the plant and the animal is alike in properties, though differences are apparent in the direction of its differentiation.

I have avoided as far as possible the discussion of controverted points, feeling that this would be out of place in a work intended to serve as an introduction to the subject. Such matters are more properly treated of in the more comprehensive works to which I have already alluded.

J. Reynolds Green.

Cambridge.

Chapter I Page

Unicellular plants ; zoogonidia, yeasts, bacteria ; multicellular plants ; the protoplast, its structure and arrangements ; characters of protoplasm ; nuclei and nucleoli ; association of protoplasts in colonies ; slime fungi ; ccenocytes ; arrangements in multicellular plants — Needs of protoplasm; its relation to water; formation of vacuoles ; relation of water to the plant in general ; the aeration of protoplasm— Connection of protoplasts with one another in the body of the plant .......... 1-16

Chapter II

Division of labour the. clue to differentiation of structure — Formation of protective tissues : epidermis, cuticle, periderm, bark— System of conducting tissues ; vascular bundles and their distributionstrengthening tissues : collenchyma and sclerenchyma ; the

different arrangements of them which are met with — The stereome of the plant — The metabolic tissues — The arrangements for the aeration of the interior ; stomata, lenticels 17-35

Chapter III

Necessity of a skeleton to support the protoplasts ; varieties of the skeleton Development of the skeleton as the plant grows — Characters of the cell-wall ; cellulose, its properties and reactions ; pectose and related substances — Arrangement of the solid matter and the water of the cell-wall ; hypotheses of Naegeli and Strasbuiger Differentiation of the substance of thickened cell-walls ;

X stratification, middle lamella — Lignin and its reactions — Cutin — Impregnation of cell-walls with various matters — Mucilage- Differences between temporary and permanent portions of the skeleton .... «

Chapter IV

the relation op water to the protoplasm of

THE CELL

Dependence of the protoplasts on water; function of the vacuole— Renewal of the water of the vacuole— Osmosis— Formation of the vacuole as the protoplast develops — Regulation of osmosis by the cell-protoplasm ; the plasmatic membranes— Movements of water from cell to cell— Evaporation into the intercellular spaces — Turgescence and its dependence on the protoplasm -Storage of water

Chapter V The Transport Of Water In The Plant

Varied needs of different plants in this respect— Transport in a terrestrial plant -The ascending sap — Condition of water in the soil; absorption of water a function of the root-hairs ; mechanics of the root-hair — Path of the ascending stream ; forces causing the movement — Evaporation of water from the interior — Influence of the stream of water upon the development of the plant . . (50-77

Chapter VI

THE TRANSPIRATION CURRENT. ROOT PRESSURE.

The ascending sap, sometimes called the transpiration current — Its path ; methods of demonstration — Rate of the transpiration current — Causes of the upward flow ; root pressure ; transpiration ; capillarity ; pumping action of living cells ; osmotic action of the parenchyma of the leaves - Root pressure ; its nature and mode of action ; bleeding of cut stems ; of entire plants ; measurement of root pressure ; conditions of the activity of roots ; diurnal variations of root pressure — Transpiration; methods of demonstration ; amount of water given off ; negative pressure in the wood vessels ; character of the evaporation of transpiration ; regulation by stomata, their mode of action ; variations in numbers of stomata ; conditions affecting transpiration ; light, temperature,

CONTENTS xi

PAGE

moisture of air, rest — The Potometer — Suction of transpiration Osmotic action of the parenchyma of the leaves and its effect— Regulation of all these forces by the protoplasm . . • 78-102

CHAPTER VI I

Necessity of admitting oxygen to the protoplasts— The intercellular space system ; its origin and development ; condition in terrestrial plants; relative extent in roots, stems, leaves— Air reservoirs in aquatic plants ; in Equisetum, grasses, rushes, &e. ; mode of formation of the reservoirs — External orifices of the intercellular space system ; stomata and lenticels — Relative dimensions of cellular tissue and intercellular spaces — Movements of air in intercellular space system — Composition of the air . . • . l(h

Chapter VIII

THE FOOD OF PLANTS. INTRODUCTORY

True nature of the food of plants — Materials absorbed by plants, and their relationship to actual food — Differences between food and food materials — Construction of food from the latter — Assimilation of food — Intricacy of the metabolic processes of plants . . 118-125

Chapter IX Absorption Of Food Materials By A Green Plant

Examination of substances absorbed from the soil ; water-culture ; destructive analysis — Classification of materials absorbed — The ash of plants — Conditions of absorption of substances in the soil — Absorption of nitrogen by leguminous plants — Insectivorous plants and their behaviour — Absorption of metallic compounds ; silicon— Absorption of carbon dioxide from the air ; its mechanism . 126-140

Chapter X

Formation of organic substances from the inorganic materials absorbed Chlorophyll — Structure of a chloroplast — Properties of chlorophyll ; its absorption spectrum — Xantliophyll— Erythrophyll — Composition of chlorophyll— Distribution of the chloroplasts — Relationship between the plastid and the colouring matter — Leucoplasts

Xll

PAGE

— Conditions of formation of chlorophyll ; light, temperature, iron — Formation of carbohydrates by chloroplasts ; conditions of their activity — Theories of photosynthesis— Relation of starch to the process — Bays of light made use of in photosynthesis ; researches of Engelmann, of Tmiriazeff — -Inhibition of the chlorophyll apparatus— Formation of organic substance in its absence . . 141-159

Chapter XI

Complexity of the composition of protein; its percentage composition — Classification of proteins ; albumins, globulins, metaproteins, proteoses, peptones, proteins soluble in alcohol — Synthesis of proteins in plants ; various hypotheses — Locality of protein construction in the plant 100-170

Chapter XII

Nature and composition of the ash — Water-culture and the limitations of its usefulness in the study of the ash - Classification of the constituents of the ash — The selective power of plants — Sulphur and phosphorus— Potassium, magnesium, calcium, iron — Sodium, silicon, chlorine, bromine, iodine, manganese — Accidental constituents of the ash— Relation of nitrogen and potassium to herbage plants . . . . ‘ 171-182

Chapter XIII

Partial or entire absence of the constructive power — Nutrition of saprophytes — Insectivorous plants — Utricularia — The pitcher-plants - Drosophyllum — Pinguicula — Dionma — Drosera — Digestion of substances by Fungi— Commensalism — Symbiosis Mycorhiza — Root parasites — Parasitism among green plants and Fungi 183-206

Chapter XIV

Conditions of the constructive processes; surplus production and storage — Necessity of circulation of food material in consequence of localisation of construction, and intermittence of consumption

CONTENTS

Xlll

PAGE

Mode of transport of substances in the plant ; osmosis and diffusion ; temporary storage— Translocatory and storage forms of food— The so-called descending sap— The pathway of translocation . 207-220

Chapter XV

Connection between transport .and storage ; forms in which food is stored— Reservoirs of storage ; stems, roots, floral organs — Storage of carbohydrates ; starch grains and their formation by chloroplasts and leucoplasts, by the cytoplasm ; glycogen ; inulin ; sugars ; cellulose and similar compounds — Storage of proteins ; aleurone grains, their composition and mode of formation ; protein crystals ; antecedents of gluten —Storage of asparagin ; glucosides ; fats and oils — Mode of formation of the last-named group . 221-241

Chapter XVI Digestion Of Reserve Materials

Nature of digestion — Its localisation in plants — Agents of digestion — Secretion of enzymes — Conditions of their action — Zymogens — Differentiation of glandular structures — Classification of enzymes — Diastase and its action on starch — Inulase— Invertase — Glucase — Cytase and cell->valls — Pectase— Proteoclastic enzymes — Rennet — Enzymes which decompose glucosides— Lipase and its action on fats — Zymase and the production of alcohol — Oxidases— Fermentative activity of protoplasm— Assimilation .... 242-257

Chapter XVII

Constructive and destructive processes; anabolism and katabolism — Constructive processes depending on katabolism— Secretion — Bye-products— Secretion of enzymes — Formation of cell-wells, of starch grains, of aleurone grains, of fat, of chlorophyll, of anthocyan — Formation of resin, of alkaloids, of acids— Decomposition-produets of cellulose ; colouring matters ; nectar ; etherial oils . . 258-274

Chapter XVIII

Preliminary considerations —The expenditure of energy in evaporation, in constructive processes, in movements, in radiation, in light — Source of the energy of plants the radiant energy of the sun ; its

XIV

PAGE

absorption by chlorophyll ; absorption of heat rays— Fixation of energy — Kinetic and potential energy — Distribution and liberation of energy — Dependence of the plant upon oxygen ; absorption of oxygen and exhalation of carbon dioxide ; apparatus to demonstrate these processes — Loss of weight during respiration — Variations in the respiratory activity — Delation between the absorption of oxygen and the exhalation of carbon dioxide ; the respiratory quotient — Exhalation of water during respiration — Respiration a function of protoplasm — Conditions affecting respiration ; temperature, light, differences of gaseous pressure and of nutritive materials — Relation of respiratory processes to local utilisation of potential energy — Oxidative actions other than respiration — Intramolecular or anaerobic respiration — Fermentation— Anaerobic plants 275-303

Chapter XIX

Relation of growth to constructive metabolism — Definition of growth — Distribution of growth — Conditions necessary for growth ; plastic materials, turgescence, temperature, oxygen — The grand period of growth — Growth of a cell and of a multicellular organ — The region of growth in the latter — Daily period of growth in length — The Auxanometer — Variations in growth ; hyponasty and epinasty ; nutation and circnmnutation — Tensions accompanying growth — llectipetality 304-318

Chapter XX

Range of temperature through which the vital processes proceed; photosynthesis, germination — Causes and effects of fluctuations of temperature — Influence of the light rays on temperature ; importance of anthocyan — Absorption of heat by conduction— Dissipation of heat in evaporation of water — Radiation — Nyctitropic movements — Loss and gain of heat by conduction— Regulation of heat —Power of resistance to extremes of temperature . . 319-328

Chapter XXI Influence Of The Environment On Plants

Characters of aquatic plants ; influence of a watery environment on structure— Xerophytes and their peculiarities— Alpine plants— Epiphytes— Parasites— Insectivorous plants . • • 329-343

CONTENTS xv

PAGE

Chapter XXII The Properties Of Vegetable Protoplasm

Adaptability of plants to their surroundings— Contractility— Ciliary and amoeboid movement — Locomotion-Movements of rotation and circulation — Turgor and its maintenance — Mobile condition of protoplasm — Rhythm and its manifestations — Irritability and its conditions — Tone— Phototonus— Thermotonus -Tonic influence of light — Etiolation — Influence of too brilliant illumination ; paraheliotropism, apostrophe and epistrophe — Photo-epinasty — Regulating action of light on growth — Conditions of health — Acclimatisation .......... 344-366

Chapter XXIII

Response of an organism to changes in its surroundings — Nature of stimulation — Purposeful character of the response — Stimulation of light -Nyetitropic movements, their conditions and purpose — Mechanism of the movements — Effect of incidence of lateral light — Heliotropism— Stimulus of gravitation; geotropism — The Klinostat — Knight’s wheel— Stimulus of contact— Behaviour of various organs in relation to this form of stimulation — The root— Twining stems and tendrils— Hydrotropism— Chemical stimuli — Chemotaxis — Induced rhythm 367-396

Chapter XXIV

The purposeful responses of plants to stimulation ; relation of stimulus to effect — Nature of nervous mechanisms— Sense organs and their differentiation— Motor mechanisms of plants— Contraction — Regulation of supply of water to the cell — Glandular organs — Conduction of impulses ; continuity of protoplasm — Co-ordination of impulses — Latent period of stimulation — After-effects — Fatigue — Anaesthetics— Comparison of nervous mechanisms of plants and animals 397-410

Chapter XXV

Distinction between the individual protoplast and the colony or plant — Process of t multiplication of protoplasts ; gemmation, karyokinesis, formation of cell-walls; free-cell formation — Vegetative

XVI

PAGE

propagation — Formation of asexual reproductive cells, spores or gonidia ; zooeoenocytes — Development of sexual cells or gametes : planogametes and conjugation ; male and female cells ; antherozoids and oospheres — Gametangia and their varieties— Fertilisation — Alternation of generations— Gametopliytes and sporophytes — Heterospory and its consequences — The seed and its formation 411-437

Chapter XXVI

reproduction ( continued )

Pollination and its mechanisms — Advantages of cross-pollination — Dichogamy, protandry, protogyny— Diclinism — Heterostylism or dimorphism — Prepotency— Self-sterility — Self-pollination ; cleistogamy— Mechanism of fertilisation; the growth of the pollen-tube — Hybridisation — Results of fertilisation ; formation and ripening of fruits and seeds — Germination of the seed— Apospory ; apogamy ; parthenogenesis 438-450

. 451

LIST OF ILLUSTRATIONS

PAGE

IG.

1. Zoospore of Ulothrix 1

2. Yeast Plants ^

3. Bacteria 3

4. Plasmodium of a Myxomycete 4

5. Vegetable Cells (Young) 5

6. Vegetable Cells (Adult) 5

7. Cells exhibiting Botation, from Elodea 7

8. Cells of Tradescantia, showing circulation 7

9. Structure of the Nucleus ......... 7

10. Colonies of Protococcus ......... 9

13. Filaments of Nostoc . . . . • • • • . . 12

14. Pediastruni 12

15. Vegetable Cells (Young) . . . . . . . . . 12

16. Vegetable Cells (Adult) 13

17. Continuity of Protoplasm in Seed . . . . . . . 16

18. Continuity of Protoplasm in Seaweed 16

19. Thallus of Pelvetia 18

20. Stem of Sphagnum . . . .19

21. Stem of Common Moss 19

23. Cork Cells 20

25. Collenchyma 21

26. Exodermis of Root .......... 21

27. Diagram of Course of Vascular Bundles in a Dicotyledonous plant . 23

29. Section of Rhizome of Fern 26

30. Section of Leaf of Pinus 27

31. Vascular Bundle of Monocotyledon 28

32. Different Arrangements of Stereome in Herbaceous Plants . . 29

33. Chloroplasts in Cell 30

34. Section of Stem of Potamogeton ....... 31

35. Cortex of Root 32

37. Stomata on Lower Surface of Leaf . . . . . . . 33

38. Section of Epidermis of Leaf 33

39. Section of a Lenticel .34

a

FIC!- PAGK

40. Section of Dicotyledonous Stems of two ages 38

41. Embryo of Orobus 39

42. Stratification in Cell-walls 44

43. Longitudinal Section of Vascular Bundle of Sunflower Stem . . 45

44. Wood-cells, showing Middle Lamella 45

45. Section of Epidermis of Leaf 48

46. Cork in Twflg of Lime 49

47. Section of a Lenticel 49

48. Crystals in Wall of Cell of Bast 50

49. Cystolith of Ficus 50

50. Apparatus to show the process of Osmosis 55

51. Young Vegetable Cells 56

52. Adult Vegetable Cells 57

53. Cells undergoing Plasmolysis 59

55. Hoot-hair in contact with particles of Soil 69

56. Section of Young Hoot 70

57. Diagram of Course of Vascular Bundles in a Dicotyledonous Plant 71

58. Veins of a leaf 72

59. Ending of a Vascular Bundle in a Leaf 73

60. Intercellular Spaces in Leaf 73

61. Stomata on Lower Surface of Leaf 75

62. Apparatus for the Estimation of Root-pressure . . . .84

63. Apparatus to demonstrate Transpiration 89

65. Apparatus to show dependence of Withering on Loss of Water . . 91

66. Stomata on Lower Surface of Leaf 93

67. Section of a Stoma 93

68. Darwin’s Potometer 98

69. Apparatus to show the Suction of Transpiration . . . . 99

70. Ending of a Vascular Bundle in a Leaf 101

71. Formation of Intercellular Spaces 105

72. Intercellular Spaces in Root 105

73. Intercellular Spaces in Leaf 106

74. Section of Leaf of Isoiiles 106

75. Section of Rhizome of Marsilea 107

76. Section of Stem of Potamogeton 108

77. Section of Stem of Equisetum 109

78. Section of Stem of Juncus 110

79. Section of a Lenticel HI

80. Apparatus to show Continuity of Intercellular Spaces in a Leaf . Ill

81. Section of Leaf of Heath 112

82. Root of a Leguminous Plant with its Tubercles .... 134

84. Absorption Spectra of Chlorophyll and Xanthophyll . . . 143

85. Chloroplasts in Cell

86. Section of Leaf of Beta ...•••••• 146

87. Section of Stem of Equisetuiu ........ 147

LIST OF ILLUSTRATIONS xix

FIG. rAQE

88. Apparatus to show the Exhalation of Oxygen by a Green Plant . 151

89. Plants of Buckwheat cultivated in various nutritive solutions . 173

90. Aleurone Grains in Cell of Ricinus ....... 177

91. Bladderworts ( Utricularia) 187

92. Traps of Utricularia 188

93. Pitcher of Sarracenia 189

94. Pitcher of Nepenthes .......... 190

97. Section of Lichen 196

98. Mycorhiza on Beech Boot ......... 198

99. Plant of Tliesium 200

100. Sucker of Tliesium 201

101. Plant infested with Dodder 203

102. Haustoria of Dodder .......... 204

103. Haustoria of Phytophthora 205

104. Starch Grains in Chloroplast 214

105. Section of Stem of Ricinus 225

106. Section of Stem of Tilia three years old . . . . . ‘ . 226

107. Starch Grains in Chloroplast 228

108. Starch Grains in Cell of Potato 229

109. Starch Grain of Potato 229

110. Compound and Semi-compound Starch Grains 230

111. Laticiferous Cell of Euphorbia 231

112. Leucoplasts of Phajus 231

113. Inulin Sphaero-crystals 233

114. Aleurone Layer of Barley ......... 234

115. Cells of Embryo of Pea . . . . . . . . 235

116. Formation of Aleurone Grains 235

117. Aleurone Grains of Lupin 236

118. Aleurone Grains of Ricinus 236

119. Section of Oat Grain 246

120. Epithelium of Scutellum 246

121. Aleurone Layer of Bariev 247

123. Corrosion of Starch Grains by Diastase ..... 249

124. Glandular Hairs of Primula ........ 269

125. Glandular Hairs of Hop 269

126. Oil Keservoirs of Hypericum ........ 272

127. Bast-Cell of Ephedra ....... 272

128. Crystals of Oxalate of Calcium in Cells . . . . . . 273

129. Cystolith of Ficus 273

130. Absorption Spectra of Chlorophyll and Xanthophyll . . . 280

131. Apparatus to show the Absorption of Oxygen by a Green Plant . 287

132. Apparatus to show Exhalation of Carbon Dioxide by Germinating

,t5eeds 288

133. Longitudinal Section of Growing Point of Root .... 305

XX VEGETABLE PHYSIOLOGY

F IG‘ _ TAGE

135. Section of Stem of Rush 307

136. Adult Vegetable Cells 300

137. Region of Growth in Root of Bean 312

138. Pfeifer’s Auxanometer 313

139. Air Passages in Potamogcton 330

141. Petiole of Water-lily 332

142. Rhizome of Marsilea 334

143. Water-gland of Saxifraga 336

144. Leaf of Heath 338

145. Suckers of Thesium 341

146. Section of a Sucker 342

147. Zoospore of TJlothrix 345

148. Plasmodium of a Myxomycete 340

149. Cells from Leaf of Elodea 349

150. Cells from Hair of Traclcscantia 349

151. Leaf of Telegraph Plant 354

152. Pulvinus of Mimosa 355

153. Dcsmodium gyrans, day and night position 372

154. Nicotiana glauca , day and night position 373

155. Pulvinus of Mimosa 375

156. Darwin’s Klinostat 381

157. Section of Sucker of Thesium 388

158. Haustoria of Guscuta 389

159. Leaf of Dioncca 390

160. The samo 401

161. Continuity of Protoplasm through the Cell-wall .... 404

162. Yeast Plants 413

163. Stages in Karyokinetic Division of the Nucleus .... 414

164. Zoospore of TJlothrix 418

165. Gonidangia of Adilya 418

166. Mycelium of Mucor 419

167. Part of Hymenial Layer of Peziza 419

168. Stylogonidia of Eurotium 420

169. Filament of TJlothrix with Gametes escaping .... 421

170. Oogonium of Fucus 423

171. Oosphere and Antlierozolds of Fucus 423

172. Procarpium of a Red Seaweed 424

173. Archegonium of Fern 425

174. Prothallium of Fern 428

175. Germination of Microspores of Salvinia 429

176. Germination of Megaspore of Salvinia 430

177. Germination of Megaspore of Selaginella 431

178. Ovule of Pinus 432

179. Ovule of an Angiosperm 432

180. Antherozo'ids of Moss and Fern 435

181. Antheridium of Fern 435

182. Archegonium of Fern , 436

Chapter I

Examination of the body of every living organism shows us that it is composed of different materials, which exhibit a great deal of variety in the ways in which they are arranged. These different materials fall very naturally into two classes, which include respectively the living substance itself, and various constituents of the body which have been constructed by it. The relative proportions in which these two classes of materials exist vary very greatly in different organisms ; in some of the simplest forms indeed we can discern nothing structural except the living substance itself. In others the materials constructed by the latter are much the greatest in amount.

When we study the life history of the simplest or the most complex plant with which we can become acquainted, we find that at some time or other in its existence it is found in the form of a minute portion of jelly-like material which is endowed with life. Sometimes this piece of living substance is motile, and can swim freely about in water by means of certain thread-like appendages which it possesses (fig. 1).

Such structures occur almost exclusively among the lowest forms of plants, particularly the seaweeds. They are known as zoosgwrcs, or zoogoniclia,

Fig. 1. — Zoospore of Ulothrix. x 500.

and are produced in large numbers. In other cases the little mass of living substance is not capable of locomotion, but may be found floating about in water, or enclosed in particular cavities in its parent plant.

The jelly-like substance of which these bodies are composed is living and capable of carrying out all the functions necessary for its life, growth, and multiplication. It is called protoplasm, and each portion of protoplasm which is thus capable of independent existence is known as a vegetable cell, or protoplast.

These free-swimming organisms are not protected by any coating, but every part of their surface is in complete contact with the water in which they live. This condition

Pig. 2. — Saccharomyces Cerevisle, on Yeast-Plant, as developed DURING THE PROCESS OP FERMENTATION. X 800.

a, b, c, d, successive stages of cell multiplication.

is, however, exceptional. Usually the protoplast is encased in a colourless homogeneous membrane of extreme tenuity which is known as its cell-wall. Examples of unicellular organisms of this kind are found in great numbers among the fungi, the Yeasts (fig. 2) and the Bacteria (fig. 3) being exceptionally numerous. Such plants may be motile or non-motile, a few of the bacteria being furnished with thread-like appendages, known as cilia or flagella, which are similar in most respects to those of the zoospoores already mentioned. These plants show a little more differentiation than the others, the protoplasm being clothed by a kind of exoskeleton, the cell-wall, which is at once supporting and protective.

More complex organisms consist of two or more protoplasts united together in various ways, The number of these masses of protoplasm, or cells, may be as small as two, or may be enormous, as is the case in such plants as the gigantic seaweeds of the tropics or the tall terrestrial trees which abound all over the surface of the globe,

Whether the plant is simple or complex, we find the same fundamental arrangement of its parts : there is a certain number of protoplasts, in close relationship with each other, supported upon a framework or skeleton which shows a wonderful variety of arrangement, its details depending on the manner of life of the whole organism of which it forms so large a part. In such an organism each

Fig. 8.— Figures of different Bacteria. (After Colin and Sachs. Very highly magnified.)

5, 6, 7, Micrococcus. (Single, in strings, and in groups.)

protoplast is usually found occupying a. particular cavity which is formed by its cell-walls, and communicating with its neighbours on all sides by delicate prolongations of living substance which extend through the walls of contiguous chambers. Each chamber is often called a cell.

In dealing with the physiology of the plant, it is the living substance which should first engage our attention, though the arrangements of the supporting structures or skeleton exhibit the greatest variety. We have seen that in the simplest forms of plants the living substance may exist without any cell-membrane, and may be freely motile, swimming in water by means of cilia. The absence of the cell-membrane can also be observed in certain peculiar fungi, which are to be found creeping over moist surfaces without such appendages (fig. 4). These are known as the slime-fungi or Myxomycetes. In many respects they approach very near to one of the humblest animals, the Amoeba. They have hardly any structure, appearing like a lump of transparent jelly, the whole mass being called a plasmodium. They have the power of extruding a certain portion of their substance in the form of a blunt protrusion

Fig. 4. — Portion op a Plasmodium of a Myxomycete. x 800.

(After De Bary.)

known as a pseudopodium , and by means of these pseudopodia they can creep slowly over the surface on which they are lying. The naked condition is, however, exceptional in plants. In most of those which are unicellular the living substance is covered by a delicate membrane or cell-wall, and it may either fill the space inside the latter, or may have in its interior a cavity or vacuole, which is filled with a watery fluid. In the multicellular plants each chamber during life contains its own protoplast or little mass of protoplasm, which is connected, as already mentioned, with its neighbours on all sides. In such cells the protoplast when young usually occupies the whole of the interior (fig. 5, a), but when they are adult it generally lies as a

Fig. 5. — Vegetable Cells.

A, very young ; B, a little older, showing commencing formation of vacuole. p, protoplasm ; n, nucleus ; v, a vacuole.

peripheral layer round the wall, to which it is closely pressed, while a central vacuole occupies the greater space of the cavity enclosed by the cell-walls (fig. 6). Sometimes the vacuole is crossed by a number of bridles or strands of protoplasm, which generally pass from a somewhat central spot to the periphery. The protoplasm is transparent, but somewhat granular in appearance, and is saturated with water. Somewhere, in its substance, whether it fills the cell-cavity or not, there exists a special differentiated portion called the nucleus.

Sometimes, but only in particular cells, the protoplasm contains other differentiated portions, distinct from the rest of the substance, which are known as plastids. The bulk of the living substance, to distinguish it from these specialised portions, is usually called the cytoplasm. It is not of the same consistency throughout, a generally firmer portion lying next to the cell-wall being known as the ectoplasm. A similar firm