Opening…

Preface

In the Preface to Volume II published in 1910 I rashly stated that it was my intention “to devote such space as is available within the limits of a text-book to the neglected subject of the geographical distribution of plants at different stages in the history of the earth,” also that Volume III would be completed with as little delay as possible. Though nearly seven years have elapsed since the publication of the second volume it may fairly be said that the delay is not entirely due to causes which it was in my power to control. The subject of geographical distribution receives no connected treatment in Volumes III and IV for the simple reason that I underestimated the space required for the description of the Gymnosperms. The alternatives were either to insert a greatly compressed survey of the successive floras of the world at the end of Volume IV or to attempt a fuller and less technical treatment of the subject in a separate book. In choosing the latter course I am conscious that a further obligation is undertaken which it may not be possible to fulfil; but the risk is deliberately taken. Volume IV is in the press and will, it is hoped, be published before the end of 1917.

It is a pleasant duty to repeat my thanks to many friends who have helped me in various ways. Dr Kidston generously and without reserve allowed me access to his splendid collection of Palaeozoic plants, and the frequent occurrence of his name in the list of illustrations shows how freely I have availed myself of his kindness. He has read some of the chapters and greatly assisted me by his friendly criticism and encouragement. By reading the proofs of this volume Dr Scott has further increased my already large debt to him. It is impossible to thank him adequately; he not only corrected many careless mistakes but by wise counsel and advice he rendered me a service which I greatly appreciate.

The exchange of views with Prof. Zeiller has been a constant source of profit and enjoyment, and it is hard to realise that the completed book will not receive his kindly criticism. He was a singularly unselfish and generous colleague, always ready to help fellow workers, and he had the faculty in an unusual degree of influencing those who had the privilege of his friendship by his sound advice and lovable personality.

The death of Count Solms-Laubach has deprived Palaeobotany of one of its most learned and strongest supporters. In common with all students of fossil plants I owe much to the critical treatment of the subject in the Einleitung in die Paläophytologie. Prof. Jeffrey has very kindly given me several photographs and sections which have been of great service, and I am similarly indebted to Prof. Zalessky. To Prof. Nathorst my thanks are due for the great interest he has taken in my work and for his generosity in providing drawings and showing to me many of the treasures in the famous Stockholm Museum. Prof. Bertrand and Prof. Lignier freely supplied photographs and drawings of specimens in their possession, and I am particularly grateful to them for the willingness with which they always responded to my requests.

Through the death of Prof. Lignier in March 1916 Palaeobotany has been deprived of another original thinker who devoted himself with whole-hearted enthusiasm to botanical research and for many years faithfully served the University of Caen: he was a generous friend to whom one never appealed in vain for assistance. Through the kindness of the Director of the Indian Geological Survey I have been able to examine several fossils from the Calcutta Museum described by Oldham and Morris and by Feistmantel. With the Director’s permission several photographs and drawings made for a forthcoming paper to be published by the Indian Survey are reproduced in this volume. I take this opportunity of thanking friends in Australia who recently afforded me facilities for examining fossil plants in their charge, and I would especially thank Mr A. B. Walkom of the University of Brisbane, who has recently undertaken an investigation of the rich plant-beds in the Ipswich district, for all that he did to enable me to make the most of a very short time available for palaeobotanical work.

For the loan of specimens and for other help I am indebted to Prof. Bayly Balfour, Prof. Bower, Prof. Margaret Benson, Prof. Oliver, Sir David Prain, Dr Smith Woodward, Prof. Weiss, the Director of the Geological Survey, Dr A. H. Church, Dr Arber, and other friends. I would also acknowledge a debt, by no means inconsiderable, to my Colleague Mr Hamshaw Thomas. Among younger friends in the Cambridge Botany School to whom I am indebted I wish particularly to thank Miss Ruth Holden, Miss Bancroft, Mr Sayers, Mr Dutt and others who have rendered me willing help.

In the List of Illustrations mention is made of Corporate Bodies and individuals from whom blocks have been obtained, and I am grateful to them for readily responding to my applications.

My Wife, though prevented by more urgent calls in the later stages of my task from giving as much time to the illustrations as in the two former volumes, has contributed several drawings, and my daughter Phyllis Seward has also given me much help in preparing drawings from previously published figures.

In spite of the vigilance and wise counsel of many friends numerous blemishes remain and for these the author is alone responsible.

A. C. SEWARD.

+Downing College Lodge+, February 10, 1917.

* * * * *

+Note.+ The letters A and B added to references in the footnotes indicate that the works will be found in the Bibliographies at the end of Volumes I and II.

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XXIX. PTERIDOSPERMEAE (continued from Volume II). Pp. 35–85

PAGE

I. =LYGINOPTERIDEAE= 35–85 =Lyginopteris= 35–70 Telangium 54, 55; Lagenostoma 55–64; Lagenospermum 64–66; Pterispermostrobus 66, 67.

=Heterangium= 70–85 Sphaerostoma 78–81.

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Chapter XXX

II. =MEDULLOSEAE= 86–165 i. Medullosa 86–109, 156–163 Myeloxylon 106–110; Neuropteris 110–117; Alethopteris and Linopteris 113; Trigonocarpus 117–124; Codonotheca 124–126; Schützia 126–128; Whittleseya 128–131; Dolerophyllum 132–139; Ottokaria 139–141; Strobilites 141, 142. ii. =Colpoxylon= 142–145 iii. =Rhexoxylon= 146–149 iv. =Sutcliffia= 149–156 General considerations suggested by the anatomical features of Medullosa 156–163

II. =A. STELOXYLEAE= 163–165 =Steloxylon= 163–165

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XXXI. PTERIDOSPERMEAE. (Foliage and seeds.) Pp. 166–174

Pecopteris Pluckeneti 166–168; Eremopteris artemisaefolia 169–172; =Wardia= 172, 173; Adiantites bellidulus and Lagenospermum Arberi 173, 174.

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XXXII. CYCADOFILICES. Pp. 175–213

I. =MEGALOXYLEAE= 175–180 =Megaloxylon= 175–180

II. =RHETINANGIEAE= 181–183 =Rhetinangium= 181–183

III. =STENOMYELEAE= 183–185 =Stenomyelon= 183–185

IV. =CYCADOXYLEAE= 185–190 =Cycadoxylon= 185–187 =Ptychoxylon= 187–190

V. =CALAMOPITYEAE= 190–200 =Calamopitys= 190–196 Kalymma 194–196 =Eristophyton= 197–200

VI. =CLADOXYLEAE= 200–210 =Cladoxylon= 201–207 =Völkelia= 208–210

VII. =PROTOPITYEAE= 210–213 =Protopitys= 210–213

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XXXIII. CORDAITALES. Pp. 214–299

A. =POROXYLEAE= 214–219 =Poroxylon= 214–219

B. =CORDAITEAE= 219–284 =Cordaites= 219–265 Noeggerathiopsis 238–243; Rhiptozamites 244, 245; Phylladoderma 244; Euryphyllum 245; Artisia 246–248; Dadoxylon 248–260; Metacordaites 260, 261; Cordaianthus 264–270; =Mesoxylon= 270–275; =Pelourdea= 277–281; =Niponophyllum= 282, 283; =Titanophyllum= 283, 284.

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Chapter XXXIV

C. =PITYEAE= 285–299 =Pitys= 285–289 =Archaeopitys= 290 =Callixylon= 291–293 =Coenoxylon= 293, 294 =Parapitys= 294, 295 =Mesopitys= 295, 296 =Antarcticoxylon= 296–299

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XXXV. PALAEOZOIC GYMNOSPERMOUS SEEDS. Pp. 300–365

I. +Lagenostomales+ 308–318 =Physostoma= 309–312; =Conostoma= 313–316; =Sphaerostoma= 316; =Lagenostoma= 316; =Gnetopsis= 317, 318.

II. +Trigonocarpales+ 318–332 =Trigonocarpus= 319–321; =Tripterospermum= 321; =Ptychotesta= 321; =Hexapterospermum= 321–323; =Polypterospermum= 323; =Pachytesta= 323–326; =Stephanospermum= 326–329; =Polylophospermum= 329, 330; =Codonospermum= 330, 331; =Aetheotesta= 331, 332; =Eriotesta= 332; =Gaudrya= 332.

III. +Cardiocarpales+ 332–356 =Cardiocarpus= 338–340; =Cyclospermum= 340, 341; =Cycadinocarpus= 341; =Rhabdocarpus= and =Rhabdospermum= 341–345; =Mitrospermum= 345, 346; =Diplotesta= 346; =Leptocaryon= 346, 347; =Taxospermum= 347; =Compsotesta= 347, 348; =Samaropsis= 348–354; =Cordaicarpus= 354–356.

IV. +Miscellaneous Seeds+ 356–365 i. =Hexagonocarpus= 356,357; =Decagonocarpus= 357; =Polypterocarpus= 357, 358; =Rhynchogonium= 358–360; =Boroviczia= 360; =Diplopterotesta= 360, 361; =Musocarpus= 361; =Holcospermum= 361–363.

ii. =Malacotesta= 363; =Thysanotesta= 364; =Carpolithus= 364, 365; Microspermum 365.

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XXXVI. CYCADOPHYTA (+Fossil+). Pp. 366–420

=BENNETTITALES= 367–420 =Cycadeoidea= 367–417 =Cycadella= 417, 418 =Amphibennettites= 418 =Vectia= 419, 420

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Chapter XXXVII

=BENNETTITALES= (continued) 421–477 =Williamsonia= 421–463 =Wielandiella= 463–467 =Williamsoniella= 467–473 =Cycadocephalus= 473–475 =Weltrichia= 475–477

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XXXVIII. CYCADOPHYTA. Pp. 478–506

i. =Bucklandia= 480–490; =Cycadeomyelon= 490, 491; =Colymbetes= 491–494; =Cycadeolepis= 494–496.

ii. =Carpolithus= 497–500; =Cycadospadix= 500, 501; =Beania= 502; =Zamiostrobus=, =Cycadeostrobus= 503–505; =Androstrobus= 505, 506.

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XXXIX. CYCADOPHYTA (+Fronds+). Pp. 507–591

I. +Bennettitales+ 512–566 =Ptilophyllum= 512–529; =Zamites= 529–537; =Otozamites= 537–545; =Dictyozamites= 546–548; =Pterophyllum= 548–558; =Cycadites= and =Pseudocycas= 558–566.

II. +Nilssoniales+ 566–587 =Nilssonia= 566–578; =Ctenis= 578–583; =Pseudoctenis= 584–587.

III. +Incertae sedis+ 587–591 =Sphenozamites= 587–589; =Plagiozamites= 589, 590; =Cycadorachis= 590, 591.

+List of Works referred to in the Text+ (Vols. III & IV). 592–639

+Index+ 640–656

LIST OF ILLUSTRATIONS

Several of the illustrations are printed from blocks for which I am indebted to learned societies or to individuals. The sources from which clichés were obtained are mentioned within square brackets. The names of donors of photographs or drawings are added after the descriptions of the figures.

+Frontispiece.+ Charles René Zeiller. From a photograph given to me by Madame Zeiller.

FIG. PAGE

377. Cycas circinalis. A. Malins Smith 2

378. Cycas revoluta. [G. R. Wieland.] 3

379. Encephalartos horridus 3

380. Cycas circinalis. [Encyclopaedia Britannica.] 4

381. Cycas revoluta and Zamia Loddigesii 5

382. Encephalartos Ghellinckii. [Camb. Phil. Soc.] 11

383. Cycas siamensis. [Encyclopaedia Britannica.] 12

384. Cycas circinalis 13

385. Cycas Micholitzii and Zamia angustifolia 14

386. Encephalartos Altensteinii; Dioon edule. S. M. Wadham. 15

387. Cycadean Fronds 16

388. Zamia Wallisii 17

389. Zamia integrifolia. [Camb. Univ. Press.] 18

390. Zamia Loddigesii 18

391. Bowenia spectabilis. [Encyclopaedia Britannica.] 20

392. Cycas pectinata, C. Riuminiana, C. angulata, Encephalartos, and Stangeria 21

393. Stangeria, Encephalartos, and Ceratozamia 22

394. Macrozamia Preissii. [Camb. Univ. Press.] 23

395. Zamia floridana. [G. R. Wieland.] 24

396. Dioon edule, Cycas circinalis, etc., Encephalartos, Bowenia, Macrozamia, Microcycas 26

397. Stangeria paradoxa. [New Phytologist; A. G. Tansley.] 29

398. Macrozamia 29

399. Cycadean foliar bundles 31

400. Cycas Micholitzii. [Camb. Phil. Soc.] 33

401. Lyginodendron Landsburgii. R. Kidston 37

402. Lyginopteris oldhamia. [Camb. Phil Soc.] 39

403–405. Lyginopteris oldhamia 41, 44, 48

406. Lyginopteris oldhamia. F. W. Oliver 49

407. Crossotheca Hoeninghausi. R. Kidston and other sources 53

408. Lagenospermum Sinclairi; Lagenostoma; Calymmatotheca Stangeri; Crossotheca Hoeninghausi 57

409. Lagenostoma 58

410. Lyginopteris oldhamia 69

411, 412. Heterangium Grievii 73, 76

413. Sphenopteris elegans 78

414. Sphaerostoma ovale 79

415. Heterangium Grievii; H. tiliaeoides; Lyginopteris root 82

416. Medullosa (stem-anatomy) 92

417. Medullosa centrofilis 96

418. Myeloxylon radiatum; Medullosa stellata; M. Leuckarti 103

419. Medullosa Leuckarti 104

420. Myeloxylon radiatum; Medullosa anglica, and Medullosa sp. 107

421. Potoniea adiantiformis; Linopteris obliqua; Neuropteris heterophylla 111

422. Neuropterocarpus Kidstoni. [Edinburgh Royal Society.] 114

423–426. Trigonocarpus Parkinsoni 118, 120, 121

427. Codonotheca caduca. [New Phytologist.] 125

428. Whittleseya elegans; W. brevifolia 128

429. Whittleseya elegans 130

430–432. Dolerophyllum Goepperti 133, 134, 137

433. Ottokaria bengalensis. [Indian Geol. Survey.] 140

434. Strobilites Milleryensis 141

435, 436. Colpoxylon aeduense 143

437, 438. Rhexoxylon africanum 146, 148

439, 440. Sutcliffia insignis 150, 151

441. Steloxylon Ludwigii 163

442. Pecopteris Pluckeneti 167

443. Eremopteris artemisaefolia; Samaropsis acuta 170

444. Samaropsis acuta. R. Kidston 171

445. Wardia fertilis 172

446–448. Megaloxylon Scotti. [Camb. Phil. Soc.] 176–178

449, 450. Rhetinangium Arberi. W. T. Gordon 182

451. Stenomyelon tuedianum 183

452. Stenomyelon tuedianum. R. Kidston 185

453. Ptychoxylon Levyi; Cycadoxylon Fremyi; C. robustum 188

454, 455. Calamopitys Saturni. M. Zalessky 191, 192

456. Kalymma grandis 195

457. Eristophyton fasciculare 197

458. Eristophyton Beinertianum. M. Zalessky 199

459. Cladoxylon mirabile; C. dubium; C. taeniatum; Syncardia pusilla 202

460. Cladoxylon Kidstoni; C. mirabile 206

461. Völkelia refracta 209

462. Protopitys Buchiana 211

463. Poroxylon Edwardsii. C. E. Bertrand and F. W. Oliver 216

464. Poroxylon Edwardsii; P. Boysseti. C. E. Bertrand 218

465. Cordaites leaf. (C. Felicis.) R. Kidston 228

466. Cordaites principalis; Artisia transversa. R. Kidston 231

467. Cordaites lingulatus 234

468. Cordaites (Rhizo-Cordaites); Cordaites circularis; Cordaites sp. cf. C. Lacoei; C. crassifolius 237

469. Cordaites aequalis; C. Clerci. M. Zalessky 239

470. Cordaites (Noeggerathiopsis) Hislopi. G. R. Wieland. 241

471. Cordaites (Noeggerathiopsis) Hislopi. [Geological Society.] 241

472. Cordaites (Noeggerathiopsis) Hislopi. [Geol. Surv. India.] 242

473. Cordaites stem 246

474, 475. Dadoxylon materiarum 257

476. Dadoxylon Pedroi 259

477. Cordaites root 262

478. Cordaites? root. T. N. Leslie 263

479. Cordaianthus. F. E. Weiss 265

480. Cordaianthus Pitcairniae; C. Volkmanni 266

481. Cordaianthus Grand’Euryi; C. Williamsoni; C. Zeilleri. C. E. Bertrand 268

482. Cordaianthus microsporangia 270

483. Mesoxylon Sutcliffii 272

484. Pelourdea vogesiaca. [Geologists’ Assoc.] 279

485. Titanophyllum Grand’Euryi 284

486, 487. Pitys antiqua 286, 287

488. Pitys primaeva 288

489. Callixylon Trifilievi. M. Zalessky 292

490. Mesopitys Tchihatcheffi 296

491. Antarcticoxylon Priestleyi 298

492. Diagram of a synthetic type of seed 303

493. Lagenostoma ovoides; L. Lomaxi; Physostoma elegans; Telangium Scotti; Stephanospermum akenioides 311

494. Stephanospermum akenioides; Conostoma oblonga; Gnetopsis elliptica; Physostoma elegans; Mitrospermum compressum 314

495. Polypterospermum Renaultii; Ptychotesta tenuis; Polylophospermum stephanense; Hexapterospermum stenopterum; Diplotesta avellana; D. Grand’Euryana; Taxospermum Grüneri 322

496. Pachytesta incrassata; Polypterocarpus anglicus 324

497. Pachytesta 325

498. Codonospermum olivaeforme; Aetheotesta elliptica 330

499. Samaropsis emarginata 337

500. Cardiocarpus drupaceus var. expansus 339

501. Cardiocarpus sclerotesta; Rhabdocarpus cyclocaryon 340

502. Samaropsis fluitans; S. emarginata; S. bicaudata; _S. Seixasi_; S. barcellosa; S. Newberryi; _Cordaicarpus Cordai_ 350

503. Samaropsis Leslii 352

504. Samaropsis Milleri. [Geol. Surv. India.] 353

505. Cordaicarpus Cordai 355

506. Holcospermum sulcatum; Codonospermum anomalum; Diplopterotesta spitzbergensis; Gnetopsis elliptica; Thysanotesta sagittula; Rhynchogonium costatum; Boroviczia Karpinskii; Hexagonocarpus Noeggerathi; Rhabdospermum tunicatum 359

507. Cycadeoidea marylandica. [G. R. Wieland.] 373

508, 509. Cycadeoidea Wielandi [G. R. Wieland.] 375, 376

510. Cycadeoidea sp. 377

511. Cycadeoidea Gibsoniana 377

512. Cycadeoidea ingens. [G. R. Wieland.] 378

513–515. Cycadeoidea. [G. R. Wieland.] 380–383

516. Cycadeoidea Dartoni 384

517–523. Cycadeoidea Gibsoniana 387–394

524–527. Cycadeoidea (Bennettites) Morierei. [O. Lignier.] 397–402

528–532. Cycadeoidea dacotensis. [G. R. Wieland.] 404–408

533. Cycadeoidea colossalis. [G. R. Wieland.] 410

534. Cycadeoidea Reichenbachiana. [A. Rothpletz.] 411

535–537. Cycadeoidea gigantea. [Geological Soc.] 413, 414

538. Cycadeoidea micromyela 415

539. Cycadeoidea Jenneyana (?) [G. R. Wieland.] 416

540. Vectia luccombensis. [Trustees of the British Museum.] 419

541–543. Williamsonia gigas 424–427

544. Williamsonia gigas. [Yorkshire Nat. Union.] 428

545. Williamsonia gigas. A. G. Nathorst. 429

546, 547. Williamsonia gigas 430, 431

548. Williamsonia gigas. O. Lignier. 432

549, 550. Williamsonia gigas. [Cambridge Phil. Soc.] 435, 436

551. Williamsonia spectabilis. A. G. Nathorst. 437

552. Williamsonia spectabilis. [Geological Soc.] 438

553, 554. Williamsonia Leckenbyi. A. G. Nathorst. 439, 440

555, 556. Williamsonia whitbiensis. A. G. Nathorst. 441

557. Williamsonia sp. [Indian Geol. Survey.] 444

558. Williamsonia Blandfordi. [Indian Geol. Survey.] 446

559. Williamsonia Carruthersi 447

560. Williamsonia Bucklandi 449

561. Williamsonia scotica 450

562–564. Williamsonia scotica. [Royal Society.] 451–453

565. Williamsonia spectabilis; W. whitbiensis; W. mexicana 459

566, 567. Wielandiella angustifolia 463, 464

568. Wielandiella angustifolia. A. G. Nathorst. 466

569–573. Williamsoniella coronata. [Royal Society.] 468–471

574. Cycadocephalus Sewardi. A. G. Nathorst. 474

575. Bucklandia anomala 482

576. Bucklandia Milleriana 484

577. Bucklandia Yatesii 485

578. Bucklandia buzzardensis. [Trustees of the British Museum.] 486

579. Bucklandia indica. [Trustees of the British Museum.] 488

580, 581. Colymbetes Edwardsi 492, 493 [Trustees of the British Museum.]

582, 583. Cycadolepis sp. 495

584, 585. Carpolithus conicus 498 [Trustees of the British Museum.]

586. Beania gracilis. [W. G. Carruthers.] 502

587. Ptilophyllum pecten. [Trustees of the British Museum.] 513

588, 589. Ptilophyllum pecten 514, 516 [Geological Surv. India.]

590. Ptilophyllum pecten and Williamsonia Blandfordi 517 [Geological Surv. India.]

591. Ptilophyllum cutchense. [Geological Society.] 518

592. Ptilophyllum pecten 519

593. Ptilophyllum pecten. [Geological Society.] 519

594. Ptilophyllum pecten 520

595. Ptilophyllum pecten 523

596. Ptilophyllum pecten. [Trustees of the British Museum.] 523

597. Ptilophyllum boreale. A. G. Nathorst. 526

598. Ptilophyllum antarcticum; P. Anderssoni 527

599. Zamites gigas. [Encyclopaedia Britannica.] 533

600. Zamites recta 534

601. Zamites Buchianus; Z. Carruthersi; Z. Zitteli 535

602. Zamites Buchianus 536

603. Otozamites Goeppertianus; O. Bechei 538

604. Otozamites Bechei 539

605. Otozamites Bechei 541

606. Otozamites Beani; O. Bunburyanus [Encyclopaedia Britannica.] 542

607. Otozamites bengalensis. [Geological Surv. India.] 542

608. Otozamites Kliptsteini. [Geological Society.] 545

609. Dictyozamites Hawelli 548

610. Pterophyllum Jaegeri 549

611. Pterophyllum Nilssoni; Nilssonia mediana 551

612. Pterophyllum Fayoli 552

613. Pterophyllum Braunianum 554

614. Pterophyllum Nathorsti; Nilssonia tenuicaulis 556 [Royal Society of Scotland.]

615. Pterophyllum Nilssoni [Trustees of the British Museum.] 557

616. Pterophyllum Lyellianum [Trustees of the British Museum.] 557

617. Pseudocycas insignis 562

618. Pseudocycas Saportae. [Encyclopaedia Britannica.] 564

619. Nilssonia pterophylloides; N. brevis; N. polymorpha 567

620. Nilssonia brevis; N. saighensis 570

621. Nilssonia rajmahalensis. [Geological Surv. India.] 571

622. Nilssonia compta. [Trustees of the British Museum.] 574

623. Nilssonia princeps. [Geological Surv. India.] 577

624, 625. Ctenis sp. [Manchester Phil. Lit. Society.] 579

626. Ctenis sulcicaulis 583

627. Pseudoctenis eathiensis [Trustees of the British Museum.] 584

628. Sphenozamites Rochei 588

629. Sphenozamites Belli. [Trustees of the British Museum.] 588

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Chapter XXVIII

CYCADALES (+recent+).

Among the fossil genera described in the last chapter of the second volume some were spoken of as true Ferns though most of them, it was added, ‘may safely be regarded as plants which will ultimately be shown to belong to some other group, in most cases that of the Pteridosperms.’ Since this was written additional evidence has been obtained in favour of the inclusion of certain genera in the Pteridosperms. In the case of Taeniopteris, one of the genera already described, there is reason to believe that at least one species is a member of the Cycadales and not a true Fern as formerly supposed.

The Pteridosperms so far described are represented for the most part by sterile leaves preserved as impressions, the genera founded on more satisfactory material having been reserved for treatment in this volume. As these genera are founded to a large extent on anatomical characters oscillating in their essential features between recent Ferns and Cycads, it is important that the student should be in possession of the anatomical characteristics of both of these classes; and for this reason a general account of recent Cycads is intercalated between the Pteridosperms already described and those reserved for treatment in this volume.

* * * * *

The section of the Gymnosperms known as the Cycadales, represented by nine recent genera and less than 100 species, is of exceptional importance phylogenetically and demands special attention from palaeobotanical students. Familiarity with the morphology of recent forms is essential not only in relation to extinct cycadean plants but also to types which, though not sufficiently close to surviving species to be included with them in one class, exhibit features regarded by many botanists as indications of an affinity either to true Cycads or to some generalised stock of which they are an offshoot. The Cycads of to-day may fairly be spoken of as anachronisms, plants appropriate to a former age but out of harmony with the present. They are confined to tropical and sub-tropical regions in both the old and new world. In habit many of them resemble tree-ferns, but the columnar stem, which may live to a great age and attain a height of 20 metres, differs from that of ferns in its gradually tapered form consequent on the presence of one or more cambial cylinders. Though often unbranched (fig. 377) branching of the main trunk is by no means unusual (fig. 378; fig. 381, B). Many Cycads are geophilous and have short tuberous stems (figs. 383, 395, 1_a_; 396, E): the genus Zamia includes a few epiphytic forms[1]. The typical cycadean stem is covered with persistent petiole-bases with or without an admixture of smaller scale-leaf bases (figs. 379, 380), while in several species a transversely wrinkled or irregularly fissured periderm forms the superficial tissue (figs. 381, B; 383). The foliage-leaves are relatively large and, with the exception of the bipinnate fronds of Bowenia (fig. 391), they are always pinnate. The fronds usually form a terminal crown (figs. 377, 379) and as many as 100 may be produced from one bud. In Zamia pygmaea[2] the fronds are only 10–12 cm. long, but in some cycads they reach a length of several metres. On both young foliage-leaves and scale-leaves long and very rarely branched[3] unicellular hairs (fig. 396, N) form a characteristic feature and take the place of the ramental scales of the majority of ferns. The apex of the stem shown in fig. 386, A is covered with a mass of woolly hairs and several scale-leaves are seen on the lower part of the bud.

All recent Cycads are dioecious. The reproductive shoots, except the megasporophylls of Cycas—which have departed to a less extent than those of other genera from the foliage-leaf plan (fig. 381, A; fig. 392, A–C) and are borne in a terminal cluster through which the stem subsequently pushes its way—consist of a varying number of micro- or mega-sporophylls in dense spirals on the axis of an elongated or oval strobilus (figs. 386, B, 393, 394). The microsporophylls are occasionally verticillate[4]. The strobili are sometimes though rarely branched[5] and generally but by no means invariably[6] terminal on the main stem which branches sympodially[7]. A striking example of lateral strobili has recently been described by Chamberlain[8] who figures a stem of Macrozamia Moorei with fertile shoots wedged among the persistent petiole-bases, a condition very similar to that in the Mesozoic Bennettitales. Pearson has also described clear cases of laterally-borne cones in Encephalartos. Cycas exhibits two kinds of branching, the female plants being monopodial while in the male the branching is sympodial. The microspores are produced in sporangia grouped in more or less well defined sori (figs. 389, A; 392, E–G). There is no definite annulus, but in the occurrence of groups of thick-walled cells some microsporangia recall those of certain ferns[9]. The ovules vary considerably in size, sometimes exceeding 5 cm. in diameter: there are usually two on each megasporophyll (figs. 393, C; 394; 395, 1_d_) but in most species of Cycas (fig. 392, B) and occasionally in other genera the number is larger[10]. A thick integument encloses the nucellus with which it is fused except in the apical region (fig. 396, A, B). Below the comparatively long micropylar tube is a well-developed pollen-chamber (fig. 396, B′, p), a striking feature of Cycadean ovules, immediately above the megaspore; the latter is filled with prothallus-tissue and bears a small apical group of archegonia on the floor of a depression (fig. 396, A–B′). In Microcycas[11] as many as 200 archegonia are recorded—a very exceptional case—and these are not confined to the apical region, though only the apical archegonia are functional. Each archegonium is characterised by a very large oval egg-cell and a much reduced neck[12]. The microspores usually produce a single prothallus-cell, a stalk-cell, and body-cell, and from the body-cell are developed two spirally ciliated spermatozoids (fig. 396, M). In this respect also the monotypic genus Microcycas is peculiar: it may have as many as 8 body-cells and 16 male gametes in a single pollen-tube (fig. 396, G), while in Ceratozamia[13] 4 gametes have been seen in one tube. The pollen-tube grows like a fungal mycelium into the nucellar tissue and the male gametes are formed in the distended proximal end which on bursting liberates the motile sperms with the watery cell-sap. Fertilisation is succeeded by the development of a homogeneous proembryo partially or completely filling the zygote (fertilised egg): by the formation of long suspensors the embryo is brought into contact with the food-store of the prothallus. In some Cycads, e.g. Encephalartos, the embryogeny exhibits a close resemblance to that of Ginkgo[14]. The embryo is dicotyledonous[15].

The single stele of the stem is characterised by a large pith which in some genera (e.g. Encephalartos, Macrozamia) contains an anastomosing system of collateral bundles. The vascular tissue of a cycadean stem forms a cylinder of secondary xylem and phloem, the primary xylem being represented only by a few, usually crushed, protoxylem elements on the inner margin of the reticulately pitted or scalariform tracheids. Both xylem and phloem are traversed by numerous broad and deep medullary rays[16]. The looser texture and more parenchymatous structure of Cycadean wood afford a ready means of distinguishing it from the wood of Conifers: for the Cycadean type the term manoxylic is proposed and pycnoxylic for the more compact coniferous wood[17]. Rims (or ‘bars’) of Sanio, of which much has been said in discussions on the phylogeny of Conifers, have recently been described in the petiolar xylem of Cycas revoluta: the rims are short and ‘cling closely to the borders of the pits,’ features which also characterise the rims found in the cones of the Araucarineae and in the root- and cone-wood of certain Pines[18]. In some Cycads the secondary xylem and phloem form a single cylinder, but in others (Cycas, Encephalartos, Macrozamia, Bowenia) the cambium is succeeded by one or several concentric cylinders of meristem which have their origin in the pericycle. The spasmodic occurrence of separate arcs of inversely orientated secondary xylem and phloem between the normal cylinders is a feature of importance from the point of view of comparison with the Palaeozoic Medulloseae[19]. The occurrence of concentric cauline strands in the cortex of Cycas is also a peculiarity worthy of notice. Successive bands of periderm, and occasionally a considerable amount of phelloderm[20], are formed in the peripheral region of the stem.

The leaf-traces in an adult stem exhibit a striking feature in their indirect or girdle-like course to the leaves (fig. 396, H, g) and in the gradual change from an endarch (fig. 396, O) to an apparently mesarch structure (fig. 400) as they pass from the perimedullary zone to the petiole: except at the base of the petiole the vascular bundles of the frond-axis consist of (i) centripetally developed xylem with a median protoxylem and a much smaller amount of centrifugal xylem (fig. 400) separated by a few parenchymatous elements from the centripetal xylem, (ii) an external arc of protophloem and within this metaphloem and parenchyma[21]. In the slender petiole of Bowenia there are a few collateral bundles arranged in the form of a circle or ellipse[22]; in Cycas and some other genera the more numerous bundles form a pattern like an inverted U, and in some species of Encephalartos the number is greater and the strands more irregularly scattered[23]. In the vegetative stems there is no centripetal xylem in the stele, but scattered centripetal tracheids occasionally occur internal to the protoxylem in the steles of the peduncles[24].

* * * * *

=Cycadeae.= Megasporophylls each bearing 2–8 ovules, borne separately like foliage-leaves and not in strobili. Pinnae have a midrib but no lateral veins (figs. 384, 387, A). Cycas (fig. 377).

=Zamieae.= Both kinds of sporophylls form strobili. Pinnae have several dichotomously branched, more or less parallel veins. Zamia (figs. 388–390), Macrozamia, Encephalartos (figs. 379, 386, C), Ceratozamia, Dioon (fig. 386, B), Microcycas.

=Stangerieae.= Strobili as in Zamieae. Pinnae fern-like, numerous dichotomously branched lateral veins given off from a midrib. Stangeria.

=Bowenieae.= Leaves bipinnate (fig. 391), strobili as in Zamieae. Bowenia.

Distribution. The most widely spread genus, Cycas, occurs in Siam, India, the Nicobar Islands, Ceylon, Madagascar, and Australia, in many of the islands in the Indian and Pacific oceans, in New Guinea, Borneo, New Caledonia, New Britain, China and Japan[25]. Zamia, the most northerly genus, extends from North Mexico and Florida through Central America and some of the West Indian islands to Ecuador, Bolivia, Chile, and Peru. Dioon and Ceratozamia are confined to South Mexico, and Microcycas flourishes on the Cuban mountains. The continent of Africa possesses two endemic genera Encephalartos and Stangeria. Encephalartos extends from Cape Colony through Natal and Zululand to Zanzibar and Mombasa[26]: a specimen in the Kew Herbarium (probably E. Hildebrandti) is said to have been collected as far north as the Soudan. Two species are recorded from the Congo[27] and E. Barteri, discovered by Barter in Central Africa, is recorded from the Gold Coast[28]. Stangeria has a much more limited range in S.E. Africa[29]; Australia possesses Macrozamia, represented by several species in Western Australia, New South Wales and Queensland, Cycas in Queensland and the Northern territory and the Queensland genus Bowenia. There are no Cycads in New Zealand. As a whole Cycads have a limited range and with the exception of Cycas and Zamia none of them extend beyond the limits of a single continent. They are as a rule not gregarious plants and play a subordinate part in the facies of the vegetation. Macrozamia forms dense thickets[30] in some districts and occurs both in exposed situations and in association with Palms in damp Queensland forests. Chamberlain[31] speaks of 100 plants of Dioon edule as visible in one view in South Mexico where the species forms a mountain forest. In Florida Zamia pumila[32] grows in dense moist woods, a habitat in contrast to that of many Cycads. The Mexican Ceratozamia is associated with luxuriant vegetation, while its compatriot Dioon[33] lives in blazing sunshine. Sir Joseph Hooker[34] speaks of Cycas living in the deepest and hottest valleys in Sikkim. Encephalartos is essentially a xerophilous genus. Stangeria paradoxa is said to be confined to forests in Cape Colony, and another species grows among the grass of the Park-lands in open country[35]. While it is true that many Cycads are characteristic of dry regions some species flourish in places where shade and moisture are abundant.

Though it is impossible in many cases to form an estimate of the age of individual plants, there are clear indications that some specimens afford notable instances of longevity. Chamberlain estimates the age of some plants of Dioon spinulosum as exceeding 400 years and mentions an example of D. edule that is probably 1000 years old. An unusually tall plant of Encephalartos in the Botanic Garden of Amsterdam is believed by Prof. de Vries to have reached the venerable age of 2000 years[36]. The restricted range and in many cases the solitary existence of recent Cycads, with their tall stems clothed with the persistent cork-covered stumps of thousands of fronds, deepens the impression of antiquity derived from a study of the geological history of this dwindling race.

Stems. The tall columnar stems of some species of Cycas, often branched or bearing numerous ovoid buds like enlarged bulbils[37], are characterised by the regular alternation of large and small leaf-bases as seen in the stem of C. circinalis reproduced in fig. 380. In older stems of this species the leaf-bases are exfoliated and the stem is covered with wrinkled and fissured cork; but in Cycas revoluta the leaf-bases are even more persistent. The columnar but relatively stout stems of Encephalartos (figs. 379, 382, 386, A) and Ceratozamia are similarly encased in a covering of petiole-bases, but in these genera the differences between foliage-leaves and bud-scales is much less obvious and there is no zonal alternation. On the stems of Macrozamia the rhomboidal leaf-bases are more uniform in size and there are no scale-leaves. The tall and often palm-like stems of Microcycas sometimes show transverse rings on the bark marking the position of former terminal buds, and in older trunks these may disappear, leaving a fissured bark[38]. In Cycas siamensis the tuberous stem is similarly covered with a rough bark (fig. 383) and the stems of Zamia are also characterised by an absence of persistent leaf-bases (figs. 381, B; 395, I_a_, a). It is pertinent to remind the palaeobotanical student of the occurrence of flowering plants with stems closely simulating those of some Cycads. Prof. Bower[39] in describing Rhynchopetalum montanum, an Abyssinian Lobeliaceous plant, drew attention to the similarity in surface-features and to some extent in anatomical structure to cycadean stems. The resemblances are further emphasised in a more recently published account of the same species under a different name, Lobelia Rhynchopetalum[40].

Fronds. A general acquaintance with the various types of fronds illustrated by recent Cycads is important to the student of fossils not only to enable him to compare existing and extinct forms but as affording safeguards against possible sources of error in the description and identification of impressions[41]. The vernation exhibits less uniformity than in Ferns: in Cycas the rachis is straight and the pinnae circinately coiled (fig. 220, B, vol. +ii.+ p. 283); in Zamia and Stangeria the rachis is bent and the pinnae straight, while in Ceratozamia and other genera both the axis and leaflets are straight. As Braun pointed out, there is as a rule no terminal leaflet, or it may be pushed to one side giving a forked appearance to the frond apex[42].

Cycas. The presence of a strong midrib and the absence of lateral veins are distinguishing features: the lower margin of the lamina is frequently decurrent (fig. 387, A). In C. circinalis the pinnae may reach 40 cm. in length with a fairly uniform breadth of 2 cm. A frond of this species in the British Museum, not quite complete, has a length of 112 cm.: on the lower part of the rachis strong spines replace the leaflets and near the apex of the leaf concrescent pinnae form a continuous lamina traversed by seven ribs and dissected at the margin into acuminate teeth (fig. 384): some of the pinnae are forked as in Cycas Micholitzii[43] (fig. 385). Several years ago I noticed a similar instance of concrescence in a small plant of C. circinalis in the Royal Gardens, Kew (fig. 387, I). In Cycas Micholitzii the pinnae, reaching a length of 20 cm., are repeatedly and deeply forked (fig. 385, A, B; fig. 400): the pinnae of C. Rumphii var. bifida[44] are also deeply dissected. Cycas Beddomei has very narrow pinnae (15 cm. × 2 mm.) similar to those of the Wealden species Cycadites Saportae, and it is noteworthy that narrow leaflets with a strongly revolute lamina would produce casts with two parallel ribs (the grooves between the midrib and the edge of the lamina) simulating the double midrib of the fossil genus Pseudocycas. In some fronds, e.g. C. Cairnsiana, the midrib is hardly visible on the upper face of a dried pinna which shows a longitudinal wrinkling simulating parallel venation.

Encephalartos. The fronds of this genus, in Encephalartos Laurentianus[45] reaching the exceptional length of 7 metres, bear alternate pinnae exhibiting a considerable range in form and breadth. In E. longifolius, E. Altensteinii (fig. 386, C), E. Lehmanni, etc., the pinnae are for the most part linear, reaching a length of 20 cm. and a breadth of 2 cm.: in E. caffer (fig. 387, D), E. latifolius, and others the pinnae are broader and shorter and often spinous. A frond of E. longifolius or E. Altensteinii may bear both entire and lobed, spinous pinnae. In E. Frederici-Guilielmi (fig. 387, G) and E. Ghellinckii[46] (fig. 382) the pinnae are very narrow and almost filiform, with revolute edges. The thick and leathery pinnae of some species are attached obliquely to the edge or to the upper sloping sides of the rachis which forms a prominent ridge between the rows of leaflets, and characteristic oval scars are left on the fall of the pinnae (fig. 387, D, G′). The lamina in most species contains several veins more or less parallel to the margins and often much more prominent on the lower than on the upper surface.

Zamia. In Zamia angustifolia (fig. 385, C) and Z. linifolia the pinnae are long and very narrow: the other extreme is represented by Z. Wallisii[47] (fig. 388) with broad ovate segments reaching a length of nearly ·5 metre and attached to the rachis by a short stalk; the veins are prominent and dichotomously branched. Other forms of pinnae are represented by Z. integrifolia, Z. floridana, and Z. Loddigesii (figs. 389, 390, 395). The broad and short pinnae of Z. furfuracea[48] bear a close resemblance, except in the absence of an auriculate base, to those of some species of the fossil genus Otozamites. The broadly linear pinnae of Z. pseudoparasitica (45 cm. × 3 cm.) often show longitudinal wrinklings on drying which suggest comparison with the corrugated lamina of the fossil species Nilssonia brevis. A basal pad or callosity on the slender bases of the pinnae is characteristic of many Zamia fronds.

Ceratozamia. The fronds bear a fairly close resemblance to those of Macrozamia: in Ceratozamia mexicana the linear pinnae reach a length of over 30 cm. and a breadth of 2–3 cm.; the lamina tapers to a narrow apex and is more abruptly contracted at the base (fig. 387, H). The veins in Ceratozamia are sub-parallel and dichotomy occurs up to the middle of the lamina[49]. A striking feature is the occurrence of two opposite stipule-like projections a short distance above the base of the petiole.

Macrozamia. A noteworthy feature in some species is the attachment of the linear pinnae along the middle line of the rachis (fig. 387, C); in others (fig. 387, B) the leaflets are attached laterally and may have a basal callosity. The parallel veins, which branch dichotomously near the base of the lamina, are often much more prominent on the lower than on the upper face. In M. heteromera[50] (fig. 396, F, F′) the narrow pinnae are deeply forked and strongly revolute. The spirally twisted rachis of M. spiralis, M. heteromera, etc., is a striking feature recalling the Rhaetic fern Camptopteris spiralis Nath[51].

Dioon. The arrangement of the linear pinnae of D. edule (fig. 386, B), D. spinulosum, and D. Purpusii[52] forms a ready means of distinguishing the fronds of this genus: the pinnae, often contiguous and at right-angles to the rachis, are attached in a lateral groove by an expanded and slightly decurrent base. The difference between the lower and upper face of a frond (fig. 387, E, F) affords a good illustration of a common source of error in the identification of fossil specimens. The leaflets of D. spinulosum, which except in their spinous margin are very similar to those of D. edule, may reach a length of 15 cm. and a breadth of 8 mm. The parallel veins are unbranched[53].

Microcycas[54]. The pinnae of this genus, very like those of the Wealden species Zamites Buchianus, reach a length of 20 cm. and a breadth of 8 mm.: on falling they leave oblong scars resembling those on the rachis of Encephalartos.

Stangeria. This genus is particularly interesting because of its fern-like habit and venation. The large fronds of S. paradoxa[55] bear broadly linear acuminate pinnae with entire, unevenly lobed, serrate, or pinnatifid margins. Some leaflets are so deeply dissected as almost to justify the appellation pinnate. Both entire and dissected leaflets may occur on one frond and the lower ones may be stalked while the upper pinnae are sessile. The venation agrees closely with that of the genus Taeniopteris[56].

Bowenia. The large fronds of this genus (fig. 391) are peculiar in being bipinnate; they may reach a length of 2 metres and have a long slender petiole: the asymmetrical lamina of the segments, entire or deeply serrate, is attached by a very short stalk; the veins branch dichotomously[57] and diverge slightly. Both entire and serrate pinnae may occur on the same plant, but Chamberlain has revived André’s specific term serrulata in preference to the generally adopted designation for the serrate forms, B. spectabilis var. serrata[58].

Reproductive shoots[59]. In Cycas circinalis, C. Rumphii, and other species the megasporophylls reach a considerable length and bear several lateral ovules each of which may be as large as a goose’s egg: the sterile distal end has the form of a spear-point with an irregularly serrate edge. In C. revoluta, C. pectinata, etc., the sterile part is deeply dissected and may break off (fig. 392, A) from the fertile portion of the sporophyll. The megasporophylls of C. Riuminiana exhibit a striking variation in form (fig. 392, B, C); some are 15 cm. long with several ovules, while others, reduced to 8 cm., bear only two ovules and resemble the sporophylls of Dioon. In all other genera the megasporophylls are aggregated into cones, but in Dioon the strobili are characterised by their more ovoid form and by the looser arrangement of the sporophylls (fig. 386, B), each of which consists of a horizontal stalk expanded distally into a broadly lanceolate upturned end covered with a thick felt of hairs and bearing at its base usually 2, rarely 5–6, ovules on cushion-like swellings. In Dioon spinulosum the cones may be 50 cm. long. Between the cones of Microcycas, over 90 cm. long, and those of some Zamias, a few centimetres long, there are many intermediate forms. The large strobilus of an Encephalartos reproduced in fig. 393, D, shows the convex ends of the sporophylls with a jagged edge, and in monstrous cones the marginal lobes may be abnormally developed and assume the appearance of pinnae[60]. Each megasporophyll bears two large ovules (fig. 393, B). In certain species of Encephalartos the swollen ends of the sporophylls have a truncate centre like the flattened umbo of some Pines (fig. 392, D). The presence of two divergent spines is a peculiarity of the megasporophylls of Ceratozamia (fig. 393, C): in Macrozamia (fig. 394) the distal ends are prolonged as tapered processes. The surface of the strobilus of Stangeria is formed by imbricate and rounded ends of sporophylls (fig. 392, H) not unlike the cone-scales of Pinus excelsa or P. cembra. The megasporophylls of Zamia are expanded into regular cushion-like hexagons with a flat central area (figs. 389, B; 395, I_b_).

The microsporophylls (figs. 389, A; 392, E) are in all genera aggregated into strobili which often bear a close resemblance to seed-cones (fig. 393, A). On a single sporophyll of Cycas circinalis there may be as many as 700 sporangia while in Zamia floridana there are only two microsporangia. The spore-output is large and in extreme cases, e.g. in Dioon spinulosum, the average number of spores in a sporangium is said to be 30,000[61].

Seeds. In the great majority of recent species the seeds may be described as large and afford a striking contrast to the small seeds of the Mesozoic Bennettitales. A feature of interest from the point of view of comparison with Palaeozoic seeds is the absence of a resting stage, germination in some cases following seed-fall without an interval. As Warming pointed out, the embryo is often undeveloped when the seeds are shed. An interesting fact is recorded by Capt. Dorrien-Smith[62] with regard to seed-dispersal: he describes the heavy pebble-like seeds of a Macrozamia as being hurled from the ripe cones a distance of 12 ft. The seeds of Cycas are platyspermic; the woody shell exposed on removal of the outer flesh is slightly flattened and has two prominent angles, but three-angled seeds may occur as in Ginkgo biloba (fig. 631, C). In other genera the seeds are radiospermic. The seed of Encephalartos Altensteinii[63] (fig. 396, D) has a square-cut distal end with a small papilla at the summit of the unusually long micropylar canal (17 mm.). The stone of this seed (fig. 396, C) shows parallel curved ridges which mark the position of vascular strands in the inner region of the outer flesh. The large ovules of Cycas circinalis[64] have an integument 1 cm. thick consisting of an outer and inner flesh and an intervening stony layer which reaches its greatest development at the base and apex. Three vascular strands enter the base of the seed, the concentric strand breaks up in the broad inner flesh into a group of bundles which embrace but do not penetrate the lower end of the nucellus. Each of the two lateral strands branches in the outer flesh near its entrance into the seed; the outer and larger collateral and mesarch bundle passes up close to the surface of the shell to the seed-apex, while the inner branch penetrates the shell and, occasionally branching, passes up the inner region of the inner flesh as far as the micropyle. In other seeds the tracheal supply of the outer flesh consists of several bundles and not two as in Cycas. The inner flesh abuts on the nucellus and is connected with it except at the apex (fig. 396, B). In ripe seeds the nucellus is reduced to a thin membrane enclosing the large megaspore at the upper end of which is a depression (fig. 396, B′) or sometimes two depressions (fig. 396, I) in the prothallus containing the archegonia. In the seed of Dioon edule[65] (fig. 396, A) the position of the absciss-layer (s) is indicated by a slight transverse constriction. In the seeds of Bowenia, constructed on the same plan, the inner series of vascular strands appears to be nucellar in position, thus differing from the strands in Dioon, Cycas, and other genera which are confined to the integument. Miss Kershaw[66] in describing Bowenia speaks of an upper and a lower pollen-chamber; the former serves as a storage-place for the microspores prior to their further development in the lower chamber. Dr Stopes[67] regards the integument as double in origin, a view suggested by Griffith[68] in 1835, and as homologous with the single integument plus the cupule of Lagenostoma. This view is supported by Mrs Thoday[69]: on the other hand Miss Kershaw’s investigation of Bowenia seeds leads her to regard the integument as single. Although there would seem to be a prima facie case in favour of the dual nature of the integument, the arguments on the other side have greater weight[70].

Recent observations point to the probability that insects play a part in the pollination of cycadean ovules. Kraus[71] drew attention to the strong smell emitted by the microstrobili of Dioon edule and noticed that small bees were attracted to the ripe strobili of Macrozamia, while odourless cones of a neighbouring Ceratozamia received no attention. Pearson[72] and Rattray[73] have obtained evidence that beetles and weevils act as pollinators to species of Encephalartos.

Anatomical features. Allusion has already been made to some of the more striking anatomical features; the large pith, the occasional occurrence of medullary vascular bundles, the presence of one or more cambiums, the large size of the medullary rays, etc. It is worthy of remark that the occurrence of an anastomosing system of medullary bundles is not a constant feature within a genus; in Macrozamia Fraseri such a system is present, but absent in M. Denisoni[74]. In the pith of stems with no medullary bundles cylinders of collateral bundles may occur in connexion with a fertile shoot. These bundles arise from the inner face of the main cylinder and pass upwards as a domical system into the base of the terminal strobilus which is eventually pushed to one side by the growth of a lateral bud[75]. The secondary xylem tracheids are usually provided with several rows of bordered pits on the radial walls and resemble those of the Araucarieae[76], but in Cycads the pits are often not contiguous and less compact in their distribution. The wood of Stangeria is peculiar in consisting of scalariform tracheids[77] (fig. 397). Chamberlain describes growth-rings in the wood of Dioon; but this is exceptional. In tangential sections of the stele leaf-trace bundles are constantly seen passing horizontally through the broad and deep medullary rays. The pith-cast of a cycadean stem reproduced in fig. 398 shows the wide meshes in the reticulum of tracheal tissue originally occupied by parenchyma, which on decay left lenticular depressions represented on the cast by tapered convex areas occasionally bearing the impress of an outgoing trace in the form of a narrow groove. The secondary phloem often rivals the xylem in breadth and is not always easily distinguishable from it; it consists of sieve-tubes, parenchyma, and fibres. The secondary cambial cylinders characteristic of Cycas, Encephalartos, Macrozamia, and Bowenia, to which reference was made in the summary of anatomical features, arise in the pericycle, and a few layers of pericyclic parenchyma occur between adjacent extrafascicular cylinders of xylem and phloem. In a stem of Cycas media 35 cm. in diameter examined by Worsdell there were 12 concentric cylinders. Matte[78] and Miss Dorety[79] have described partially flattened arcs of extrafascicular xylem and phloem in the hypocotyl of Ceratozamia mexicana. Worsdell[80] first drew attention to the occasional occurrence of short tracheids on the inner edge of the secondary wood and to the spasmodic development of cambial arcs in the tissue between the extrafascicular cylinders forming strands of inversely orientated xylem and phloem. More recent work by Matte gives support to Worsdell’s comparison between Medullosan stems and those of recent Cycads with inversely orientated arcs or concentric vascular cylinders. The French author draws attention to the close resemblance between the seedling stems of such species as Encephalartos Barteri (fig. 396, K) and Cycas siamensis (fig. 396, L) with their polystelic type of structure and the adult stems of Medullosa[81]. In the stems of Dioon, Microcycas, Stangeria, and Zamia no extrafascicular cylinders are recorded. Two main vascular bundles enter the cortex from each leaf-base and in most stems these diverge right and left and more or less completely encircle the stele before passing through the medullary rays and joining the inner portion of the xylem of the stele either as double or single bundles. These girdle-bundles (fig. 396, H) first described by Karsten and Mettenius form a very characteristic cycadean feature[82]. Adjacent girdles are joined by connecting cortical bundles and, in addition, there are cauline collateral bundles in the cortex which form an anastomosing system. In some cases, e.g. species of Macrozamia and occasionally in Stangeria, the female peduncle of a Ceratozamia, and in seedlings of Bowenia and Cycas revoluta[83], the leaf-traces pursue a direct course from petiole to stele as in stems of Bennettitales. It is noteworthy that in seedlings of Microcycas[84], a genus characterised by a large number of male gametes—presumably a primitive feature—the leaf-traces are of the girdle-type. The two bundles at the base of a petiole by repeated subdivision give rise to the numerous collateral strands of the rachis. A leaf-trace in its passage to the leaf is like that of a Conifer in having the protoxylem on its inner edge, whereas in the petiole and elsewhere in the frond it is characterised by an arrangement of the xylem that has usually been described as mesarch. A typical vascular bundle from a cycadean frond is seen in fig. 399, C; by far the greater part of the xylem is centripetal, the centrifugal xylem being confined to an arc of scattered tracheids or a small strand separated by a few parenchymatous cells from the protoxylem.

As considerable stress has been laid on the anatomical features of the cycadean foliar bundles in discussions on the affinities and phylogeny of certain Palaeozoic genera, it is important to consider the facts more closely[85]. French anatomists described the cycadean bundle as diploxylic on the ground that the centripetal and centrifugal xylems are distinctly different things, the centripetal xylem being primary—a relic of a former organisation—and the centrifugal xylem secondary and homologous with the normal wood of the cauline bundle. The term mesarch has in recent years been applied to the cycadean type of bundle. A mesarch bundle is, however, one in which centripetal and centrifugal xylem are alike in origin, both being primary structures derived from a desmogen strand. Typical mesarch bundles occur in several recent ferns; in the stele of the Osmundaceae, Gleichenia, and other genera; but in these plants the xylem is all produced directly from one primary desmogen region and there is no question of ‘primary’ and ‘secondary’ as in the two portions of the xylem of a cycadean bundle. Recent researches into the development of cycadean foliar bundles show that they do not conform to the mesarch type as generally understood. A leaf-trace at the base of a petiole (fig. 399, A) comprises centrifugal xylem only, and this consists of regular rows of tracheids separated by medullary rays: in the lower part of the petiole the structure is gradually modified, the centrifugal xylem is reduced and the formation of centripetal xylem is initiated. At a higher level (fig. 399, B) the centripetal xylem is in excess of the centrifugal and the latter, for a time connected with the former, eventually becomes separated by a few parenchymatous cells from the protoxylem and persists as a small strand or arc of tracheids. Fig. 399 illustrates stages in the transformation of a typical collateral bundle, at the base of a Stangeria petiole, into one in which the xylem is almost wholly centripetal at a higher level in the axis of the frond. A cambium is present in all: in B the centrifugal xylem is more or less clearly differentiated into two portions, loosely arranged tracheids near the phloem, and the more compact groups abutting on the centripetal xylem: figs. C–E show a further reduction in the centrifugal tracheids. The conclusion drawn from developmental study is that the two xylem portions of the bundle are independent in origin[86]. Marsh has, however, shown that in Stangeria bundles near the base of the petiole the centrifugal xylem consists of rows of secondary tracheids and an inner portion not in rows which connects the centrifugal with the centripetal elements; this connecting portion, he adds, is ‘probably primary and connects up the Cycadean foliar bundle with the truly mesarch bundle of the Cycadofilices.’

In the xylem portion of the bundle from the midrib of a forked pinna of Cycas Micholitzii shown in fig. 400 the centrifugal xylem elements are unusually numerous: the space between the two xylems is occupied by parenchyma and the whole strand is enclosed by a sheath of crystal-containing cells, s, with thick inner walls. Fig. 400, 1–4, illustrates the gradual change in the form of the bundle in the region of dichotomy[87]. The ground-tissue of the petiole is abundantly supplied with secretory canals and in the hypodermal region is a cylinder of stereome. In some petioles, e.g. Macrozamia heteromera[88], the ground-tissue cells are lignified and reticulately pitted, a feature met with in some Mesozoic cycadean leaves[89]. In Cycas media Worsdell noticed a tendency of the leaf-trace bundles towards a concentric arrangement and similar vascular strands are recorded in the peduncle of Dioon edule, in various sporophylls[90] and in other cases. It is possible, as Worsdell believes, that the fairly frequent occurrence of concentric bundles in plants characterised by collateral bundles may have a phylogenetic significance.

The pinnae are dorsiventral and the veins exarch or pseudomesarch: secretory canals occur between (Encephalartos), above, or below the veins. The mesophyll of Cycas is characterised by the presence of isolated xylem-elements passing from the midrib to the edge of the lamina and, as Lignier[91] suggests, these may be regarded as a reduced system of lateral conducting strands.

The epidermal cells of the leaflets have straight or slightly curved walls except in Stangeria where they are undulate and fern-like[92]. The stomata, with few exceptions confined to the lower epidermis, are larger than in other gymnosperms (on the average ·075 × ·034 mm.) and are more or less depressed below the surface; the guard-cells are usually surrounded by 4–6 subsidiary cells.

The roots exhibit no feature to which attention need be called: the pericycle is several cells broad and as in the stem there may be extrafascicular cylinders of xylem and phloem.

Spotted a typo or a problem in this chapter? Tell us

XXIX. PTERIDOSPERMEAE

I. LYGINOPTERIDEAE.

LYGINOPTERIS.

The genus Lyginopteris is selected for the first place in this chapter simply on the ground that we have a fuller knowledge of its morphology than in the case of other types. It is not regarded as the most primitive member of its class. Lyginopteris may be described in a few words as a plant having the habit and to a large extent the anatomical features of a Fern, but differing from existing ferns in the possession of integumented megasporangia or seeds and in the power of secondary growth in thickness by means of a cambium in both stem and root. The seed (Lagenostoma) agrees with those of recent Cycads and Gnetales more closely than with the corresponding organs in Conifers or any other group, while the structure of the secondary wood is practically identical with that of Cycads. The microsporangia occur as groups of small bilocular sporangia, or synangia, at the tips of fertile pinnae of highly compound fronds.

Nomenclature and Historical Summary. In 1866 E. W. Binney[93] of Manchester published a short description of a small petrified stem from the Lower Coal Measures of Lancashire and named it Dadoxylon oldhamium, employing Endlicher’s term Dadoxylon which that author substituted for Pinites as previously used by Witham[94]. Three years later Williamson[95] drew attention to certain features in which Binney’s type differs from the genus Dadoxylon and substituted a new name Dictyoxylon, suggested by the reticulate pitting on the walls of the tracheids. In a subsequent paper Williamson[96] gave a fuller description of Binney’s species and spoke of it as ‘one of the most common plants in the calcareous nodules of the Lower Coal Measures’ of Lancashire and Yorkshire. He connected certain casts of arborescent dimensions with Binney’s type on the ground that the surface-features of the casts are such as would be produced by partially decorticated stems having a hypodermal reticulum of mechanical tissue like that preserved in the small petrified specimen described by Binney (fig. 402). Mr Carruthers called Williamson’s attention to a paper by Mr Gourlie[97] in which the generic name Lyginodendron is instituted for stem-casts identical in surface-features with the fossils figured by Williamson. In spite of the much larger dimensions of the reticulum on the casts described by Gourlie as compared with that in the outer cortex of Binney’s stem, Williamson concluded that Lyginodendron is ‘undoubtedly an inorganic cast of the prosenchymatous layer of the bark of Dictyoxylon.’ It is but fair to add that Williamson was influenced in coming to this conclusion by a discovery by Mr Nield of a piece of a large petrified stem believed to be generically identical with Binney’s type, but subsequently referred to a distinct genus[98], which was comparable in size with the stems responsible for Gourlie’s Lyginodendron casts. The type-specimen of Gourlie’s Lyginodendron Landsburgii[99], from Carboniferous rocks at Stevenston in Ayrshire, Scotland, is represented in fig. 401. The convex areas represent casts of depressions in a reticulum of cortical tissue, originally occupied by comparatively delicate cells, which decayed or shrunk more quickly than the enclosing framework of stronger fibrous elements that remained as a prominent reticulum and produced the depressions bounding the raised portions of the cast. Such a cast would undoubtedly be formed by the stem on which Binney founded his species: the radially disposed bands of thick-walled cells seen in the outer part of the section (fig. 402) are portions of an irregular anastomosing mechanical system, the reticulate arrangement of which is seen in the impression of a rachis of a Lyginopteris frond shown in fig. 405, E, and indicated in the more slender axis reproduced in fig. 404, A, b. This reticulate form of cortical stereome on which Brongniart founded the genus Dictyoxylon[100], a term since applied by Solms-Laubach and other authors to a certain type of cortex not confined to a single genus of plants, occurs also in some Palaeozoic lycopodiaceous stems[101] and in itself cannot be regarded as a safe criterion of botanical affinity. The largest example of Gourlie’s Lyginodendron that has come under my notice is an incomplete sandstone cast from Upper Carboniferous strata near Harrogate reaching a length of 100 cm. and with convex areas 13 cm. long. A similar cast, 36 cm. broad, has recently been figured by Nathorst[102] from the Culm of Spitzbergen, and from the Upper Devonian of Ellesmere Land the same author has described impressions of a cortical reticulum under the name Lyginodendron Sverdrupi[103]. These specimens are interesting as pointing to the former occurrence in the Arctic regions of stems—probably Lepidodendroid—reaching the dimensions of a fairly large tree. As Potonié[104] pointed out, Gourlie’s generic name serves a useful purpose for casts of stems of the type shown in fig. 401 that cannot be assigned to a definite systematic position. The genus was first used for a specimen which has nothing to do with the plant usually spoken of as Lyginodendron oldhamium (Binney). Though loath to give up a name by which Binney’s type has long been known, in spite of its retention in the second volume of this work I feel compelled so far to conform to the recognised principles governing nomenclature as to adopt Potonié’s generic term Lyginopteris.

Lyginopteris oldhamia (Binney).

i. Stem.

1866. Dadoxylon oldhamium, Binney, Proc. Lit. Phil. Manchester, vol. +v.+ p. 113.

1869. Dictyoxylon oldhamium, Williamson, Monthly Micros. Journ. vol. +ii.+ p. 66.

1873. Lyginodendron oldhamium, Williamson, Phil. Trans. Roy. Soc. vol. +clxiii.+ p. 404.

1899. Lyginopteris oldhamia, Potonié, Lehrbuch der Pflanzenpalaeontologie, p. 171.

ii. Leaf.

1828. Sphenopteris Hoeninghausi, Brongniart, Prodrome, p. 51.

1872. Edraxylon, Williamson, Proc. R. Soc. vol. +xx.+ p. 438.

1874. Rachiopteris aspera, Williamson, Phil. Trans. R. Soc. vol. +clxiv.+ p. 684.

1877. Calymmatotheca Hoeninghausi, Stur, Culm Flora, +ii.+ p. 266.

1905. Crossotheca Hoeninghausi, Kidston, Proc. R. Soc. vol. +lxxvi.+ p. 358.

iii. Seed.

1877. Lagenostoma, Williamson, Phil. Trans. R. Soc. vol. +clxvii.+ p. 234.

1903. Lagenostoma Lomaxi, Oliver and Scott, Proc. R. Soc. vol. +lxxi.+ p. 477.

iv. Root.

1876. Kaloxylon Hookeri, Williamson, Phil. Trans. R. Soc. vol. +clxvi.+ p. 23.

i. Stem.

The petrified stem on which Binney founded the species was first figured by Dr Arber[105] from a section in the Binney collection in the Sedgwick Museum, Cambridge: this section (13 mm. in diameter) is reproduced in fig. 402. The most striking features are: (i) the pith consisting of an unusually large and irregular group of dark thick-walled parenchyma, (ii) the broad cylinder of manoxylic secondary xylem characterised by multiseriate medullary rays, (iii) the outer cortex composed of dark radially disposed and oblique bands of mechanical tissue separated from one another by partially destroyed and tangentially elongated parenchymatous elements. It is this outer cortex that Williamson aptly compared with the Roman numerals on a clock-face. In the perimedullary region and in contact with the inner edge of the secondary-xylem cylinder are six strands of primary xylem representing the xylem halves of collateral bundles separated from the primary phloem strands by the intervening cylinder of secondary wood. Two of the primary xylem strands in lateral contact are seen in fig. 404, C; the other four occur as separate bundles. Each primary xylem strand contains a small group of spirally thickened protoxylem elements (px) associated with a few parenchymatous cells. The large primary tracheids internal to the protoxylem are characterised by multiseriate bordered pits on their walls, while those external to the protoxylem, which are in contact with the innermost secondary tracheids, have scalariform pitting. The dark patch s (fig. 404, C) is a portion of the large group of sclerenchymatous cells, shown in figs. 402, 403. The perimedullary xylem strands of mesarch structure are the lower portions of leaf-traces and, as Scott points out, ‘each of the bundles surrounding the pith is, in fact, a sympodium, composed of the united lower ends of successive adjacent leaf-traces.’ The larger of the two bundles shown in fig. 404, C, is on the point of passing out to a leaf, while the smaller strand is on its way to a higher level before bending outwards through the secondary wood. Slightly beyond the middle of the secondary xylem there is an arc of narrower tracheids comparable with an incomplete annual ring. Although zones or arcs of narrow tracheids are not uncommon in the wood of Lyginopteris there is no satisfactory evidence of regularly recurring seasonal changes. On the outer face of the secondary wood are a few leaf-trace strands pursuing a vertical course in the pericycle region; but the structure and behaviour of these bundles are more clearly illustrated in the stem reproduced in fig. 403. The tissue between the crushed phloem and pericycle and the outer cortex (fig. 402) consists of radially compressed parenchyma with scattered secretory cells separated from the more internal tissue by a narrow band of periderm formed by a phellogen in the outer part of the pericycle.

A larger and better preserved stem, 3·7 cm. in diameter, is seen in fig. 403. In this stem the pith of parenchyma and scattered sclerenchymatous nests is larger in proportion to the stele than in Binney’s type-specimen. From the inner edge of the secondary xylem several primary xylem-strands project as rounded wedges or tangentially elongated groups where two traces are laterally united in the perimedullary zone. The cylinder of secondary wood is partially interrupted at r by the bending outwards of the stele of an adventitious root cut across transversely as it bends down after emerging from the outer cortical region. In more or less close association with the outer surface of the secondary xylem are four pairs of leaf-trace bundles and one larger trace at d containing two widely separated protoxylem strands and faced externally with an arc of secondary xylem: this is a leaf-trace which shows by the slight constriction on the outer edge of its primary xylem that it is beginning to divide into a pair of equal strands. A precisely similar strand is shown on a larger scale in fig. 404, D. The twin bundles seen at b, fig. 403, represent a divided leaf-trace at a slightly higher level than the partially severed trace at d, and the arcs of secondary xylem are narrower. The appearance of the double leaf-trace at a still higher level is shown at c: the two strands are farther apart and the secondary xylem has almost disappeared, while those at e, nearer their entrance into the leaf-stalk, consist exclusively of primary xylem and phloem. At a the two strands of a leaf-trace, still nearer to the petiole, are inclined towards one another preparatory to reunion after reaching the leaf-stalk. A slender root is seen in transverse section at r′ immediately outside the two leaf-bundles. As Williamson and Scott[106] have pointed out, there are always five leaf-traces beyond the xylem cylinder of a Lyginopteris stem as seen in transverse section, and these traces in the pericycle, separated from one another by ⅔ of the circumference, alternate in position with the lower portions of leaf-traces in the perimedullary region of the same stem. The phyllotaxis is thus seen to be ⅖.

The secondary wood is succeeded by a cambium of normal structure passing gradually into a narrow band of secondary phloem which in well-preserved stems is seen to consist of sieve-tubes and parenchyma with medullary rays rather broader than those in the xylem. Beyond the phloem is the comparatively broad pericycle consisting of parenchyma with nests of sclerenchyma like those in the pith and scattered secretory cells. In the outer layers of the pericycle a phellogen was formed at an early stage in the growth of the plant, producing several layers of secondary tissue, which is regarded as periderm and forms a conspicuous feature in Lyginopteris stems; it appears as a comparatively dark sinuous band where it bends outwards to wrap round the leaf-traces in their almost vertical course through the pericyclic region (fig. 403). The periderm is clearly seen at p close to the crushed secondary phloem of the dividing leaf-trace in fig. 404, D. All the leaf-traces seen in fig. 403 beyond the secondary wood are still within the deep-seated periderm and, as Williamson and Scott showed, each leaf-trace after emerging from the secondary wood remains in the pericycle-zone for a length of five internodes as it very gradually inclines outwards. Once free from this region the twin bundles bend more sharply towards the petiole. Stated briefly, the history of each leaf-trace from the perimedullary region to the leaf-base is as follows: at the outer edge of the pith a single trace consists of a mesarch xylem bundle with one protoxylem strand; it passes vertically through five internodes and then bends out through a foliar gap in the xylem-cylinder, and the primary tracheids receive additions from the cambium of the stele on their outer face. As the trace leaves the secondary xylem it bends upwards and, as seen at d, fig. 403, begins to divide into twin bundles. As the trace passes higher the bisection of the protoxylem and metaxylem is completed and the secondary xylem-arcs are gradually lost until the separate strands of each pair are reduced to single mesarch bundles composed wholly of primary tracheids. As the trace bends outwards through the cortex the phloem gradually encircles each xylem-strand until a concentric structure is substituted for the collateral disposition of the conducting tissue. At the same time the protoxylem strands divide and occupy a position near the inner edge of the metaxylem. On reaching the petiole or after passing some distance up the axis of the frond, the twin bundles unite and usually form a V-shaped vascular strand (figs. 404, E; 405, A). The single meristele subsequently divides into two equal portions preparatory to the bifurcation of the petiole (fig. 406).

The inner cortex, consisting of parenchymatous tissue and many secretory cells with an occasional group of sclerenchymatous elements in place of the abundant nests of this tissue in the pericycle, has been invaded in the stem shown in fig. 403 by numerous rootlets of Stigmaria and Lyginopteris, some of which are seen interrupting the continuity of the outer cortex. The greater width of the cortical region at f, fig. 403, is due to the decurrent base of a petiole the meristele of which is not included in the section. The lighter and broader bands between the cross-sections of the stereome-network in the outer cortex are occupied by remains of tangentially stretched parenchymatous cells, and beyond this zone in a younger stem there are a few layers of parenchyma forming the superficial tissue, but there appears to be no well-defined epidermal layer.

Young stems have been recognised in which there is very little secondary xylem and phloem: in these the stereome bands in the outer cortex are closer together than in the stretched hypodermal tissue of older shoots and the scattered sclerous nests are represented by unthickened cells. In addition to young stems Williamson and Scott described a distinct type of small stem in which the primary xylem forms an almost complete ring[107] comparable with the primary xylem of some adult Sigillarian stems (vol. +ii.+ p. 220) but distinguished by its mesarch structure and by the reticulate pitting of the centripetal xylem.

A characteristic feature of the stem is the occurrence of emergences from the outer cortex which have the structure either of spinous processes, broadly linear or flask-shaped, or of stalked glands[108]. A portion of a glandular emergence is shown in fig. 405, B: the group of small cells immediately below the blunt apex is in this instance still intact though showing signs of disorganisation in the centre; but in many cases the secretory tissue has not been preserved and the head of the emergence is occupied by a space. A single stoma is seen at s in longitudinal section. Further reference to the emergences is made in the description of the leaf.

It occasionally happens that a meristematic layer is formed in the parenchymatous tissue immediately internal to some of the perimedullary xylem strands of a Lyginopteris stem from which either secondary parenchyma is produced or a zone of secondary xylem and phloem, the phloem facing the centre of the pith. An example of such internal xylem was figured by Williamson[109] and similar occurrences are more fully dealt with by Williamson and Scott[110] who consider that the perimedullary cambium may represent an internal extension through a leaf-gap of the normal cambial cylinder. In the stem represented in fig. 403 there are two perimedullary xylem strands to the left of the bottom of the V-shaped gap in the secondary xylem-cylinder, r, and on the inner face of one of these, as shown in fig. 405, C, there is a narrow arc of internal secondary xylem, c, between the xylem-strand and the outer edge of one of the sclerous nests. The sporadic occurrence of arcs of inversely orientated secondary vascular tissue affords an interesting parallel with a similar morphological feature in some recent Dicotyledonous genera such as Tecoma and Iodes. As Williamson and Scott point out, this similarity affords ‘a striking warning against the indiscriminate use of even conspicuous anatomical characters[111].’ While admitting the necessity of guarding against the danger of attaching importance to occasional and abnormal characters they may have some significance as collateral evidence in comparisons of different types of stems. It is conceivable that these anomalous arcs of secondary tissue on the inner side of the primary xylem strands may, as Worsdell[112] maintains, be reversions to an ancestral character and in this sense comparable with the strands of inverted vascular tissue in some recent Cycadean stems. The question of relationship of Lyginopteris and allied types to recent Cycads and the Palaeozoic Medulloseae is considered in a later chapter.

In 1902 Lomax[113] described two branching specimens of Lyginopteris, and more recently two others have been discovered at a locality near Bacup in Lancashire which have been thoroughly investigated by Miss Brenchley[114] who constructed models from drawings of serial sections[115]. One specimen shows six leaf-bases in a length of 4¾ inches and branches spring from the axils of five of them: some of the branches show secondary ramifications. The phyllotaxis of the leaf-bases on a branch is always the reverse of that on the main stem, a divergence to which no parallel was found in a selection of trees and shrubs examined by Miss Brenchley. The secondary wood of the stem swells below the point of exit of a branch and frequently a fairly large amount of wood occurs in the pith when a branch is given off: this anomalous wood may help to close the branch-gap.

ii. Leaf.

In his account of Lyginopteris stems published in 1873 Williamson[116] suggested that the vascular bundles met with outside the xylem-cylinder might be the leaf-traces of large fronds and expressed the opinion that the ‘stems or petioles’ previously described by him under the generic name Edraxylon might belong to Lyginopteris. A year later he substituted the name Rachiopteris aspera for the petioles previously referred to Edraxylon and inclined to the view that this type of Rachiopteris may be the petiole of the Carboniferous fronds known as Sphenopteris Hoeninghausi Brongn., an inference based to a large extent on the occurrence of emergences on Rachiopteris aspera (fig. 404, E) preserved as petrifactions like those on impressions of Sphenopteris Hoeninghausi as figured by Brongniart (figs. 404, A; 405, D, D’). In 1890 Williamson was able to demonstrate the truth of the surmise that Rachiopteris aspera and Lyginopteris oldhamia are respectively the petiole and stem of the same plant, which he believed to be an arborescent fern[117]. The petioles of Lyginopteris fronds, which may reach a diameter of 1 cm., are attached by a broad base to the stem, and as already suggested by the number of internodes traversed by each leaf-trace, the leaves are comparatively far apart. A transverse section of a petiole is shown diagrammatically in fig. 405 A. The hypodermal stereome is a prominent feature, but the narrow radial plates of the stem-cortex tend to be replaced in the rachis by broader and confluent masses of strengthening elements: the upper surface of the petiole is slightly grooved. Glandular and spinous emergences are often very abundant, as in the section reproduced in fig. 404, E. A glandular emergence is seen at a in fig. 405, A. The spinous emergences may be compared with those of Davallia (Odontosoria) aculeata[118], a West Indian fern of climbing habit and with the prickles on Hemitelia and other recent Cyatheaceous fronds[119], while capitate glands, though simpler than those of Lyginopteris, occur on the leaf-stalks of some recent Polypodiaceous species[120]. The concentric meristele may consist in the lower part of the petiole of two separate and slightly curved strands like those seen in fig. 404, E, m: sooner or later the two strands unite to form a wide-open V or a W-shaped bundle with several slightly internal protoxylem groups close to the lower edge. The two sections represented in fig. 406, A and B show the gradual divergence of the two meristeles of a petiole as they approach the level where it divides into two equal branches, a characteristic feature of Sphenopteris Hoeninghausi and allied fronds. At a lower level than that represented in fig. 406 the vascular strand of the petiole would have the form of a W as figured by Williamson in one of his earlier memoirs[121]. The phloem with scattered secretory sacs and the adjacent tissue of the leaf-stalk are occasionally preserved in wonderful perfection[122]. No endodermis has been recognised. Sclerous nests are scattered in the ground-tissue as are also secretory sacs (figs. 404, E; 405, A). A small root r has penetrated the parenchyma of the rachis shown in fig. 405, A.

Sphenopteris Hoeninghausi Brongn.[123], founded on material from English Coal Measures, was regarded by Williamson as the foliage of Lyginopteris chiefly on the ground of the occurrence of emergences on the axes (figs. 404, A, B) and laminae of the impressions like those on the petrified stems, and this comparison received support from the resemblance of the fragments of pinnules associated with Lyginopteris and its petioles in the calcareous nodules to the leaflets of Brongniart’s type. This identification is supported by subsequent work. The quadripinnate fronds, which attain a considerable size, resemble those of recent species of Davallia and other ferns, but the forking of the rachis and branches of the frond is a striking feature: the pinnae may reach a length of 15 cm.[124] The portion of carbonised rachis shown in fig. 405, E, reveals the existence of a hypodermal reticulum like that in the outer cortex of a Lyginopteris stem and the same feature is seen in the more slender axis represented in fig. 404, A, at b.[125] The pinnules are usually deeply lobed and the segments may be comparatively broad and blunt or narrow[126] (fig. 290, C, vol. +ii.+ p. 399; fig. 404, A, a, B; fig. 405, D’). The lamina has a well marked dorsiventral structure: the palisade-tissue next the upper surface is separated from the epidermis by small hypodermal cells, possibly functioning as a water-storage layer, and the central part of the mesophyll consists of loose aerenchyma: the veins are collateral as in recent ferns and stomata occur in the lower epidermis. Emergences are seen both on impressions (fig. 405, D’) and on petrified specimens. A striking feature of the pinnules is the rounded surface caused by the revolute edge of the lamina as seen in the section reproduced in fig. 404, B. This character coupled with the occasional occurrence of groups of short tracheal elements at the termination of the veins denotes a tendency to a xerophilous habit.

On the strength of a very close resemblance between Sphenopteris Hoeninghausi and Calymmatotheca Stangeri (fig. 408, E, F)—characterised by fertile pinnules bearing stellate groups of small linear valves, regarded by Stur as the open lobes of an indusium—Zeiller included Brongniart’s type in the genus Calymmatotheca. The resemblance in general habit between the two species extends to the presence in their rachises of the Dictyoxylon form of cortex[127]. The view formerly held by some authors that the valve-like appendages to the fertile segments of Calymmatotheca are sporangia is incorrect: a re-examination of Stur’s specimen (fig. 408, E, F) has confirmed the original description[128]. The stellate lobes are now regarded as portions of a cupular investment of a seed similar to Lagenostoma Lomaxi, the female reproductive apparatus of Lyginopteris oldhamia. The axes of the fertile pinnae bear small thorn-like emergences probably identical with those on the cupule of Lagenostoma and on the petioles of Lyginopteris oldhamia. It was stated in vol. +ii+ that the fronds known as Sphenopteris Linkii (Goepp.) represent, with other closely allied forms, leaves belonging to Heterangium stems. This statement was based on a misconception: the rachis of Sphenopteris Linkii, as I have satisfied myself by an examination of impressions shown to me by Dr Kidston, exhibits the reticulate pattern characteristic of Lyginopteris and not the transverse ribs characteristic of Heterangium.

It is not an easy task even for those most familiar with Carboniferous fronds to distinguish clearly between species agreeing generally with Sphenopteris Hoeninghausi, a species regarded by some authors as the type of a group of very similar and closely allied forms all of which were probably borne on stems referable to the genus Lyginopteris. The species Lyginopteris oldhamia as generally understood probably includes more than one specific type, and it is safe to assert that in the Carboniferous period Lyginopteris was represented by several forms characterised by highly compound fronds with forked rachises like Sphenopteris Linkii, S. Hoeninghausi, and others. The features characteristic of fronds included in the Sphenopteris Hoeninghausi group have recently been described by Gothan[129]. Stur’s generic name Calymmatotheca originally applied to the species C. Stangeri was applied to Sphenopteris Hoeninghausi by Zeiller, and although the fronds of the latter type have not been found with fertile appendages of the Calymmatotheca type there can be no doubt as to the generic identity of these, barely distinguishable, species both of which belong to stems of Lyginopteris. Prof. Johnson[130] has recently described some impressions from the Coal Measures of Ireland, which he refers to S. Hoeninghausi, bearing stellate groups of lobes like those of Calymmatotheca, and in one case he describes a seed in the middle of the carbonised remains of a stellate group of cupular segments. An examination of the specimen in Dublin convinced me that there is no satisfactory evidence of the seed-nature of the appearance on the rock believed by Johnson to be an elliptical Lagenostoma-like seed. The actual attachment of the stellate lobes to the pinnae of the frond is not clearly demonstrated.

iii. Microsporangia.

In 1905 Kidston[131] announced the discovery of microsporangia on fronds of Lyginopteris: he described specimens from the Coal Measures of Dudley identified by him with Sphenopteris Hoeninghausi showing sterile and fertile pinnae in organic connexion. The fertile pinnules (fig. 407, B) are slightly expanded distally into an oval limb about 2 mm. long bearing 6 to 7 bilocular fusiform microsporangia 3 mm. long and 1·5 mm. broad: in the immature condition the sorus is hemispherical, the summit being formed of the incurved apices of the sporangia. At maturity the sporangia spread out, the sorus assuming the form of an epaulet. Fig. 408, H, shows a sorus in transverse section and in fig. 408, G, the limb and two pendulous sporangia are shown. The microspores, 50–70 µ in diameter, are studded with numerous blunt spines and each spore shows a triradiate ridge. The section reproduced in fig. 407, A, from the Coal Measures of Oldham is probably a bilocular sporangium of the same type as those described by Kidston from Dudley. Dr Kidston[132] describes a second type of microsporangial sorus as Crossotheca Hughesiana which agrees closely with C. Hoeninghausi, but the fertile segments are not associated with any sterile pinnae. The generic name Crossotheca, founded by Zeiller[133] in 1883, was substituted for Sphenopteris on the ground that Brongniart’s species S. Hoeninghausi is shown to possess sporangia of the Crossotheca type. If Kidston’s specific determination is correct, his discovery demonstrates that Lyginopteris fronds bore microsporangia having the characters of Crossotheca, a type characteristic of several Carboniferous species belonging both to the form-genera Sphenopteris and Pecopteris. Reference has already been made to the difficulty of distinguishing between impressions of fronds of the Sphenopteris Hoeninghausi group, a difficulty that is illustrated by Dr Gothan’s statement[134] that the Dudley specimens of Crossotheca are not identical in the character of the sterile pinnules with Sphenopteris Hoeninghausi. An examination of Dr Kidston’s specimens led me to agree with his determination; but, it may be asked, have we any evidence of the association with Lyginopteris fronds of sporangia other than those of the Crossotheca type? Prof. Chodat[135] believes that certain petrified fragments of pinnules occasionally met with in the calcareous nodules bearing sessile and apparently annulate sporangia belong to Lyginopteris fronds. These sporangia appear to be identical with those named by Scott Pteridotheca Butterworthi[136] and regarded by him as filicean sporangia that cannot be referred to any known Carboniferous genus. The piece of lamina bearing an empty sporangium, which may or may not have possessed an annulus, reproduced in fig. 407, C, occurs in association with the larger specimen shown in fig. 404, B, and it would seem not unreasonable to regard both as parts of the same frond, namely a frond of Lyginopteris. As Prof. Weiss[137] points out, the accurate determination of small pieces of petrified pinnules is exceedingly difficult and without more decisive evidence we are hardly justified in asserting that the sporangia figured by Chodat and Scott and that shown in fig. 407 belong to the genus Lyginopteris. Although the available data appear to favour the view generally held that Kidston’s conclusion is correct additional evidence would be welcome.

Telangium. Reference was made in vol. +ii.+[138] to the genus Telangium instituted by Dr Benson for some petrified sporangia from the Coal Measures regarded by her as the microsporangia of a Pteridosperm, probably Lyginopteris. The sporangia of Telangium are similar to those of Crossotheca. Scott points out that they are borne on a flat disc or lamina ‘quite comparable to a fertile pinna of Crossotheca[139],’ and he concludes that these sporangia are not generically distinct from the impressions on which the genus Crossotheca was founded. Kidston[140] regards Telangium Scotti, Benson, as the microsporangium of a Pteridosperm though not of Lyginopteris, on the ground that the microsporangia described by Miss Benson are not attached to a limb and that they have a single loculus in place of the double loculus (fig. 407, A) of Crossotheca. The presence of a limb in Telangium recognised by Scott removes one of these distinguishing features. There are, however, no adequate reasons for regarding Telangium Scotti as specifically identical with Crossotheca Hoeninghausi. The synangium of Telangium Scotti, 5 mm. in length, consists of 6–12 sporangia united basally and opening when ripe by longitudinal dehiscence. Fig. 493, E, shows eight sporangia of a synangium in transverse section: the two sporangia at the lower end of the section are less distinct than the others, some are full of spores and others have shed their contents by the splitting of the thin inner walls of the loculi. The sporangial walls are composed of an outer layer of large cells with dark contents succeeded by 2–3 layers of smaller and crushed cells. The spores, 5–6 µ × 3·5–4 µ in diameter, have a reticulately sculptured exine: Dr Benson[141] states that they agree closely with pollen-grains found in the pollen-chamber of Lagenostoma ovoides except in their slightly smaller size; she notes the association of Telangium with fragments of the vegetative organs of Lyginopteris and draws attention to resemblances in the structure of the tissues; but the most interesting comparison, at least in an academic sense, is with the seed Lagenostoma, the integumented megasporangium of Lyginopteris. Dr Benson points out that a transverse section of a Lagenostoma in the plane of the canopy, showing the nucellar apex surrounded by radially disposed chambers (fig. 409), presents a certain resemblance to a synangium of Telangium Scotti; and it is suggested that the chambers encircling the nucellus may represent sterilised sister-sporangia[142]. ‘The seed in fact is assumed to be a synangium in which all but one of the sporangia are sterile, and form an integument to the one fertile sporangium which has become a megasporangium with one large megaspore.’ This view, though clearly incapable of confirmation in the present state of our knowledge, is not merely an ingenious hypothesis but a stimulating suggestion as to possible homologies: as an argument in favour of associating Lagenostoma and Telangium as the spore-bearing organs of the same plant it has but little weight.

iv. The Seed. =Lagenostoma= Williamson.

Lagenostoma Lomaxi, Oliver and Scott ex Williamson, +MS.+

In 1877 Williamson[143] proposed the generic name Lagenostoma for some petrified seeds from the Lower Coal Measures of Lancashire and described two species, Lagenostoma ovoides and L. physoides: in his +MS.+ Catalogue a third type is referred to as Lagenostoma Lomaxi. It is this third type that Prof. Oliver was the first to recognise as the megaspore-bearing organ of Lyginopteris oldhamia. Its structure has been thoroughly described by Oliver and Scott[144] and these authors contribute a judicial summary of the evidence on which Lagenostoma and Lyginopteris are believed to stand for one and the same plant. The evidence is based chiefly on the following considerations: an agreement in the structure of the vascular bundles in the investments of the seed with those in the leaves of Lyginopteris; the presence in the outer envelope of the seed of stalked glands identical with those on the stems and petioles. The evidence does not as yet amount to absolute proof, as the seeds, which occur either with or without a stalk, have not been found attached to a Lyginopteris frond. But ‘where vegetative and reproductive organs presenting identical structural features, not known to occur in other plants, are thus found in close and constant association, the inference that the one belonged to the other appears irresistible.’ While most botanists believe that a satisfactory case is established there are a few[145] who refuse to believe in a connexion between Lagenostoma and Lyginopteris until an actual union has been demonstrated. The discovery by Kidston[146] of seeds attached to pinnae bearing Neuropteris pinnules and the demonstration of organic continuity between seeds and the pinnules of other Palaeozoic fern-like fronds supply abundant confirmatory evidence that leaves no doubt as to the occurrence of seeds on modified pinnae of Sphenopteris Hoeninghausi and of other closely allied fronds which represent the foliage of different forms of Lyginopteris. In this connexion it is pertinent to add that Grand’Eury[147] has found seeds of the Lagenostoma type in close association with impressions of Sphenopteris Dubuissonis and other leaves of similar habit.

A seed of Lagenostoma Lomaxi reaches a length of 5·5 mm. with a maximum diameter of 4·4 mm.; it is broadly oval or barrel-like (fig. 408, C) and when immature was invested by a loose irregularly lobed glandular envelope (fig. 408, B) from which the seed eventually freed itself by a natural process of abscission. The central body or nucellus, except in the apical region, is concrescent with a fairly stout integument or testa (fig. 408, C, f) the outer portion of which is characterised by regular longitudinal rows of palisade-like cells comparable with the broad palisade-layer in the sporocarp of Pilularia. On the exposed surface of this palisade-tissue are small dark structureless pegs[148], possibly the remains of a mucilaginous layer such as occurs on the seed-coats of some recent Flowering plants. At the base of the nucellus the chalazal region, fig. 408, C, ch, is provided with sclerous elements and forms a hard investment to the axial vascular strand from the pedicel. It is at the base of this chalazal region that the seed is eventually cut off by an absciss-layer. The integument is supplied throughout its length by nine vascular bundles of endarch, or approximately endarch, structure. The free portion of the integument seen from the outside (fig. 408, B) has the form of a fluted cone with a circular opening at its summit. The greater part of this domical apex, as seen in longitudinal section in fig. 408, C, appears to be hollow, but in the living state the dome, or canopy as Williamson called it, was filled with parenchyma in which the vascular bundles were embedded and, as shown in the transverse section in fig. 409, the canopy is divided into compartments by radial septa which in its basal region are replaced by regular and deep furrows on the inner face. Enclosed by the canopy, with its outer surface fluted as the result of the partial collapse of the outer wall of each compartment due to the decay of the filling tissue, is the flask-shaped apex of the nucellus; between the apical cone of nucellar parenchyma and the superficial layer is an annular cavity which Williamson[149] called the lagenostome. The parenchymatous core tapers to a narrow summit which slightly overtops the integument and is constricted at the broad base (fig. 493, A, B; page 311). The bottle-shaped apical tissue is separated by an annular space, c, fig. 493, B, from the limiting layer of the nucellus: this space is the pollen-chamber formed in the living seed by the disorganisation of the subepidermal cells of the nucellar apex. The pollen-chamber is a feature characteristic of recent cycadean ovules (see p. 6). In Lagenostoma the annular form of the pollen-chamber is a peculiarity distinguishing this type of seed from those of recent Gymnosperms and most other Palaeozoic seeds. As Oliver says, it marks an ‘advance in precision’[150] over other forms as the microspores which fall into the chamber are brought direct to the surface of the underlying megaspore and presumably to the archegonia which, it is reasonable to believe, were disposed in a circle at the base of the annular crevice. Microspores frequently occur in the pollen-chamber and some have been discovered apparently in the act of liberating male gametes[151].

The outer wall of the nucellus is bounded externally by a similar circular space (d, figs. 408, C; 493, B) which separates it from the domical canopy. In the great majority of specimens the central tissue of the seed is not preserved and an empty sac supported from the base of the nucellus-apex occupies nearly the whole of the interior: the shrunken wall of the sac is all that remains of the large megaspore. It would seem, then, that the nucellus was almost completely destroyed as a consequence of the growth of the megaspore or embryo-sac, which eventually occupied nearly the whole of the seed-body.

In an exceptionally well preserved specimen recently described and admirably illustrated by Mr McLean[152] part of the parenchymatous tissue of the prothallus which originally filled the megaspore is clearly seen: its surface-layer consists of small cells succeeded by a broad band of radially elongated elements closely resembling the alveoli in the prothalli of some recent Gymnosperms, particularly certain Conifers. No archegonia have been discovered. The cupular envelope of immature seeds, compared by Oliver and Scott with the lobed and glandular husk of Corylus colurna L.[153], receives several vascular bundles of collateral and mesarch structure from the axial strand, and these subdivide as they ascend. The glands which occur on all parts of the cupule are sessile or stalked and identical with those on the vegetative organs of Lyginopteris. Assuming that pollination occurred at a comparatively early stage in the development of the seed when the cupule was still intact, it is conceivable, as Sir Joseph Hooker suggested, that the glandular secretion may have attracted insects and so aided in the transport of pollen which were perhaps drawn down the narrow pollen-chamber by exuded mucilage as in recent Conifers. The evidence obtained in recent years in favour of insect-pollination in certain Cycads and in Welwitschia lends support to this view: the dragon flies hovering over a fertile Lyginopteris frond in a recent restoration[154] may be a legitimate addition.

A striking feature of Lagenostoma as of other Palaeozoic seeds is the absence of an embryo: this and other considerations have led certain authors, notably Chodat[155], to question the justification for the use of the term seed. Various suggestions have been offered in explanation of this fact. In recent Cycads, as already pointed out, the development of the embryo does not always occur before seed-fall. It may be that these older seeds had no resting-period or there may have been a period of rest after fertilisation and not as now at a stage subsequent to the formation of the embryo[156]; it is also suggested by Scott that ‘the nursing of the embryo had not yet come to be one of the functions of the seed, and that the whole embryonic development was relegated to the germination stage[157].’ In this connexion reference may be made to a statement by Miss Gibbs[158] who speaks of seeds of a Podocarpus picked up from the ground in apparently a mature state and with the associated bracts coloured and swollen as though ready to aid in dispersal but with no embryo: the seeds had matured before fertilisation and fell from the tree after pollination. Whatever may be the true explanation of the absence of embryos this negative character should not be allowed to outweigh the evidence furnished by morphological features as to the applicability of the term seed. As Prof. Oliver says, ‘there is a long chapter in evolution to be deciphered before we can connect ... the seed of Lyginodendron with the sporangium of any fern at present known to us’[159].

The cupule of Lagenostoma has been homologised with the outer part of the integument of a recent cycadean seed[160] which, it is suggested, consists of an inner and an outer envelope that have become concrescent, and this hypothesis is supported by another author by a comparison between Lagenostoma and Gnetalean seeds[161]. A comparison has also been made between the collar of a Ginkgo seed and the much more conspicuous cupule of Lagenostoma[162]. Dr Benson and Miss Welsford[163] institute a comparison between the vascular supply of the outer integument of the ovules of Carpinus and Morus and that of the cupule of Lagenostoma, a comparison suggested by Miss Kershaw’s remarks[164] on the similarity between the vascular system of the ovules of Myrica Gale and Trigonocarpus. In 1908 Dr Benson[165] described some germinating microspores in the pollen-chamber of another species of Lagenostoma, L. ovoides, and recognised what she believed to be antherozoids. I am indebted to this author for allowing me to make a drawing from her section (fig. 408, D). Two microspores are seen with thick outer walls showing irregular holes probably of secondary origin and not part of a regular reticulum as Dr Benson suggests. Close to the upper microspore is a hemispherical body, s, described as a male gamete, and a similar body is seen still enclosed by the lower microspore. It is by no means improbable that these are antherozoids: they were presumably ciliate like those of Ginkgo and recent Cycads (fig. 396, M). The microspores are approximately 70μ in length and the supposed antherozoids have a maximum diameter of 45μ, the latter being about ⅔ the size of the sperms of Microcycas and ⅙ the diameter of those of Zamia. The smaller and more delicate cells near the lower microspore (fig. 408, D) are no doubt fungal cells as Miss Benson suggests. With reference to the difficulty of determining the nature of Miss Benson’s supposed gametes it is worth calling attention to some figures given by Zopf[166] of vesicular cells and sporangia of the Phycomycetous genera Rhizophidium and Lagenidium in the pollen of Flowering plants and Pines. It has been suggested by Burlingame[167] that the ‘gametes’ may be prothallial cells; but this is very improbable.

Lagenostoma ovoides Williamson.

In the memoir in which the genus was founded Williamson described two species from the Lower Coal Measures of Lancashire, Lagenostoma ovoides and L. physoides[168]. The seeds described under the latter name had previously been assigned by him to another new genus, Physostoma, and named P. elegans[169]. Lagenostoma physoides was afterwards figured by Butterworth[170] who recognised some new features. For this species Prof. Oliver[171] has adopted Williamson’s earlier name, Physostoma elegans. The former species, which has recently received exhaustive treatment by Miss Prankerd[172], agrees generally in its morphological characters with L. Lomaxi, but differs in the structure of the surface-tissue of the integument and in some anatomical features. Moreover no cupules have been found and there is ‘very little trace of a layer of separation’ such as occurs in L. Lomaxi. Over the surface of the integument is a layer of prismatic cells, much shorter and less palisade-like than those in L. Lomaxi, and there are none of the pegs which are a constant feature in that species. There are, however, indications that mucilage was poured out by the rupture of the distended cells. Some microspores were found in the pollen-chamber with an average size of 72 × 53μ; they may be as much as 90μ long. None were observed with sperm-like contents like those described by Dr Benson. Miss Prankerd discusses the morphology of the integument in relation to that of cycadean seeds and makes an instructive comparison between the lagenostome (that is the modified nucellar apex) and such fern sporangia as those of Angiopteris, Osmunda, and Schizaea, but especially the sporangia of Senftenbergia[173] with their multiseriate annulus.

An interesting feature is shown in the longitudinal section reproduced in fig. 493, A (p. 311). The apex of the nucellar cone appears to be composed of thick-walled, dark cells and it is suggested that this may have served as a stopper blocking up the circular orifice of the pollen-chamber (seen below the apex between the nucellar cone and the thick surface-layer of the nucellus) and serving as a protection to the embryo. A comparable closing-up of the micropyle occurs in the seeds of Gnetum Gnemon[174] and in the beak of cycadean seeds. At the time of pollination, when the pollen-chamber must have extended to the apex of the lagenostome, the tip of the nucellar cone may have secreted some sticky substance to which the microspores would adhere.

Prof. Lignier[175] has recently described some large megasporangia from the Westphalian Coalfield of Ostrau in Austrian Silesia which he made the type of a new genus Mittagia, after Herr Mittag, Director of Mines. Two sporangia, between 2 and 3 mm. in diameter, were found in close association as though belonging to a single sorus; one was empty and the other contained four megaspores. The structure of the thick wall of the sporangia is very similar to that of the testa of Lagenostoma Lomaxi, but it apparently split into two valves. Lignier refers the new type, Mittagia seminiformis, to some unknown Palaeozoic group of heterosporous Filicineae, possibly the ancestral stock of the Pteridosperms, and he thinks it probable that the sporangia resembled seeds in their facilities for dispersal. In the structure of the sporangial wall Mittagia also resembles the sporocarp of Pilularia.

Seeds presented as impressions, without internal structure, superficially resembling Lagenostoma.

Lagenospermum Nathorst.

This generic name is adopted for seeds represented by casts or impressions agreeing in external features with Lagenostoma but which on the available evidence cannot be confidently assigned to that genus[176]. Two types of seed were described by Arber[177] from the Lower Coal Measures of Scotland as Lagenostoma Kidstoni and L. Sinclairi: the former has been removed by Oliver[178] to Physostoma and both are included by Arber[179] in a recent paper in the genus Radiospermum. This new generic term is proposed by Arber for a number of small sub-cylindrical seeds founded on impressions including ‘small seeds which, when the structure is preserved, are known as Lagenostoma, Physostoma, and Conostoma.’ The question of nomenclature is invariably raised by cases in which impressions resemble in their superficial characters genera founded on anatomical characters: the seeds originally referred to Lagenostoma Sinclairi afford a good example of this difficulty. Nathorst has recently proposed the generic name Lagenospermum as preferable to Lagenostoma and Radiospermum in the case of Lagenostoma Sinclairi and similar seeds which afford no proof of the possession of such morphological characters as would justify their inclusion in the genus Lagenostoma but which may be examples of that genus. As Nathorst points out, the adoption of Radiospermum for L. Sinclairi is inadvisable on the ground that it is also applied to seeds of a different type. The type-species of Lagenospermum is L. Sinclairi and Nathorst describes additional species from Lower Carboniferous rocks in Spitzbergen.

Lagenospermum Sinclairi (Arber ex Kidston +MS.+).

Although it is not certain that these seeds are morphologically identical with the genus Lagenostoma, a brief description is intercalated here as the habit of the seed-bearing axes supplies a probable key to the habit of the fertile fronds of Lyginopteris. The type-specimens were collected by Mr Sinclair from the Lower Coal Measures of Ayrshire, Scotland, and recorded by Kidston as Lagenostoma sp.: they were afterwards named by him in manuscript L. Sinclairi and handed to Dr Arber for description. The seeds are elliptical-oblong, 4–5·5 × 1·5–3 mm., radially symmetrical and enclosed by a loose envelope which is longitudinally ribbed and divided distally into several linear-lanceolate lobes (fig. 408, A, A′). This covering, though much longer than the cupule of Lagenostoma Lomaxi, is probably a homologous structure. The most interesting point is the attachment of the seeds to slender branches of a compound axis (fig. 408, A). It is probable that the seeds were borne on a frond characterised by the reduction or complete abortion of the sterile lamina or perhaps, as in the recent Fern Thyrsopteris elegans[180], some of the pinnae of a large compound frond were fertile. It is worthy of note that Arber recognised pinnules of Sphenopteris obtusiloba[181] in association with L. Sinclairi, a fact, as he says, in itself of no value but which acquires significance in view of the discovery by Carpentier[182] of cupules in close proximity to the same species of frond. Specimens described by Dr Stopes from Westphalian rocks of New Brunswick as Pterispermostrobus bifurcatus[183] bear a close resemblance to L. Sinclairi.

Lagenospermum oblongum (Kidston).

The species recently described by Dr Kidston[184] as Lagenostoma oblonga from the South Staffordshire coal-field appears to be closely allied to Arber’s L. Sinclairi: it is represented by pairs of seeds borne at the ends of forked branchlets: the seed is 2·5 mm. long by 1·5 mm. broad and is surrounded by a longer oblong cupule divided distally into 6 free lobes.

A larger type of seed, 3 cm. long and 2·5 cm. broad, is described by Kidston[185] from the same coal-field as Lagenostoma? urceolaris. A characteristic feature is the truncate apex surrounded by a prominent canopy formed of the expanded apical free portion of the integument. The lack of anatomical data in both these seeds is a reason for the substitution of some less committal term than Lagenostoma.

Grand’Eury[186] and Carpentier[187] have published accounts of impressions of seeds from the Coal Measures of France compared by them with species of Lagenostoma though not assigned to new species. These and similar seeds should be referred to Nathorst’s genus Lagenospermum.

The difficulty of recognising the true nature of seed-like impressions is illustrated by some specimens in the Goldenberg collection in Stockholm described by Arber[188] as Carpolithus Nathorsti: these consist of pieces of Sphenopteris pinnae probably, as Zeiller suggested, Sphenopteris Schaumburg-Lippeana (Stur) bearing at the ends of the segments of deeply divided pinnules what appeared to be seeds 1 mm. long, oval and longitudinally ribbed, and possibly enclosed in a cupule. Arber considered the ‘seeds’ to be related to Lagenostoma, probably belonging to some member of the Lyginopterideae. An examination of the specimens by Nathorst[189] showed that the supposed seeds are collections of spores; but whether the spores of a true Fern or the microspores of a Pteridosperm cannot be determined.

Pterispermostrobus Stopes.

Pterispermostrobus bifurcatus Stopes.

Dr Stopes[190] has recently called attention to a resemblance between specimens from the Westphalian of New Brunswick, described by her as Pterispermostrobus bifurcatus, and Lagenospermum Sinclairi. The New Brunswick fossil is made the type of a new genus Pterispermostrobus, which is employed for fructifications of Pteridosperms that cannot be associated with a known species of parent and may be either seeds or complex male organs borne on a definitely branched rachis. The type-species is represented by a slender axis bearing lateral branches divided into two widely divergent arms each of which bears a terminal body, 4 × 3 mm., characterised by 3–5 apical lobes extending 2 mm. beyond the distal end of the seed-like organ and resembling a cupule. In this as in many other cases it is impossible to determine the true nature of the reproductive bodies, whether they are small seeds or groups of microsporangia: the new generic name serves a useful purpose though it is not always possible definitely to refer doubtful fructifications of this kind to a Pteridosperm. The organs in question may also be compared with Codonotheca[191].

Pterispermostrobus pusillus (Nathorst).

The name, Codonotheca pusilla, is given by Nathorst[192] to some doubtful specimens from the Culm of Spitzbergen representing short stalks bearing linear-lanceolate scale-like bodies, 9–10 mm. long by 1 mm. broad, coalescent at the base. Nathorst compares them with Sellard’s species, Codonotheca caduca, but adds that they may be cupules of some Pteridosperm and calls attention to their resemblance to some fossils figured by Carpentier as Calymmatotheca acuta. Both Nathorst’s species and the French specimens described by Carpentier[193] as cupules may be referred to Dr Stopes’ genus Pterispermostrobus as their morphological nature cannot be determined.

+v.+ Roots.

In 1876 Williamson[194] described some petrified vegetative organs from the Lower Coal Measures of Lancashire under the name Kaloxylon Hookeri characterised by a division of the secondary xylem into cuneate masses (fig. 415, C) like those in some recent Bignoniaceous stems. Williamson at first believed Kaloxylon to be a stem, but in a later memoir he expressed the opinion that ‘it is difficult to believe that these organs have been other than roots’[195]. Felix[196] had meanwhile described a specimen from the Coal Measures of Westphalia as Kaloxylon cf. Hookeri and suggested that it might be a waterplant. In 1894 Williamson and Scott[197] demonstrated that Kaloxylon Hookeri is the root of Lyginopteris, a conclusion independently reached by Hick[198].

The roots of Lyginopteris arise on all sides of the radially symmetrical stem in the pericycle region; they are copiously branched as is shown by the abundance of roots of various sizes in close association. No roots have been discovered exceeding 1 cm. in diameter nor have any been recorded with secondary periderm-tissue. The absence of root-hairs and the more or less lacunar structure of the cortex are indicative of swampy ground. It is seldom that the palaeobotanist has an opportunity of investigating the growing-points of Palaeozoic plants, and for this reason some well-preserved apices of Lyginopteris roots, attributed to that genus on the ground of constant association with fragments of stems in the calcareous nodules of Dulesgate, are of special interest. One of these specimens was figured by Dr Stopes and Mr Watson[199] in their account of plant-bearing nodules, and a description of that and other examples has since been published by Prof. Weiss[200]. A longitudinal section of a root-tip, ·21 mm. in diameter at its broadest part, shows a root-cap which suggests an origin from a single cell, but an examination of the plerome-cylinder in a slightly tangential section does not afford conclusive evidence of the occurrence of a single initial cell. Weiss on the whole inclines to the view that Lyginopteris possessed a single apical cell like the roots of Leptosporangiate Ferns, though he prefers to leave the decision open. Attention is drawn to the fact that the plate of tracheidal tissue in emerging lateral rootlets is vertical as in Phanerogams and not horizontal as in recent Pteridophyta.

The vascular tissue of a Lyginopteris root (fig. 410) consists of from three to eight alternate strands of centripetal xylem and phloem, and with the metaxylem is associated a small amount of conjunctive parenchyma which does not form a central pith. The pericycle, one to several layers broad, is succeeded by an endodermis which occasionally shows the characteristic thickenings on the radial walls. A broad cortex of thin-walled lacunar tissue with numerous secretory cells is bounded externally by a superficial cylinder of two or more layers of comparatively large and thin cells, the outermost of which are radially elongated. This superficial tissue forms a striking feature by which a Lyginopteris root may often be recognised at a glance. The root represented in fig. 410, approximately 2 mm. in diameter, has a heptarch stele divided into seven xylem-groups by crushed bands of parenchyma and a protoxylem strand occupies the apex of each projecting angle (fig. 410, px). The superficial cylinder of clear cells is seen at a. A very small root is seen at r in fig. 410.

Fig. 415, C, represents part of an older root in which the pentarch primary xylem is enclosed by broad wedge-like groups of secondary xylem and phloem separated by conspicuous medullary rays opposite the protoxylem strands (px). Crushed primary phloem arcs, p, are often clearly recognisable beyond the cambium. The secondary thickening, as Williamson and Scott state, ‘takes place exactly in the manner typical of roots of Dicotyledons, so that this fossil might very well be used for purposes of demonstration as illustrating the secondary growth of a root with diagrammatic clearness’[201]. The young roots of Lyginopteris resemble in many respects those of Marattiaceous Ferns, though the presence of a single apical cell, if such occurs, is a distinguishing feature; but in the presence of secondary conducting tissue they agree with those of Phanerogams.

Distribution of Lyginopteris.

The frequency with which petrified fragments of Lyginopteris stems occur in the calcareous nodules of the English coal seams shows that the genus must have been plentifully represented in the Upper Carboniferous vegetation, and the occurrence in both North American[202] and European localities of fronds identical with or closely resembling Sphenopteris Hoeninghausi affords evidence of wide geographical range. Petrified specimens were recorded by Felix[203] from Westphalia in 1886, and Zalessky[204] has recently discovered Lyginopteris in the Donetz coal-basin of Russia. An investigation by Kubart[205] of the calcareous nodules, to which attention was first drawn by Stur, in the Ostrau Coal Measures led to the discovery of several examples of Lyginopteris stems. The descriptions and figures so far published are hardly sufficient to enable us to estimate the degree of relationship to the English type, but some of the stems appear to be new species and Kubart considers them all to be specifically distinct from Lyginopteris oldhamia. Lyginopteris heterangioides contains scattered tracheids in the pith and thus affords an interesting transitional type between Lyginopteris and Heterangium. In L. lacunosum the inner cortex is lacunar and the primary xylem bundles pursue an independent course in the stele in contrast to the anastomosing arrangement in L. oldhamia and in another Hungarian species L. tristichum. The species recorded by Kubart occur in the Millstone grit and the Coal Measures.

The geological range of Lyginopteris as represented by petrified stems does not extend beyond the limits of the Carboniferous system.

=HETERANGIUM.=

The generic name Heterangium was first used by Corda[206] for a piece of stem from the Coal Measures of Radnitz, Bohemia, represented by part of the vascular axis of a stem consisting of strands of large reticulately pitted tracheids intermixed with parenchyma and exhibiting structural features differing apparently from those of any known type. Corda’s material has been re-examined by Kubart[207] who figures a section from it. Heterangium is a genus closely allied to Lyginopteris both in habit and in general anatomical characters. The stem is monostelic; the vascular cylinder prior to secondary thickening resembles the protostele of certain recent species of Gleichenia and may be compared also with Trichomanes scandens[208]. It agrees with that of Lyginopteris in the possession of primary mesarch bundles but differs in the substitution of a cauline axial mass of metaxylem for the pith of Lyginopteris. The secondary vascular tissue agrees closely with that of recent Cycads and Lyginopteris. A characteristic feature is the occurrence of numerous horizontal bands of sclerous cells in the cortex (fig. 412) of the stem and in the ground-tissue of the rachis and larger branches of the fronds. The stem was erect and rarely branched ‘giving off large foliar appendages at somewhat distant intervals and from its entire circumference’[209]. Our knowledge of the reproductive organs is less precise than in the case of Lyginopteris; but we are justified in asserting that Heterangium is a Pteridosperm which in all probability bore fern-like microsporangia and seeds similar in general plan to Lagenostoma.

The association of some seeds included in Williamson’s genus Conostoma with Heterangium Grievii in the Pettycur beds and their resemblance to Lagenostoma, the seed of Lyginopteris, suggested the possibility of actual connexion: further evidence in support of this view has recently been brought forward by Dr Benson[210] in the case of a species of Conostoma which she transfers to a new genus Sphaerostoma.

The two species Heterangium Grievii and H. tiliaeoides are described in illustration of the genus and reference is made to a few other types.

Heterangium Grievii Williamson.

(Stem.)

1872. Dictyoxylon Grievii, Williamson, Brit. Ass. Rep. (Edinburgh Meeting), p. 112.

1873. Heterangium Grievii, Williamson, Phil. Trans. R. Soc. Vol. 162, p. 404.

(Leaf.)

[1720. Fumaria officinalis, Volkmann, Silesia subterranea, p. 111, Pl. +xiv.+ fig. 2.]

1822. Filicites (Sphenopteris) elegans, Brongniart, Class. Vég. p. 233.

1828. Sphenopteris elegans, Brongniart, Hist. Vég. Foss. p. 172.

1836. Cheilanthites elegans, Goeppert, Foss. Farnkr. p. 233.

1877. Diplothmema elegans, Stur, Culm Flora, +ii.+ p. 130.

i. Stem.

At the Edinburgh meeting of the British Association Williamson[211] gave a brief account of some petrified stems from the Lower Carboniferous strata of Burntisland on the Firth of Forth which he included in the genus Dictyoxylon. One of these was named D. Grievii after Mr Grieve the discoverer of the specimens. In a later and more complete description Williamson adopted Corda’s generic name on the ground of the close resemblance of the Scotch stem to the Bohemian fragment Heterangium paradoxum. In 1873[212] Williamson added new facts in regard to H. Grievii and in 1890[213] he described a very closely allied type from the Lower Coal Measures of Lancashire. Five years later his descriptions were considerably extended and modified in the joint memoir with Dr Scott[214].

The great difference in age between the English Upper Carboniferous stem and the Scotch specimens from the Lower Carboniferous beds of Burntisland suggests a probable specific difference. Dr Scott has recently adopted the name Heterangium Lomaxi, proposed but not published by Williamson, for the English type. Though in the following account the species Heterangium Grievii is treated in the broader sense it should be recognised that the geologically younger stem is worthy of specific recognition; it is characterised, to quote Scott[215], by ‘the great distinctness of the primary xylem strands, by their nearly exarch structure, with little primary centrifugal wood, by the abundant secretory sacs of the stele, and by the rather scattered leaves.’

Heterangium Grievii has a radially symmetrical stem bearing compound leaves with decurrent petioles which give to the otherwise cylindrical axis an angular outline as seen in transverse section (fig. 411, A). The phyllotaxis appears to be ⅜. The stem rarely exceeds 1·5 cm. in diameter: in the centre is a comparatively large stele consisting in young stems of primary xylem and phloem, but in older stems these are separated by a cylinder of secondary vascular tissue which in this species is always narrower than in Lyginopteris oldhamia and, as Williamson pointed out, often of unequal thickness on different radii. The medullated stele of Lyginopteris is replaced by a solid xylem-cylinder consisting mainly of groups of large tracheids, reaching ·3 mm. in diameter, with multiseriate bordered pits (fig. 411, D) embedded in an anastomosing parenchymatous tissue-system. In the stele reproduced in fig. 411, B, which with the exception of a very narrow zone of secondary xylem, _x_², consists entirely of primary xylem, _x_¹, the parenchyma is represented by a darker reticulum (cf. fig. 415, B) dividing the metaxylem into islands as in Gleichenia. In the peripheral portion of the xylem the tracheids are rather narrower and arranged in more definite groups in many of which is a single strand of narrow spiral elements (fig. 411, A′, px) close to the outer margin. These peripheral primary bundles in which protoxylem is recognisable may be described as leaf-traces of mesarch structure consisting of centripetal xylem and, to a much less extent, of smaller centrifugal elements for the most part with dense spiral bands in place of the multiseriate pits of the rest of the metaxylem. The structure of these leaf-traces is practically identical with that of the primary bundles of Lyginopteris. There is, however, a difference to which attention is drawn by Williamson and Scott. While in Lyginopteris in any transverse section the primary bundles in the stele are equal in number to the leaf-traces in the pericycle and cortex, in Heterangium the peripheral groups in the stele may be as many as twenty, a number considerably in excess of the leaf-traces beyond the limits of the primary xylem of the stele. It may be that the leaf-trace of each leaf, which joins the stele at a distance of 6–10 internodes below its entrance into the cortex from the leaf-stalk, may branch in its descent in the axial region, or some of the primary groups of xylem may be confined to the axial region and independent of the leaf-traces. Portions of the peripheral region of the stele may be occupied by metaxylem groups without protoxylem and identical with those which make up the bulk of the metaxylem.

Scott[216] has recently published a note in which he states that most of the British Coal Measures Heterangiums were polydesmic. Two bundles, and not a single strand as in the Scotch H. Grievii, leave the stele for each leaf, and these divide into four, in some cases at least, before entering the petiole.

The secondary xylem is continuous at its inner edge with the outermost primary tracheids (fig. 411, A′) and consists of rows of tracheids, 1–3 elements broad, alternating with numerous broad medullary rays of radially elongated parenchyma. Beyond a typical cambium-zone the secondary phloem consists of parenchyma and sieve-tubes bounded by crushed arcs of primary phloem. Abutting on the phloem is a pericycle composed of several layers of small parenchymatous cells (fig. 411, A, p) and in the outer layers of this tissue a phellogen (fig. 411, C, p) and some periderm are usually present though, as Williamson and Scott point out, the periderm is less regular and narrower than in Lyginopteris. The inner cortex, composed of short parenchymatous cells, is traversed by numerous narrow bands of dark, thick-walled cells similar in the structure of the elements, though peculiar in the horizontal elongation of the groups, to the sclerous nests in the pericycle and pith of Lyginopteris. These characteristic bands are chiefly seen in the oblique longitudinal section of a stem represented in fig. 412. In this section, 25 mm. in length, the lighter band, p, is the pericycle and in it a few obliquely cut leaf-traces are shown as dark patches. The horizontal bands are similar in structure and shape to the diaphragms of thick cells in the pith of Abies magnifica[217], and in both plants they probably serve as supports to the softer parenchyma. There may be as many as 46 bands in a vertical length of cortex of 1 inch (about 19 per centimetre). It was the occurrence of precisely similar transverse lines on the carbonised impressions of the rachis of Sphenopteris elegans that led Kidston[218] to suggest a connexion between that species and the stem of Heterangium Grievii.

The outer cortex, consisting of alternate strands of parenchyma and stereome similar to that of Lyginopteris, is much narrower and a less conspicuous feature than in Heterangium; the stereome bands do not form so regular a hypodermal network and extend much further vertically without anastomosing. The epidermis has been described as a layer of fairly thick cells showing in one case an appearance of a depressed stoma[219]. There are no secretory canals like those of Cycads but, as in Lyginopteris, scattered cells with dark contents in the stem-tissues probably represent secretory sacs.

The leaf-traces on leaving the stele pursue a very gradually ascending course to the petiole; they retain their collateral structure in the pericycle and cortex and have no secondary xylem, but become concentric as they enter the base of a leaf.

Before passing to the description of the leaves, the more striking features in the stem may be summarised with reference to the diagrammatic sketches shown in fig. 411. Fig. 411, A, represents a section of Heterangium Grievii approximately 2 cm. in its maximum diameter; at the periphery of the primary xylem, _x_¹, and close to its outer margin are several protoxylem groups, not shown in the drawing, each of which marks the position of a mesarch trace. The zone of secondary xylem, _x_², is interrupted by the exit of leaf-traces and one of these is seen at a in fig. 415, A, separated from the central primary xylem by a foliar gap filled with parenchyma. The pericycle is shown at p in fig. 412 and its outer boundary at p in fig. 411, A. Beyond the pericycle is the broad parenchymatous cortex with leaf-traces, lt, and some sclerenchymatous patches, s. The vascular strand a is passing into the base of a leaf-stalk. In the stem shown in fig. 411, B, 1·4 cm. × 7 mm., a decurrent petiole is seen at the upper end with its single vascular strand, a, and two sclerous nests; a similar though detached leaf-base occurs at the opposite end of the long diameter. Other leaf-traces are seen at b and c. From the left-hand side of the stele a curved strand of tracheids is passing out to supply a root, r.

ii. Root.

Adventitious roots of endogenous origin are occasionally met with in Heterangium stems, but we have less information as to their anatomy than in the case of Lyginopteris. In a specimen of Heterangium Lomaxi figured by Williamson and Scott[220] three roots are seen in a vertical series growing outwards through the cortex of a stem. The roots agree generally with those of Lyginopteris but the outermost cortical layers possess no special features.

iii. Leaf.

The large compound fronds long known as Sphenopteris elegans were recognised by Kidston as the leaves of Heterangium by the closely arranged transverse striae or narrow ribs on the rachis and pinnae which are the expression on the carbonised impressions of the horizontal plates of sclerous tissue in the petrified stems and petioles of Heterangium. The dichotomously branched fronds are included by Stur in his genus Diplotmema and that author figures several typical examples in his ‘Culm Flora’[221]. Fig. 413, A, shows a forked axis with the bases of more slender branches and the characteristic transverse bands and in fig. 413, B, part of a pinna is reproduced. In general appearance, except in the bifurcating pinnae, the fronds resemble those of Davallia tenuifolia with which Brongniart compared the Palaeozoic species. There is little doubt that Sphenopteris dissecta and some other species were also borne on Heterangium stems. The rachis and petioles differ from those of Lyginopteris fronds in the absence of emergences (cf. fig. 404, E). The petioles (fig. 411, B, a) have a single concentric vascular bundle with internal protoxylem.

iv. Reproductive organs.

As yet no satisfactory evidence has been published with regard to the nature of the microsporangia but in all probability these were constructed on the same plan as those of Lyginopteris. There is a strong prima facie case for assigning the seed Sphaerostoma to Heterangium: absolute proof of organic connexion is still lacking though Dr Benson’s recent account of the seeds associated with Heterangium Grievii almost amounts to demonstration of continuity between vegetative organs and seeds.

Sphaerostoma ovale (Williamson).

In 1877 Williamson described some detached petrified seeds from the Lower Carboniferous rocks of Fifeshire, Scotland, as Conostoma ovale and C. intermedium. Dr Benson’s investigation[222] of these two forms leads her to confirm Williamson’s doubts as to the validity of a specific separation and she assigns the single type to the new genus Sphaerostoma. The seeds are always associated with the vegetative organs of Heterangium Grievii. In 1909 Oliver[223] expressed the opinion that the Burntisland species of Conostoma (= Sphaerostoma) is probably the seed of Heterangium. The seed consists of a central body representing the nucellus, an inner integument, and an enveloping cupule or outer integument: most specimens have lost the cupule and in this condition they are 3·5 mm. long with a maximum breadth of 2·2 mm. In the middle the seed is circular in transverse section and octagonal near the base and apex. The free apical part of the integument forms a frill (canopy) round the micropyle and extends beyond the nucellar apex which consists of a relatively flat plinth surmounted by a central dome or lagenostome (fig. 414). The lagenostome is surrounded by an annular pollen-chamber on to the lower surface of which abuts the large embryo-sac, and remains of archegonia were noticed below the pollen-chamber. The roof of the chamber in the young state consists of a layer of thin-walled cells extending across the flattened apex of the nucellus, n, but as the pollen-chamber becomes differentiated from the nucellar tissue by the disorganisation of the zone of cells its roof-cells thicken their vertical walls and assume the structure of a multiseriate annulus, which acts as a mechanism for opening the pollen-chamber by a circular dehiscence in such a way that the edge of the ruptured roof of the pollen-chamber slightly overlaps the periphery of the central column of nucellar tissue after it has returned to its original position subsequent to the entrance of the microspores. The micropylar region is surrounded by eight lobes of the integument and each is characterised by a crest of radially elongated cells, fig. 414, f, especially prominent on the outer side. External to this is the slightly longer cupular sheath (fig. 414, c) which may also have been lobed. The surface of the integument below the terminal crests consists of a layer of cells with small papillae which eventually ruptured and discharged mucilage. Both integuments have a vascular supply, that of the inner integument being represented by eight vascular bundles, some of which were found to have mesarch xylem, given off from the single strand in the pedicel. Fig. 414 shows the apical region of a seed of Sphaerostoma: the flat-topped nucellar cap, n, is surrounded by the annular pollen-chamber, pc, below which are indicated the archegonia: the wall of the megaspore (embryo-sac) is seen at m and external to this vascular bundles, v, run up the inner portion of the integument accompanied by some large cells (aqueous tissue). The elongated epidermal cells at the apices of the lobes of the integument form the frill, f, and at a lower level the cells of the same layer are much smaller and papillate (e): the outer integument, c, forms the so-called cupule. The transverse section shown in fig. 414, B, is taken at the level of the roof of the pollen-chamber and of the nucellar cap; it illustrates the contrast between the ‘multiseriate annulus’ and the central column of small parenchyma.

Sphaerostoma differs from Lagenostoma in the whorl of crests around the micropyle, in the nearly hemispherical form of the lagenostome and in the relatively wider pollen-chamber with its peculiar form of dehiscence. Miss Benson, while regarding Sphaerostoma as similar to Lagenostoma in general plan, believes the distinguishing features of the former to be such as are consistent with a more primitive form.

An important argument in support of connecting this seed with Heterangium is derived from the juxtaposition of some seeds and portions of Heterangium petioles, a juxtaposition that is believed to demonstrate original continuity.

Grand’Eury[224] has recorded the association of two species of leaves, Sphenopteris elegans and S. dissecta, with small seeds compared by him with Lagenostoma. In the absence of petrified specimens it would be practically impossible to distinguish between Lagenostoma and Conostoma or Sphaerostoma.

Carpentier[225] has described some impressions from French Westphalian beds as Conostoma and he records cupules without seeds on fronds of Sphenopteris obtusifolia which he speaks of as having transverse striations like those of Heterangium. Dr Kidston pointed out to me that the surface-features of the Sphenopteris rachis are probably due to ramental scales and not to the presence of horizontal sclerous bands. Carpentier’s seeds may be compared with Lagenospermum Sinclairi.

Heterangium tiliaeoides Williamson, Phil. Trans. R. Soc. Vol. 178, p. 289.

This species, founded by Williamson on material from the Lower Coal Measures of Halifax, Yorkshire, while agreeing in the structure of the primary stele and in the general features of the cortex with the older Heterangium Grievii, is clearly distinguished by certain well-marked characters. Sclerous groups occur in the inner cortex as in H. Grievii but they are present also in the pericycle. The peripheral leaf-traces in the stele show the mesarch structure rather more distinctly than in H. Grievii, and the secondary xylem, which forms a much broader cylinder than in the Scotch type, is divided by broad medullary rays into characteristic cuneate masses each of which rests at its base on the centrifugal tracheids of a leaf-trace strand of xylem (fig. 415, B). The most striking distinctive feature is afforded by the secondary phloem, which is often preserved in wonderful perfection; this is unusually thick and owing to the tangential expansion of the principal medullary rays the secondary phloem is divided into separate masses which decrease in breadth towards the external arcs of primary phloem. The triangular form of the phloem rays, composed of tangentially stretched parenchyma, suggested the specific name tiliaeoides on account of their striking resemblance to the rays of Tilia. The leaf-traces are nearly always in pairs as they pass out through the cortex; they subsequently divide and appear as four vascular strands in the petiole. The portion of stem reproduced in fig. 415, B, 8 mm. broad, shows clearly the separation of the secondary xylem and phloem into wedge-shaped groups: in each group there are several narrow medullary rays. The extrastelar tissues are represented by a few fragments only. Several layers of crushed periderm occur in the pericyclic region but the more external tissues have been almost completely exfoliated[226].

Reference has already been made to Heterangium Lomaxi, the English type originally included by Williamson in Heterangium Grievii. The provisional species Heterangium cylindricum Williamson and Scott[227] differs, as Scott says, in no important respect from H. Lomaxi and should not be retained. A new species, H. minimum Scott[228], has been founded on a very small stem from the Coal Measures of Dulesgate in which the leaf-traces leave the stele as single bundles as in the Scotch H. Grievii.

The French species Heterangium Duchartrei[229] Ren. from Permian rocks was originally referred by Renault to the genus Poroxylon: it is represented by little more than the xylem of the stele and bears a close resemblance to H. tiliaeoides. Heterangium punctatum Ren. and H. Renaulti[230] (Brongn.) also from the Permian of France were originally placed in the genus Lycopodium and afterwards recognised as stems of Heterangium. A fourth French Permian species, H. bibractense[231], is peculiar in the possession of a very small primary stele encircled by deep wedges of secondary xylem, but without more information it is impossible to speak with confidence as to its systematic position. Kubart[232] has recently published brief descriptions of some stems from the Ostrauer coal-basin in Moravia all of which he regards as specifically distinct from the English types. In Heterangium Sturi the primary xylem is almost exarch and the peripheral xylem groups are not very clearly defined: in H. alatum, so called from the presence of lateral wings on the petioles, the leaf-trace strands are more sharply differentiated from the rest of the stele. H. polystichum is a similar type, and H. Andrei, with a relatively larger amount of parenchyma in the stele and thicker stems forms an additional link between Heterangium and Lyginopteris[233]. Prof. Johnson[234] has described a species of Heterangium, H. hibernicum, from Upper Devonian and Lower Carboniferous beds in Co. Cork, Ireland, based on some impressions of frond fragments without any pinnules. The occurrence of numerous transverse striae on the rachis and lateral branches suggests comparison with Heterangium fronds, but an examination of the specimens led me to suspect that some at least of the striae are cracks and not original features. The presence of spur-like appendages from the lower surface of the pinnae near their origin from the rachis is recorded as a peculiar character, and some obscure oval bodies, the nature of which is extremely doubtful, are considered to be seeds. The imperfection of the material hardly justifies the institution of a new species of Heterangium.

Heterangium ranges from the Lower Carboniferous to the Permian strata and is thus older than Lyginopteris which in the form of petrified stems is not recorded from the Lower beds of the Carboniferous system. Heterangium has been described as having a ‘great preponderance of fern-like characters,’ but having regard to the resemblance of the primary xylem of the latter to that of the Osmundaceae it would seem doubtful whether in their relation to the Ferns there is any important difference. Heterangium may safely be spoken of as the more primitive genus. The polydesmic character of the petioles of most species is particularly interesting as it brings the genus nearer to the Medulloseae and to Rhetinangium[235].

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