FOSSIL PLANTS
CAMBRIDGE UNIVERSITY PRESS
~London~: FETTER LANE, E.C.
C. F. CLAY, MANAGER
[Illustration]
~Edinburgh~: 100, PRINCES STREET
~London~: H. K. LEWIS, 136, GOWER STREET, W.C.
~Berlin~: A. ASHER AND CO.
~Leipzig~: F. A. BROCKHAUS
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~Bombay and Calcutta~: MACMILLAN AND CO., LTD.
_All rights reserved_
[Illustration: Part of a transverse section of a Permian Osmundaceous
Fern stem, _Thamnopteris Schlechtendalii_ (Eichwald). _a_, outer
xylem; _b_, inner xylem. For description, see page 329. (After
Kidston and Gwynne-Vaughan. Very slightly reduced.)]
FOSSIL PLANTS
A TEXT-BOOK FOR STUDENTS
OF BOTANY AND GEOLOGY
BY
A. C. SEWARD, M.A., F.RS.
PROFESSOR OF BOTANY IN THE UNIVERSITY; FELLOW OF ST JOHN’S
COLLEGE AND HONORARY FELLOW OF EMMANUEL COLLEGE, CAMBRIDGE
WITH 265 ILLUSTRATIONS
VOL. II
CAMBRIDGE:
AT THE UNIVERSITY PRESS
1910
~Cambridge~:
PRINTED BY JOHN CLAY, M.A.
AT THE UNIVERSITY PRESS.
PREFACE
I regret that pressure of other work has prevented the completion of
this Volume within a reasonable time since the publication of Volume
I. Had Volume II been written ten years ago, the discoveries made in
the course of the last decade would have given an out-of-date character
to much of the subject-matter. It is more especially in regard to the
Ferns and the extinct members of the Gymnosperms that our outlook has
been materially altered by recent contributions to Palaeobotany. It is,
however, some satisfaction to be able to add that recent progress has
been relatively slight in that part of the subject dealt with in the
first volume.
The original intention was to complete the whole work in two volumes.
Soon after the second volume was begun, it became evident that the
remaining divisions of the plant-kingdom could not be included within
the compass of a single volume. I decided, therefore, to take the
consequences of having embarked on too ambitious a plan of treatment,
and to preserve uniformity of proportion by reserving the seed-bearing
plants for a third volume. The third volume will include the
Pteridosperms, other than those briefly described in the final chapter
of the present volume, and other classes of Gymnosperms. I propose also
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. It is my intention to
complete Volume III with as little delay as possible. As I have written
elsewhere, the past history of the Flowering plants needs special
treatment, and anything more than a mere compilation can be adequately
attempted only after considerable research and with the assistance
of botanists possessing a special knowledge of different families of
Angiosperms. The need of a critical examination of available data in
regard to the geological history of this dominant group will not be
lost sight of.
I am well aware that while certain genera have received an undue share
of attention in the present volume, others have been ignored or treated
with scant consideration. For this inconsistency I have no excuse
to offer, beyond the statement that the subject is a large one, and
selection is necessary even though the work consists of three volumes.
The publication in 1909 of a collection of excellent photographs of
Palaeozoic Plants, with brief descriptive notes, by Mr Newell Arber, as
one of a series of popular “Nature Books,” bears striking testimony to
the remarkable spread of interest in the study of the vegetation of the
past, which is one of the outstanding features in the recent history of
botanical science.
In the list of illustrations I have mentioned the source of all figures
which have been previously published. I would, however, supplement the
statement of fact with an expression of thanks to corporate bodies and
to individuals who have allowed me to make use of blocks, drawings, or
photographs.
I wish to thank my colleague, Mr A. G. Tansley, for placing at my
disposal several blocks originally published in the pages of the _New
Phytologist_. To Professor Bertrand of Lille and to his son Dr Paul
Bertrand I am indebted for several prints and descriptive notes of
specimens in their possession. My friends Dr Nathorst of Stockholm
and Dr Zeiller of Paris have generously responded to my requests for
information on various points. I wish especially to thank Dr Kidston
for several excellent prints of specimens in his collection and for the
loan of sections. I have profited by more than one examination of his
splendid collection at Stirling. Professor Weiss has generously allowed
me to borrow sections from the Manchester University collections, more
especially several which have been reproduced in the chapter devoted
to the genus _Lepidodendron_. To Professor F. W. Oliver my thanks are
due for the loan of sections from the collection under his charge at
University College. I have pleasure also in thanking Dr Scott, not
only for lending me sections of a Lepidodendron and for allowing me
to use some drawings of _Miadesmia_ originally made by Mrs Scott for
reproduction in his invaluable book, _Studies in Fossil Botany_, but
for kindly undertaking the laborious task of reading the proofs of this
volume. It would be unfair to express my gratitude to Dr Scott for many
helpful suggestions and criticisms, without explicitly stating that
thanks to a friend for reading proofs must not be interpreted as an
attempt to claim his support for all statements or views expressed. The
General Editor of the Series, Mr A. E. Shipley, has also kindly read
the proofs. I am under obligations also for assistance of various kinds
to Prof. Thomas of Auckland, New Zealand, to Mr Boodle of Kew, to Mr
D. M. S. Watson of Manchester, to Mr T. G. Hill of University College,
and to Mr Gordon of Emmanuel College, Cambridge. I am indebted to the
kind offices of Miss M. C. Knowles for the photograph of the specimen
of _Archaeopteris hibernica_ in the Irish National Museum, Dublin,
reproduced on page 561.
Many of the illustrations are reproduced from drawings by my wife:
those made from the actual specimens are distinguished by the addition
of the initials M. S. I am grateful to her also for some improvements
in the letter-press. For the drawings made from sections and for some
of the outline sketches I am responsible. I have availed myself freely
of the facilities afforded by Professor McKenny Hughes in the Sedgwick
Museum of Geology for the examination of specimens under the charge of
Mr Newell Arber, the University Demonstrator in Palaeobotany. It is
a pleasure to add that, as on former occasions, I am indebted to the
vigilance of the Readers of the University Press for the detection of
several errors which escaped my notice in the revision of the proofs.
A. C. SEWARD.
BOTANY SCHOOL, CAMBRIDGE.
_March_ 12, 1910.
TABLE OF CONTENTS
CHAPTER XII
=SPHENOPHYLLALES= (continued from Volume I.). Pp. 1–16.
PAGE
_Sphenophyllum_ 1–7
_Cheirostrobus_ 7–12
=Sphenophyllales and Psilotaceae= 12–16
CHAPTER XIII
=PSILOTALES.= Pp. 17–29.
_Psilotum_ and _Tmesipteris_ 17–24
Fossils described by authors as being closely allied to
_Psilotum_ 24–26
_Psilophyton_ 26–29
CHAPTER XIV
=LYCOPODIALES.= Pp. 30–91.
=Recent Lycopodiales= (General) 30–33
Lycopodiaceae (Recent) 33–49
Selaginellaceae (Recent) 49–58
Isoetaceae (Recent) 58–66
=Fossil Lycopodiales= 66–91
Isoetaceae (Fossil) 66–68
_Pleuromeia_ 68–73
Herbaceous fossil species of Lycopodiales 73–91
_Lycopodites_ 76–84
_Selaginellites_ 85–88
_Lycostrobus_ 88–91
_Poecilitostachys_ 91
CHAPTER XV
=ARBORESCENT LYCOPODIALES.= Pp. 92–195.
=Lepidodendron= 93–181
i. General 93–97
ii. Leaves and Leaf-cushions 97–105
iii. _Lepidophloios_ 105–109
iv. The anatomy of _Lepidodendron vasculare_ 109–123
v. Lepidodendron stems as represented by casts and
impressions of partially decorticated specimens 123–128
_a._ _Knorria_ 124–126; _b._ _Bergeria_ 126, 127;
_c._ _Aspidiaria_ 127, 128.
vi. Lepidodendroid axes known as _Ulodendron_ and
_Halonia_ 128–139
_a._ _Ulodendron_ 128–135; _b._ _Halonia_ 135–139.
vii. Anatomical characters of vegetative Lepidodendron
shoots 139–181
1. _Lepidodendron esnostense_ 139, 140; 2. _L.
rhodumnense_ 140; 3. _L. saalfeldense_ 141;
4. _L. fuliginosum_ 141–160; 5. _L. Harcourtii_
160–163; 6. _L. Wünschianum_ 163–171; 7. _L.
macrophyllum_ 171; 8. _L. Veltheimianum_
171–177; 9. _L. Pedroanum_ 177, 178; 10. _L.
australe_ 178–181.
viii. Fertile shoots of Lepidodendreae 181–195
A. _Lepidostrobus_ 181–191. i. _Lepidostrobus
variabilis_ 187, 188; ii. _L. oldhamius_
188–190; iii. _L. Brownii_, etc. 190, 191.
B. _Spencerites_ 192–195.
CHAPTER XVI
=SIGILLARIA.= Pp. 196–226.
i. General 196–210; ii. Leaves 210–215; iii. Fertile
shoots 215–218; iv. The structure of Sigillarian
stems 218–224; v. _Sigillaria Brardi_ 224–226.
CHAPTER XVII
=STIGMARIA.= Pp. 227–247.
CHAPTER XVIII
=BOTHRODENDREAE.= Pp. 248–270.
_Bothrodendron_ 248–264. _a._ _B. minutifolium_ 251–253;
_b._ _B. punctatum_ 254, 255; _c._ _B. kiltorkense_
255–259. Anatomy of vegetative shoots of
_Bothrodendron_ 260–262; Cones of _Bothrodendron_
262–264.
_Pinakodendron_ 264
_Omphalophloios_ 264–266
General considerations 266–270
CHAPTER XIX
=SEED-BEARING PLANTS CLOSELY ALLIED TO
MEMBERS OF THE LYCOPODIALES.= Pp. 271–279.
i. _Lepidocarpon_ 271–275; ii. _Miadesmia_ 275–279.
CHAPTER XX
=FILICALES.= Pp. 280–323.
I. =Leptosporangiate Filicales= 283–316
Eufilicineae 284–316. Osmundaceae 285, 286;
Schizaeaceae 286–291; Matonineae 291–293; Loxsomaceae
293; Hymenophyllaceae 294; Cyatheaceae 294–296;
Dennstaedtiinae 296; Polypodiaceae 296; Parkeriaceae
297; Dipteridinae 298.
The habit, leaf-form, and distribution of ferns
300–309; The anatomy of ferns 309–316.
II. =Marattiales= 316–321
III. =Ophioglossales= 321–323
CHAPTER XXI
=FOSSIL FERNS.= Pp. 324–394.
=Osmundaceae= 324–346; =Schizaeaceae= 346–351; =Gleicheniaceae=
351–355; =Matonineae= 355–363; =Hymenophyllaceae= 363–365;
=Cyatheaceae= 365–375; =Polypodiaceae= 375–380; =Dipteridinae=
380–394.
CHAPTER XXII
=MARATTIALES (FOSSIL).= Pp. 395–411.
_Ptychocarpus_ 397; _Danaeites_ 398; _Parapecopteris_ 398;
_Asterotheca_ 398–400; _Hawlea_ 400; _Scolecopteris_ 401, 402;
_Discopteris_ 402–404; _Dactylotheca_ 404–406; _Renaultia_ 406;
_Zeilleria_ 407; _Urnatopteris_ 407; _Marattiopsis_ 407–409;
Danaeopsis 409; Nathorstia 410, 411.
CHAPTER XXIII
=PSARONIEAE.= Pp. 412–426.
CHAPTER XXIV
=OPHIOGLOSSALES (FOSSIL).= Pp. 427–431.
CHAPTER XXV
=COENOPTERIDEAE.= Pp. 432–472.
I. Botryoptereae 434–443
II. Zygoptereae 443–470
CHAPTER XXVI
=HYDROPTERIDEAE AND SAGENOPTERIS.= Pp. 473–483.
Marsiliaceae 473–475; Salviniaceae 475–477; _Sagenopteris_ 477–483.
CHAPTER XXVII
=GENERA OF PTERIDOSPERMS, FERNS, AND=
_PLANTAE INCERTAE SEDIS_. Pp. 484–580.
_Taeniopteris_ 485–494; _Weichselia_ 494–496; _Glossopteris_
496–512; _Gangamopteris_ 512–517; _Lesleya_ 517–519;
_Neuropteridium_ 519–523; _Cardiopteris_ 523–525; _Aphlebia_
525–529; _Sphenopteris_ 529–532; _Mariopteris_, _Diplotmema_,
_Palmatopteris_ 532–537; _Cephalotheca_ 537; _Thinnfeldia_
537–544; _Lomatopteris_ 544–546; _Cycadopteris_ 546;
_Ptilozamites_ 546–548; _Ctenopteris_ 548–550; _Dichopteris_
550–552; _Odontopteris_ 552–556; _Callipteris_ 557–559;
_Callipteridium_ 560; _Archaeopteris_ 552–565; _Neuropteris_
565–571; _Cyclopteris_ 571, 572; _Linopteris_ 572, 573;
_Alethopteris_ 573–576; _Lonchopteris_ 576; _Pecopteris_
576–580.
INDEX Pp. 609–624
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.
FRONTISPIECE. _Thamnopteris Schlechtendalii_ (Eich.). From a
photograph given to me by Dr Kidston and Mr Gwynne-Vaughan. (Page 329.)
FIG. PAGE
112. _Sphenophyllostachys_ 2
113. ╭ _Sphenophyllostachys Römeri_ 3
┤ _Sphenophyllum trichomatosum_
╰ _S. majus_
114, 115. _Sphenophyllostachys fertilis_ 4, 5
[Council of the Royal Society of London.]
116. _Sphenophyllostachys Dawsoni_ 6
[Mr A. G. Tansley, Editor of the New Phytologist.]
117. ╭ _Cheirostrobus pettycurensis_ 8
╰ _Pseudobornia ursina_
118. _Psilotum triquetrum_ 18
119. _Psilotum triquetrum_ (anatomy) 20
120. _Tmesipteris tannensis_ 22
121. _Lycopodium_ (seven species) 35
122. _Lycopodium squarrosum_ 36
123. _Lycopodium cernuum_ 37
124. _Lycopodium obscurum_ 38
125. _Lycopodium_ (anatomy of stem) 41
126. _Lycopodium_ (anatomy of cones) 45
127–129. _Lycopodium cernuum_ (cone) 47–49
[Council of the Royal Society of Edinburgh.]
130. _Selaginella grandis_ 50
131. _Selaginella_ (anatomy) 52
132. ╭ _Isoetes echinospora_ 59
╰ _I. lacustris_
133. _Isoetes lacustris_ (anatomy) 62
134. _Pleuromeia Sternbergi_ 70
135. _Selaginellites_ and _Lycopodites_ 80
136. _Lycopodites lanceolatus_ 81
[Council of the Geological Society of London.]
137. _Lycopodites falcatus_ 83
138. _Selaginellites primaevus_ 86
139. _Lycostrobus Scotti_ 89
140. _Picea excelsa_ 94
141. _Lepidodendron Sternbergii_ 97
142. _Sigillaria_ (leaves) 98
143. _Lepidodendron_ (leaves) 99
144. _Lepidodendron Veltheimianum_ 101
145. Lepidodendron leaf-cushion 102
146. Lepidodendron and Lepidophloios leaf-cushions 104
147. Lepidophloios leaf-cushion 108
148–155. _Lepidodendron vasculare_ 112–122
156. _Knorria mirabilis_ 125
157. _Lepidodendron Veltheimianum_ (_Ulodendron_) 129
158. Diagrammatic section illustrating the branch-theory of the
Ulodendroid scar 132
[Council of the Manchester Literary and Philosophical Society.]
159. _Pinus clausa_ 134
160. _Lepidophloios scoticus_ 135
161. _Halonia tortuosa_ 136
162–167. _Lepidodendron fuliginosum_ 143–147
[Council of the Cambridge Philosophical Society.]
168. _Lepidodendron vasculare_ and _L. fuliginosum_ 148
169. _Lepidodendron fuliginosum_ 149
170–172. _L. fuliginosum_ 150–152
173. _Lepidodendron obovatum_ 154
174–176. _Lepidodendron aculeatum_ 155, 156
[Oxford University Press: Annals of Botany.]
177. _Stigmaria radiculosa_ 157
178. Stigmarian rootlet 158
179. _Lepidodendron Harcourtii_ and _L. fuliginosum_ 162
180. _Lepidodendron Wünschianum_ 163
181, 182. _L. Wünschianum_ 165, 166
183, 184. _L. Wünschianum_ 168, 169
[Editor of the New Phytologist.]
185. _Lepidodendron Veltheimianum_ 173
186. _L. Veltheimianum_ and _L. macrophyllum_ 176
187. _Lepidodendron australe_ 179
[Dr H. Woodward, Editor of the Geological Magazine.]
188, 189. _Lepidostrobus_ 183, 184
190. _Lepidodendron_ and _Lepidostrobi_ 186
191. _Lepidostrobus_ 188
192. _Spencerites insignis_ 193
[Oxford University Press: Annals of Botany.]
193. _Sigillaria elegans_, _S. rugosa_, _S. tessellata_,
_Omphalophloios anglicus_ 197
194. _Sigillaria McMurtriei_ 199
195. _Sigillaria mammillaris_ 199
196. _Sigillaria Brardi_, _S. laevigata_, and _Lepidodendron
Wortheni_ 200
197. _Carica_ sp. 202
198, 199. _Sigillaria_ 205, 206
200. _Sigillaria Brardi_ 212
201. _Sigillariostrobus_ 216
202. _Sigillaria elegans_ and _S. elongata_ 220
203. _Sigillaria Brardi_ 225
204, 205. _Stigmaria ficoides_ 227, 228
206. _Cyperus papyrus_ 230
207, 208. Stages in the development of _Sigillaria_ 236
209. _Stigmariopsis_ 237
210. _Stigmaria_ 241
211. _Bothrodendron punctatum_ 250
212. _Bothrodendron minutifolium_, _B. punctatum_, _B.
kiltorkense_ and _Lepidostrobus Olryi_ 252
213. _Bothrodendron minutifolium_ 254
214. _Bothrodendron Leslei_ 258
[Trustees of the British Museum.]
215. _Bothrodendron mundum_ 259
216. _Bothrostrobus_ 263
[Council of the Manchester Literary and Philosophical Society.]
217. _Omphalophloios_ 265
218. _Lepidocarpon Lomaxi_ 273
219. _Miadesmia_ and _Bothrodendron_ 276
220. _Angiopteris evecta_ and _Cycas revoluta_ 283
221. _Osmunda cinnamomea_, _O. regalis_, and _Todea barbara_ 286
222. _Schizaea elegans_ 287
223. _Aneimia rotundifolia_ 288
224. _Aneimia flexuosa_, _A. phyllitidis_, _Hymenophyllum_,
_Matonia pectinata_, _Thyrsopteris elegans_, _Gleichenia_ 289
225. _Gleichenia dicarpa_ 290
226. _Gleichenites Rostafinskii_, _Gleichenia dicarpa_, _G.
dichotoma_ 290
227. _Matonia pectinata_ 292
[Council of the Royal Society.]
228. _Matonia pectinata_ 293
229. _Thyrsopteris elegans_, _Cyathea spinulosa_, _Dicksonia
coniifolia_, _D. culcita_, _Davallia concinna_, _Alsophila
excelsa_ 294
230. _Dicksonia Bertervana_ 295
[Trustees of the British Museum.]
231. _Dipteris quinquefurcata_, _D. conjugata_, _D. Wallichii_,
and _Polypodium quercifolium_ 297
232. _Davallia aculeata_ 299
233. _Polypodium Billardieri_ 302
234. _Polypodium quercifolium_ 303
235. _Hemitelia capensis_ 304
236 _a_, 236 _b_. _Pteris aquilina_ 305, 306
[Council of the Linnean Society of London.]
237. _Matonia pectinata_, _Matonidium_, _Gleichenia dicarpa_, and
_Trichomanes reniforme_ (anatomy) 310
238. _Trichomanes scandens_ 311
[Editor of the New Phytologist.]
239. _Platyzoma microphylla_ 312
[Editor of the New Phytologist.]
240. _Cyathea Imrayana_ 313
[Editor of the New Phytologist.].
241. _Angiopteris evecta_ and _Marattia fraxinea_ 317
242. _Angiopteris evecta_ and _Danaea_ 318
243, 244. _Angiopteris evecta_ 319
245. _Marattia fraxinea_, _M. Kaulfussii_, _Kaulfussia_, and
_Marattiopsis Münsteri_ 320
246. _Ophioglossum vulgatum_ 322
247. _Botrychium virginianum_ 322
248. _Zalesskya gracilis_ 327
249. _Zalesskya diploxylon_ 328
250. _Thamnopteris Schlechtendalii_ 330
251. _Lonchopteris virginiensis_ 331
252. _Osmundites Dunlopi_ 333
253, 254. _Osmundites Kolbei_ 334, 335
[Editor of the Geological Magazine.]
255. _O. Kolbei_ 336
256. _Cladophlebis denticulata_, _Todites Williamsoni_,
_Discopteris Rallii_, _Kidstonia heracleensis_, and
_Todeopsis primaeva_ 340
257, 258. _Cladophlebis denticulata_ 342, 345
259. _Klukia exilis_ 348
[Council of the Cambridge Philosophical Society.]
260. _Ruffordia Goepperti_ 349
261. _Chrysodium lanzaeanum_, _Lygodium Kaulfussi_, _Marattia
Hookeri_ 350
262. _Gleichenites longipennis_, _G. delicatula_, _G.
Nordenskioldi_ and _G. Zippei_ 354
263. _Gleichenites hantonensis_ 356
[Council of the Palaeontographical Society.]
264. _Laccopteris elegans_ 357
[Council of the Royal Society.]
265. _Matonidium Wiesneri_, _Marattiopsis marantacea_,
_Gleichenites_ _gracilis_, _Laccopteris Goepperti_, and
_L. Muensteri_ 358
266. _Laccopteris polypodioides_ 359
[Trustees of the British Museum.]
267. _Laccopteris_ 359
[Trustees of the British Museum.]
268. ? _Laccopteris polypodioides_ 360
[Trustees of the British Museum.]
269. _Matonidium Goepperti_ 362
[Editor of the Encyclopaedia Britannica.]
270. _Senftenbergia elegans_, _Oligocarpia Brongniartii_,
_Trichomanes sp._, _Hymenophyllum tunbridgense_,
_Sphenopteris (Hymenophyllites) quadridactylites_ 364
271. _Coniopteris hymenophylloides_ 368
[Council of the Manchester Literary and Philosophical Society.]
272. _C. hymenophylloides_ 369
273. _Coniopteris quinqueloba_ 370
274. _Coniopteris arguta_ 371
275. _Coniopteris arguta_ and _C. hymenophylloides_ 372
276. _Oncopteris Nettvalli_ 373
277. _Protopteris punctata_ 373
278. _Laccopteris polypodioides_, _L. Muensteri_, _Dicksonia_,
_Onychiopsis Mantelli_, _Hausmannia Sewardi_, _H.
Kohlmanni_, and _Protopteris Witteana_ 374
279. _Adiantides antiquus_ and _A. lindsayoides_ 376
280. _Onychiopsis Mantelli_ 379
281. _Dictyophyllum exile_ 381
282. _Dictyophyllum Nilssoni_, _Rhizomopteris Schenki_,
_Camptopteris spiralis_, and _D. exile_ 382
283. _Dictyophyllum rugosum_ 384
[Trustees of the British Museum.]
284. _Thaumatopteris Münsteri_ 386
285. _Clathropteris meniscoides_ 387
286. _Clathropteris egyptiaca_ 388
[Editor of the Geological Magazine.]
287. _Camptopteris spiralis_ 389
288. _Hausmannia dichotoma_ 391
289. _Hausmannia sp._ 393
290. _Alethopteris lonchitica_, _Lonchopteris rugosa_,
_Sphenopteris Hoeninghausi_, _Parapecopteris
neuropteroides_,and _Pecopteris (Dactylotheca) plumosa_ 399
291. _Ptychocarpus unita_, _Asterotheca Sternbergii_, _Danaeites
sarepontanus_, _Hawlea Miltoni_, _H. pulcherrima_,
_Scolecopteris elegans_ 400
292. _Dactylotheca plumosa_ 405
293. _D. plumosa_ 406
294. _Nathorstia angustifolia_ and _N. latifolia_ 410
295. _Psaronius_ 414
296. _Psaronius infarctus_, _P. coalescens_, _P. musaeformis_,
and _P. asterolithus_ 416
297. _Pecopteris Sterzeli_ 419
298. _Caulopteris peltigera_ and _Megaphyton insigne_ 421
299. _Ptychopteris_ 423
300. _Dicksonia antarctica_ 424
301. _Rhacopteris sp._ 427
302. _Noeggerathia foliosa_ 429
303. _Chiropteris Zeilleri_ 430
[Annals of the South African Museum.]
304. _Tubicaulis solenites_ 435
[Editor of the New Phytologist.]
305. _Botryopteris cylindrica_ 439
306. _Botryopteris ramosa_ 441
307. _Botryopteris antiqua_ 442
308. _Clepsydropsis antiqua_, _Etapteris Scotti_, _Diplolabis
forensis_, _Zygopteris primaria_, _Stauropteris
oldhamia_ 444
309. _Diplolabis forensis_, _Botryopteris forensis_,
_Corynepteris coralloides_, _Schizopteris pinnata_ 445
310. _Metaclepsydropsis duplex_, _Stauropteris oldhamia_,
_Ankyropteris scandens_ 450
311. _Ankyropteris Grayi_ 451
312. _Thamnopteris Schlechtendalii_, _Ankyropteris corrugata_,
_A. bibractensis_ 453
313. _Ankyropteris bibractensis_ 454
314. _Ankyropteris corrugata_ 457
315. _Ankyropteris corrugata_ 458
[Editor of the New Phytologist.]
316, 317. _Ankyropteris corrugata_ 459, 460
318. _Etapteris Scotti_ 462
[Editor of the New Phytologist.]
319. _Etapteris_, _Botryopteris forensis_ 463
320. _Stauropteris oldhamia_ 464
[Editor of the New Phytologist.]
321. _Stauropteris oldhamia_ 467
322. _Stauropteris oldhamia_ 468
[Editor of the New Phytologist.]
323. _Stauropteris_ 469
[Editor of the New Phytologist.]
324. _Asterochlaena laxa_ 472
[Editor of the New Phytologist.]
325. Sporocarp-like bodies (? _Sagenopteris_) 478
326. _Regnellidium diphyllum_, _Sagenopteris rhoifolia_ 479
327. _Sagenopteris Phillipsi_ 480
[Trustees of the British Museum.]
328. _Sagenopteris Phillipsi_ 481
[Council of the Manchester Literary and Philosophical
Society.]
329. _Taeniopteris multinervis_, _Lesleya Delafondi_ 487
330. _Taeniopteris Carnoti_, _T. spatulata_, _T. coriacea_ 490
331. _Taeniopteris Carruthersi_ 491
[Annals of the South African Museum.]
332. _Taeniopteris vittata_ 493
333. _Weichselia Mantelli_, _W. erratica_ 495
334. _Glossopteris Browniana_ 499
[Council of the Geological Society of London.]
335, 336. _Glossopteris Browniana_ 500, 501
[Trustees of the British Museum.]
337. _Vertebraria indica_ 502
338. _Vertebraria indica_, _Onoclea struthiopteris_ 503
339. Glossopteris fronds attached to rhizome 504
340, 341. _Glossopteris indica_, _G. angustifolia_ 506, 507
[Trustees of the British Museum.]
342. _Glossopteris angustifolia_ var. _taeniopteroides_ 508
[Council of the Geological Society.]
343. _Blechnoxylon talbragarense_ 509
344. _Glossopteris retifera_ 511
[Trustees of the British Museum.]
345. _Gangamopteris cyclopteroides_ 515
[Trustees of the British Museum.]
346. _Arberia sp._ 517
347. _Lesleya simplicinervis_ 518
348. _Neuropteridium validum_ 520
[Trustees of the British Museum.]
349. _Neuropteridium intermedium_ 522
350. _Cardiopteris frondosa_ 524
351. _Gunnera manicata_ 527
352. _Sphenopteris obtusiloba_, _Pecopteris arborescens_,
_Sphenopteris furcata_ 529
353. _Sphenopteris affinis_ 531
354. _Palmatopteris_, _Mariopteris_, _Diplotmema Zeilleri_,
_Neuropteris macrophylla_, _N. heterophylla_, _N.
Scheuchzeri_, _Alloiopteris Essinghii_ 535
355. _Cephalotheca mirabilis_ 536
356. _Thinnfeldia odontopteroides_, _Ptilozamites_ 539
[Council of the Geological Society.]
357. _Thinnfeldia odontopteroides_ 540
[Council of the Geological Society.]
358. _Thinnfeldia odontopteroides_ 541
[Annals of the South African Museum.]
359. _Thinnfeldia rhomboidalis_ 542
360. _Lomatopteris jurensis_, _L. Schimperi_, _Thinnfeldia
rhomboidalis_ 544
361. _Ptilozamites Heeri_ 547
362. _Ctenopteris cycadea_ 549
363. _Dichopteris visianica_ 551
364. _Alethopteris lonchitica_, _Mariopteris muricata_,
_Odontopteris_ cf. _alpina_ 553
365. _Odontopteris minor_ 554
366. _Odontopteris genuina_, _Callipteridium gigas_, _Callipteris
Pellati_, _C. lyratifolia_ 557
367. _Callipteris conferta_ 559
368. _Archaeopteris hibernica_ 561
369. _Archaeopteris hibernica_, _A. archetypus_, _A.fissilis_, _A.
fimbriata_ 564
370. _Neuropteris_ with Cyclopteris leaflets 566
[From a block received from Mr Carruthers.]
371. _Neuropteris heterophylla_ 568
372. _Neuropteris macrophylla_ 569
373. _Neuropteris Scheuchzeri_ 570
374. _Linopteris neuropteroides_ 573
375. _Alethopteris Serlii_ 575
376. _Pecopteris arborescens_ 578
ERRATA IN VOL. I
Page 16, line 4. For “The North American Tulip tree” read The Tulip
tree of North America and China.
„ 66, line 2 from the bottom. For “Browera” read Berowra.
„ 127, line 3 and 4 from bottom. For _Achyla_ and _Palaeachyla_ read
_Achlya_ and _Palaeachlya_.
„ 145, lines 4 and 5. For “Upper Greensand” read Lower Eocene.
„ 162, line 3 from bottom. For “_Corallina barbata_” read
_Cymopolia barbata_.
„ 170, line 20. For “sporangiaphore” read sporangiophore.
„ 185, line 2. The genera _Udotea_ and _Halimeda_, members of the
Siphoneae, are incorrectly included under the Corallinaceae.
„ 191, line 11 from bottom. Omit _Chondrus crispus_, which is one of
the Florideae and not a Brown Alga.
„ 202, line 13. For “_Halmeda_” read _Halimeda_.
„ 250, line 11. For “three” read the.
„ 381, line 10. For “_Calamopytus_” read _Calamopitys_.
CHAPTER XII[1].
SPHENOPHYLLALES (_concluded_).
_Sphenophyllum_.
The account of the Sphenophyllales given in the first volume[2] of this
work must be extended and somewhat modified in the light of recent work
on the fertile shoots of _Sphenophyllum_.
_Sphenophyllostachys Dawsoni_ (Will.) was described as consisting of
an axis bearing superposed whorls of bracts connate at the base in
the form of a shallow funnel-shaped collar giving off from the upper
surface and close to the axis of the cone two concentric series of
sporangiophores. Occasionally there are three series, as represented in
fig. 112. In another type of strobilus, _Sphenophyllostachys Römeri_[3]
each sporangiophore terminates in two pendulous sporangia (fig. 113,
A; see also fig. 107, C, vol. I.). It has already been pointed out
that the common occurrence of detached strobili necessitates their
description under distinct specific names; it is only by a rare
accident that we can assign fossil cones to their vegetative shoots.
There are, however, reasons for believing that _Sphenophyllostachys
Dawsoni_ is the strobilus of the plant originally described by
Sternberg[4] from impressions of foliage-shoots as _Rotularia
cuneifolia_. Another difficulty presented by petrified material is
that of determining, with certainty, whether two imperfect specimens,
differing from one another in features which do not appear to be of
sufficient importance to warrant specific separation, are forms of one
species or portions of specifically distinct cones. It has been pointed
out by Scott[5] that the strobilus known as _Sphenophyllostachys
Dawsoni_ probably includes two distinct species, one being the cone
of _Sphenophyllum cuneifolium_ Sternb., and the other the cone of
_S. myriophyllum_ Crép[6]. The stem of _S. myriophyllum_ agrees
anatomically with the type known as _Sphenophyllum plurifoliatum_ Will.
and Scott[7].
[Illustration: FIG. 112. Sketch of a radial longitudinal section of
_Sphenophyllostachys_. There are usually two concentric series of
sporangia on the sporophylls, not three as shown in the figure.
The upper figure (after Zeiller) shows the linear bracts in
surface-view.]
In addition to the two types of cone already mentioned,
_Sphenophyllostachys Dawsoni_ and _S. Römeri_, others have been
described by Kidston from carbonised impressions. One of these is the
fertile branch of _Sphenophyllum majus_[8]. The basal portions of the
bracts of each whorl form a narrow collar round the axis of the cone;
the free portion of each bract consists of a lamina divided into two
equal bifid lobes bearing on its upper surface one group, or possibly
two groups, of four sessile sporangia between the narrow coherent bases
of the laminae and the sinus between the terminal lobes (fig. 113, C).
Another characteristic feature is the greater length of the internodes;
this renders the cone less compact and less sharply differentiated from
the vegetative shoots than those of other species. A specimen in Dr
Kidston’s collection illustrates the peculiar character of the fertile
portion of this species; it consists of an axis bearing a succession of
lax sporophylls succeeded above and below by whorls of sterile leaves.
In this species, therefore, we cannot speak of a compact strobilus at
the end of a shoot of limited growth, but of axes in which sterile and
fertile leaves are borne alternately[9], a condition recalling the
alternation of foliage leaves and sporophylls in _Tmesipteris_ and in
_Lycopodium Selago_.
[Illustration: FIG. 113.
A. _Sphenophyllostachys Römeri._ (Solms-Laubach.)
B. _Sphenophyllum trichomatosum_ Stur.
C. _Sphenophyllum majus._ Bronn. (A–C. After Kidston.)]
Another form of cone, also from the Middle Coal Measures, is referred
by Kidston to _Sphenophyllum trichomatosum_ Stur[10] (fig. 113, B):
this is characterised by the more horizontal position of the bracts,
which “do not appear to be so much or so suddenly bent upwards in their
distal portion as in some other species of _Sphenophyllum_,” and by
sessile sporangia borne singly on the upper face of each bract.
[Illustration: FIG. 114. _Sphenophyllostachys fertilis_ (Scott).
(After Scott.) Diagram of a node in longitudinal section, showing
one sporophyll and the base of the opposite one. _v.l._ ventral
lobe of sporophyll; _v.s._ one of the segments into which it
divides; _v.s′._ stump of another segment; _d.l._ dorsal lobe;
_d.s._, _d.s′._ segments of dorsal lobe.]
A more recent addition to our knowledge of the fertile shoots of
_Sphenophyllum_ is due to Scott who has described a new type of cone
under the name _Sphenophyllum fertile_[11]. The petrified specimen
on which the species was founded was discovered by Mr James Lomax in
the Lower Coal Measures of Lancashire; it represents a portion of a
cone 6 cm. long and approximately 12 mm. broad. The axis contains a
single vascular cylinder agreeing in essentials with the type of stem
structure known as _Sphenophyllum plurifoliatum_. The nodal regions,
which exhibit the slight swelling characteristic of the genus, bear
several (probably twelve) appendages connate at the base and forming
a narrow flange encircling the axis. Each bract, the base of which
forms part of the narrow collar surrounding the axis, consists of two
lobes, ventral and dorsal, divided palmately into several (sometimes
four) segments or sporangiophores (fig. 115). Each sporangiophore
terminates distally in an oblong or oval lamina bearing two sporangia
on its adaxial face (fig. 114). The space between the axis and the
periphery of the cone is thus occupied by crowded peltate laminae,
each with its pair of sporangia. A single vascular bundle supplies
each sporangiophore and bifurcates in the distal lamina into two
branches which extend to the bases of the sporangia. The sporangia
agree in structure with those of other species of _Sphenophyllum_: the
spores are of one size and elliptical, characterised by the presence
of several sharp ridges or flanges encircling the spore-wall in the
direction of the major-axis. _Sphenophyllostachys fertilis_ differs
from all previously recorded types in the absence of sterile bracts.
The appendages of the cone-axis are all fertile, a striking contrast
to the differentiation into protective and sporangia-bearing bracts
which constitutes a constant feature in the cones of _Sphenophyllum_
and _Calamites_. It is possible, as Scott suggests, that the absence of
sterile segments is the result of modification of the more usual type
of strobilus; instead of the dorsal and ventral lobes of the bracts
sharing between them the duties of protection and spore-production,
the whole of each bract is constructed on the plan of the maximum
spore-output, the laminar terminations of the sporangiophores
serving the purpose of protection. The cone may be described as
more specialised than the normal type of strobilus for reproductive
purposes[12].
[Illustration: FIG. 115. _Sphenophyllostachys fertilis_ (Scott).
(After Scott.) Diagram of a single sporophyll as it would appear
in a transverse section of the cone; showing one lobe (dorsal or
ventral). _ax_, part of axis to which the sporophylls are attached.]
[Illustration: FIG. 116. _Sphenophyllostachys Dawsoni._ (After
Thoday.) A. Larger spores; B, abortive spores; C, mature spores
showing the characteristic spines.]
It has been stated, on evidence which is unsatisfactory, that
_Sphenophyllum_ possesses two kinds of spores. While regarding the
genus as homosporous on the evidence before us, it is interesting
to find that cases occur in which the spores in the same sporangium
exhibit a marked difference in size. Attention has been called by
Williamson and Scott[13] to variation in the dimensions of spores: a
more pronounced difference in size has been recorded by Mr Thoday[14]
who gives 120μ as the maximum and 90μ as the minimum diameter of the
spores in a cone of _Sphenophyllostachys Dawsoni_. The presence of
several abortive spores in the sporangium (fig. 116) containing the
larger spores favours the view that this difference in size may be the
first step towards the development of heterospory.
It is clear that the types of strobilus designated
_Sphenophyllostachys_ (figs. 112–114) present a divergence of
characters too great to be comprised under one genus; but in the
absence of fuller information, we cannot do otherwise than follow the
only logical custom of grouping them together as examples of strobili
borne by plants which, in the present state of our knowledge, are most
conveniently referred to the genus _Sphenophyllum_.
_Cheirostrobus._
This generic name was applied by Dr Scott[15] to a calcified cone
obtained by Mr James Bennie in 1883 from the Lower Carboniferous
plant-beds of Pettycur near Burntisland on the Firth of Forth.
_Cheirostrobus_ is distinguished from _Sphenophyllostachys_ by its
greater breadth (3.5 cm.); externally it agrees more closely with the
fertile shoots of _Lepidodendron_ than with those of _Sphenophyllum_.
A single vascular cylinder having the form of a fluted Doric column
(fig. 117, B, _x_) occupies the axis of the cone: it consists for
the most part of reticulate tracheae which tend to assume a short
or isodiametric form in the central region; the smaller protoxylem
tracheids with the spiral form of pitting constitute the sharp and
prominent ridges at the periphery of the xylem-cylinder. In the
outer part of the cylinder the metaxylem[16] consists exclusively of
tracheae, but towards the centre of the axis these are associated with
numerous parenchymatous cells.
The xylem is therefore centripetal in origin as in _Sphenophyllum_
and in nearly all recent and fossil members of the Lycopodiales. In
the type-specimen of _Cheirostrobus_ the vascular cylinder of the
cone consists entirely of primary xylem, but secondary xylem has been
found in a more recently discovered specimen[17]. Secondary xylem
occurs also in the peduncle of the cone. No appreciable remains of
phloem have been found. The cortex consists of slightly elongated
rather thick-walled tissue containing secretory sacs. Crowded
superposed whorls of bracts (or sporophylls), usually twelve in each
whorl, are borne on the axis and each sporophyll receives a single
vascular bundle from one of the vertical ridges of the xylem column
(fig. 117, A, _lt_). The members of each whorl are connate at the
base: from this narrow collar each sporophyll branches into an upper
or dorsal and a lower or ventral limb (fig. 117, A, _f_ and _s_).
Each limb divides palmately at a short distance from its origin into
three slender segments, which extend in a horizontal direction and
terminate in large laminar expansions (fig. 117, B, _s_) to afford
a protective covering to the surface of the cone. The upper set of
three segments, constituting sporangiophores (fig. 117, A, B, _f_) or
fertile divisions of the sporophyll, expand distally into comparatively
bulky laminae; each of these bears on its adaxial face four diagonally
placed outgrowths which form the short pedicels of very long and narrow
sporangia. The three lower segments—the sterile divisions of the
sporophylls—(fig. 117, A, B, _s_) are similar to the upper set except
in their greater length and in the kite-shaped form of their distal
laminae which are provided with lateral lobes. The single vascular
strand which supplies each sporophyll is represented at _lt_ in fig.
117, B; at _lt′_ the strand has divided into four, the three upper
bundles in the figure supply the sterile segments and the single lower
bundle ultimately divides into three which supply the fertile segments.
A pair of blunt processes (fig. A, _s_) extend downwards over the ends
of the underlying fertile lamina and two slender prolongations extend
upwards through several internodes.
[Illustration:
FIG. 117. A, B. _Cheirostrobus pettycurensis_ Scott. (After Scott.)
C, D. _Pseudobornia ursina_ Nath. (After Nathorst.)
A. Diagrammatic radial longitudinal section of part of the
cone-axis and two sporophylls. _lt_, bundle passing out to
sporophyll; _f_, fertile segment of sporophyll showing two
sporangia; _s_, sterile (lower) segment.
B. Part of transverse section. _x_, stele; _lt_, _lt′_, bundles on
their way to sporophylls; _a_, tips of sterile segments of lower
sporophylls.
C. Palmately branched leaf (½ natural size).
D. Node of stem showing leaf-bases.]
An economical arrangement of the long and narrow sporangia and of the
sporophyll-segments between the axis and the periphery of the cone
is rendered possible by the interlocking of the sterile and fertile
segments by means of a groove in the upper face of the latter for
the accommodation of the former. The sporangia are characterised by
their unusually long and narrow form: the length of a sporangium may
reach 1 centimetre. In the structure of the wall the sporangia of
_Cheirostrobus_ agree closely with those of _Calamostachys_[18] and
_Sphenophyllostachys_. The spores are of one size only. The vascular
cylinder of the peduncle, originally described by Williamson[19] as
the peduncle of a large _Lepidostrobus_ (the cone of _Lepidodendron_),
is characterised by the presence of a short radially disposed zone
of secondary tracheids, a feature, as Scott points out, which may
extend into the axis of the cone. It is noteworthy that the protoxylem
elements are not always external, but occasionally occur internal to
one or two of the outermost metaxylem tracheae: the usual exarch[20]
structure of the central cylinder is not therefore absolutely
constant, but may be replaced by a mesarch arrangement.
The presence of a few sterile leaves on the peduncle below the fertile
portion of the cone, which agree in their lobed laminae with the
sporophylls, is the only fact which we possess as to the form of the
vegetative characters of the genus.
The above description is sufficient to indicate the extraordinary
complexity and high degree of specialisation of _Cheirostrobus_. The
sporophylls, with their trilobed segments, and the crowded sporangia of
exceptional length attached only by a narrow base constitute striking
peculiarities of the genus.
It is unfortunate that we are still without any satisfactory evidence
as to the nature of the plant the cones of which have been made the
type of a new genus and a new family. _Cheirostrobus_ affords an
interesting example of a type of reproductive shoot constructed on
a plan _sui generis_, and may be classed with some other extinct
genera as instances of the production in the course of evolution
of architectural schemes which appear to have been ill adapted for
competition with equally efficient though much simpler types. But the
discovery of these isolated forms of restricted geological range among
the relics of the Palaeozoic vegetation frequently supplies a key to
phylogenetic problems. _Cheirostrobus_ by its complex combination of
features characteristic of the Equisetales, the Lycopodiales and the
genus _Sphenophyllum_ throws a welcome light on the inter-relationships
of groups which represent divergent series. The combination of
morphological features in this generalised type led the author of the
genus to describe it as a descendant of an old stock which existed
prior to the divergence of the Equisetales and Lycopodiales.
The discovery of this new type of strobilus naturally led to a search
among Lower Carboniferous plants for vegetative shoots exhibiting
characters conformable with the whorled and branched leaves of
_Cheirostrobus_. In _Sphenophyllum_ we have a genus obviously
comparable with _Cheirostrobus_ as regards the form and disposition
of the leaves, but the differences between the cones and the striking
similarity of the vascular cylinder of the latter to that of
_Lepidodendron_ demonstrate conclusively that we must look elsewhere
for the vegetative members of the plant which produced cones of the
_Cheirostrobus_ type.
[Sidenote: PSEUDOBORNIA]
In 1902 Professor Nathorst[21] instituted the generic name
_Pseudobornia_ for plants of which imperfect examples had previously
been referred by Heer[22] to _Calamites_ under the name _C. radiatus_.
Heer’s plants were obtained from Upper Devonian rocks of Bear Island
in the Arctic seas and additional specimens were brought from the
same locality by the Swedish Polar Expedition of 1898. _Pseudobornia_
possesses jointed stems (fig. 117, D) bearing whorled and shortly
stalked leaves, often four in number, at each node. The leaves are
palmately branched with fine serrated edges (fig. 117, C). Certain
specimens, which are no doubt correctly described by Nathorst as
cones, are characterised by a thick axis bearing whorled leaves
with sporangia on their lower surfaces, but the material is not
sufficiently well preserved to render possible a recognition of
structural details. It has been suggested by Scott that _Pseudobornia_
may possibly be referable to the Sphenophyllales and that the stem of
_Cheirostrobus_ “may have had something in common with” Nathorst’s
genus[23]. The beds in which the stems occur are of Upper Devonian age,
while _Cheirostrobus_ was found in Lower Carboniferous rocks: this
difference in age is not, however, a serious objection to the validity
of the comparison. We cannot do more than express the view that
_Pseudobornia_, so far as can be ascertained without an examination of
petrified material or of more perfect impressions of strobili, exhibits
vegetative features not inconsistent with the morphological characters
of the fertile shoots known as _Cheirostrobus_.
• • • • •
The institution of a special group-name for the reception of
_Sphenophyllum_ is justified by the sum of its morphological features,
which do not sufficiently conform to those of any existing group of
Pteridophytes to warrant its inclusion in a system of classification
based on recent genera. In the case of _Cheirostrobus_ we are limited
to the characters of the cone and its peduncle. The suggestion that
the Devonian fossils known as _Pseudobornia_ may represent the foliage
shoots of a plant closely related to _Cheirostrobus_ has still to
be proved correct. Although we may find justification in the highly
complex and peculiar structure of _Cheirostrobus_ for the recognition
of the genus as a type of still another group of Pteridophytes, it
would be unwise to take this step without additional knowledge.
The undoubted similarity between _Cheirostrobus_ and _Sphenophyllum_
coupled with striking points of difference favours the inclusion of the
two genera in distinct families placed, for the present at least, in
the group Sphenophyllales.
Group =SPHENOPHYLLALES=.
=Sphenophylleae=: genus _Sphenophyllum_.
=Cheirostrobeae=: genus _Cheirostrobus_.
It has recently been proposed to include the family Psilotaceae,
comprising the two recent genera _Psilotum_ and _Tmesipteris_, as
another subdivision of the Sphenophyllales. This proposal had been made
by Professor Thomas[24] primarily on the ground that the sporophylls
of _Tmesipteris_ and _Psilotum_ appear to afford the closest parallel
among existing plants to the peculiar form of sporophyll characteristic
of the Sphenophyllales. The morphological interpretation of the
sporophylls of both _Sphenophyllum_ and _Cheirostrobus_ has been the
source of considerable discussion[25]. If we regard each sporophyll
as a leaf with two lobes, one fertile and one sterile, except in the
case of _Sphenophyllostachys fertilis_ in which both are fertile, an
obvious comparison may be made with the fern _Ophioglossum_; but the
difference between a single fern frond, consisting of a comparatively
large sterile lamina bearing a fertile branch composed of a long axis
with two rows of sporangia embedded in its tissues, and the whorled
sporophylls of _Sphenophyllum_ is considerable.
[Sidenote: PSILOTACEAE]
A brief reference may be made to the principal reasons which have
led to the suggestion that the Psilotaceae should be included in
the Sphenophyllales. The shoots of _Tmesipteris_ bear simple foliage
leaves spirally disposed on a slender axis, and in association with
these occur sporophylls consisting of a short axis bearing a pair of
small lobes and a bilocular synangium[26] (fig. 120, B). The synangium
is seated on a very short stalk given off from its sporophyll at the
base of the pair of laminae: the synangium with its short stalk may be
spoken of as the sporangiophore. In most cases the synangium appears
to be sessile on the sporophyll, but occasionally the much reduced
stalk is prolonged and forms an obvious feature. Dr Scott[27] suggested
that the Tmesipteris synangium with its axis may correspond to the
ventral lobe (or sporangiophore) of _Sphenophyllum_. In the latter
genus the whorled sporophylls consist in most species of a dorsal
and a ventral lobe, the latter serving as a sporangiophore bearing
one or more sporangia; in _Tmesipteris_ the sporophylls are spirally
disposed and each consists of a bilobed sterile portion bearing a
septate sporangium or bilocular synangium on a very short ventral lobe.
Professor Bower[28], in his account of the development and structure of
the sporophylls of _Tmesipteris_, drew attention to the comparatively
frequent occurrence of abnormal sporophylls and spoke of the plant as
unstable. More recently Professor Thomas[29] of Auckland has carefully
examined living plants, with the result that variations of different
kinds are proved to be exceedingly common. He finds that sporophylls
occur which exhibit repeated dichotomy of the axis (fig. 120, D, F) and
thus each may bear four instead of two leaf-lobes and three synangia,
one at the first fork and one at each of the forks of the second
order[30].
Other abnormalities occur in which the synangium is raised on a
distinct stalk instead of being more or less sessile at the point from
which the leaf-lobes diverge. A third form of departure from the normal
is that in which there is no synangium on the bilobed sporophyll, its
place being taken by a leaf-lobe. The deduction from the occurrence of
these abnormalities is that the synangium of _Tmesipteris_ represents
a ventral leaf-lobe, as Scott suggested. Professor Thomas draws
attention to the resemblance between _Tmesipteris_ sporophylls and
the foliage-leaves of _Sphenophyllum_, which are either simple with
dichotomously branched veins or the lamina is deeply divided into two
or more segments. In some types of _Sphenophyllostachys_ the bracts
are simple (_S. Dawsoni_), but in others (_Sphenophyllum majus_, fig.
113, C) they are forked like the foliage-leaves and bear a close
resemblance to the abnormal sporophylls of _Tmesipteris_. Moreover,
in _Sphenophyllostachys Römeri_ (fig. 113, A) each ventral lobe of
a sporophyll bears two sporangia, a condition almost identical with
that represented by the occasional occurrence of a synangium on a
comparatively long stalk in _Tmesipteris_. Similarly the more elaborate
sporophylls of _Cheirostrobus_ may be compared with the branched
sporophylls of _Tmesipteris_ (fig. 120). This agreement between the
sporophylls of the Palaeozoic and recent genera acquires additional
importance from the very close resemblance between the exarch stele
of _Sphenophyllum_ and that of the genus _Psilotum_, which conforms
to the Palaeozoic type not only in the centripetal character of the
primary xylem and in its exarch structure, but also in the occasional
occurrence of secondary xylem[31], and in the stellate form of its
transverse section. The occasional mesarch structure of the stele of
_Cheirostrobus_ finds a parallel in the mesarch xylem groups in the
stem of _Tmesipteris_. It is thus on the strength of these resemblances
that Thomas and Bower would remove the Psilotaceae from the group
Lycopodiales and unite them with _Sphenophyllum_ and _Cheirostrobus_
in the Sphenophyllales. While admitting the validity of the comparison
briefly referred to above, I prefer to retain the Psilotaceae
as a division of the Pteridophyta including only _Psilotum_ and
_Tmesipteris_.
[Sidenote: SPHENOPHYLLUM]
In his recent book on _The Origin of Land Flora_, Prof. Bower raises
objection to the use of the term ventral lobe in speaking of the
sporangium-bearing stalk or sporangiophore borne on the sporophyll
of _Sphenophyllum_. He points out that the use of this term implies
the derivation of the sporangiophore by metamorphosis of part of a
vegetative leaf, an opinion untenable in the absence of proof. The
designation sporangiophore is no doubt preferable to that of ventral
lobe as it carries with it no admission of particular morphological
value; as a further concession to a non-committal attitude we may
provisionally at least regard a sporangiophore as an organ _sui
generis_ “and not the result of modification of any other part[32].”
The view put forward by Prof. Lignier[33] that the Sphenophyllales are
descendants of primitive ferns is not convincing, and his comparison
of _Sphenophyllum_ with _Archaeopteris_ lacks force in view of our
ignorance as to the nature of the reproductive organs of the latter
genus. That the Sphenophyllales are connected with the Equisetales and
with the Psilotales by important morphological features is clear; but
the comparison between the sporophylls of the extinct genera with those
of the existing genus _Tmesipteris_, though helpful and possibly based
on true homology, cannot be considered as settling the morphological
value of the sporangiophores of _Sphenophyllum_ and _Cheirostrobus_.
I do not propose to discuss at length the different views in regard
to the morphological nature of the sporangiophore of _Sphenophyllum_.
The comparison, which we owe in the first instance to Scott, with the
synangium of the Psilotales with its short stalk, though not accepted
by Lignier as a comparison based on true homology, is one which
appeals to many botanists and is probably the best so far suggested.
The further question, whether these sporangiophores are to be called
foliar or axial structures is one which has been answered by several
authors, but it is improbable that we shall soon arrive at a decision
likely to be accepted as final. Discussions of this kind tend to
assume an exaggerated importance and frequently carry with them the
implication that every appendage of the nature of a sporangiophore
can be labelled either shoot or leaf. We treat the question from an
academic standpoint and run a risk of ignoring the fact that the
conception of stem and leaf is based on morphological characteristics,
which have been evolved as the result of gradual differentiation of
parts of one originally homogeneous whole. There is much that is
attractive in the view recently propounded by Mr Tansley that a leaf is
not an appendicular organ differing _ab initio_ from the axis on which
it is borne, but that it is in phylogenetic origin a “branch-system of
a primitive undifferentiated sporangium-bearing thallus[34].” Admitting
the probability that this view is correct, our faith in the importance
of discussions on the morphological nature of sporangiophores is
shaken, and we realise the possibility that our zeal for formality and
classification may lead to results inconsistent with an evolutionary
standpoint[35].