IAN FLC)R,AS

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^?ell

DEVONIAN FLORAS

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DEVONIAN FLOKAS

A STUDY OF THE ORIGIN OF CORMOPHYTA

BY

E. A. NEWELL ARBER, M.A., Sc.D.

TRINITY COLLEGE, CAMBRIDGE
UXIVERSITY DEMONSTRATOR IN PALAEOBOTANY

WITH A PREFACE BY
D. H. SCOTT, M.A., LL.D., Ph.D., F.R.S.

WITH A FRONTISPIECE AND FORTY-SEVEN
FIGURES IN THE TEXT

NEW YORK

DOTANICAL

GARDEN

CAMBRIDGE

AT THE UNIVERSITY PRESS

1921

■Al

PRINTED IN GREAT aWTAJN

NOTE

The present critical review of our knowledge of the two Devonian
land floras was one of the last pieces of work which my husband
undertook. Both text and illustrations are embodied in this
book substantially as he left them. His health was already
failing when he finished the manuscript in January, 1918 — six
months before his death — but his delight in his subject remained
unabated. I do not think that anything in his scientific life gave
him a keener intellectual pleasure than the development of the
idea — the Leitmotiv of the present essay — that the transition
from the Algae to the Vascular Cryptogams no longer remains
a matter of jDure conjecture, but that, in the fossil plants of the
Devonian rocks, we witness, actually occurring beneath our eyes,
the passage from the Thallophyta to the Cormophyta. He
welcomed this conclusion as exemplifying a generalisation to
which his experience in research had gradually led him — namelj%
that although the apj)arent insolubility of a problem may be
for many years unhesitatingly ascribed to lack of data, yet, when
the solution is found, it often becomes obvious that the essential
data were all the time under one's hand, and that it was merely
the recognition of their significance that was lacking.

Since the Author left this memoir as a first draft which he

was never able to revise, I must assume the responsibility for

its final form. I am deeply indebted to Dr D. H. Scott, F.R.S. —

who during my husband's life-time was closely associated with

f^ his study of the Devonian floras— for reading the manuscript

C'^ and proofs and suggesting a number of emendations, and for

1

>

vi NOTE

writing the preface and the footnote on p, 45 to which his name
is appended. I am also under great obhgations to Professor A. C.
Seward, F.R.S., for vahiable criticism, and to Dr W. T. Gordon
for advice on questions relating to Scottish Geology. The por-
trait which forms the frontispiece of this book is reproduced by
kind permission of the Editor of the Geological Magazine.

I have to express my gratitude to the Council of the Royal
Society for a grant in aid of the preparation and publication of
my husband's manuscripts.

AGNES ARBER.

PREFACE

The present memoir, which I had the advantage of reading in
MS. and of fully discussing with the author, seems to me of the
utmost interest. A survey of these early Floras of the land is a
most useful undertaking and one much wanted at the present
time, when important new discoveries have called general atten-
tion to the plant-life of the Devonian period. Such a survey is
all the more valuable, when, as in this case, there is an under-
lying theory giving a definite point of view to the exposition of
the facts, and animating the whole.

It is a matter for deep regret that the work never received
the final rc\'ision of the author, and that he never saw the later
results of Kidston and Lang's researches on the Rhynie fossils.
As it seems to the present writer, the views of those investi-
gators, though differently and more tentatively expressed, are
yet in substantial agreement with Arber's, so far as regards the
general question of the systematic position of the Psilophyton
Flora. However this may be, it must be recognised that Arber's
conclusions, which were reached altogether independently, bear
the stamp of true originality and are absolutely his own.

In the Introduction, the author lays down the essential dis-
tinction between the earlier and the later Devonian Floras, the
former, called the Psilojyhyton Flora, consisting chiefly of Pro-
cormophyta or Propteridophyta, while the latter, the Archaeo-
pteris Flora, was chiefly composed of true Pteridophytes. In
the first two chapters the geological age and distribution of the
two Floras are discussed, and useful tables of genera and species
are given.

Chapter III, "Recent Advances in our Knowledge of the
Psilophyton Flora," gives a clear account of each genus, with
the aid of abundant illustrations, collected from various sources.
This chapter, and the corresponding one (the fifth) on the
Archaeopteris Flora, will be of the greatest value to the student,

viii PREFACE

Avho will find here, in a small compass, an excellent comparative
description of ancient fossils, often little known, the literature
on which is much scattered and not always readily accessible.
So far as I know, nothing of the kind has been attempted before,
for the Devonian Floras, and I can testify to the great utility
of the author's critical summary.

Attention may be specially called to the evidence, which the
author adduces, in favour of the generic identity of Psilophyton
and Bhynia, an important conclusion, which will have to be
taken into serious consideration. On the other hand the remarks
on Halle's genus S-porogonites, would undoubtedly have been
modified, if the author could have been acquainted with the
later evidence; this point is dealt with in a footnote on p. 45.

Chapter IV, "A Discussion of the Natiu-e and Affinities of
the Psilophyton Flora," is of great theoretical importance.. The
author expresses his conviction that Psilophyton and all the
other genera of that Flora "were much more probably Thallo-
phyta than Pteridophy ta " (p; 47). But he also points out that
'' Psilophyton,... while still Thallophytic in habit, may occup}^
anatomically a place half-way between the Thallophyta and
Pteridophyta " (p. 49). This was written early in 1918; it agrees
very nearly with the statement by Kidston and Lang (Part ii,
1920, p. 622) that "The facts are consistent with the Rhyniaceae
finding their place near the beginning of a current of change
from an Alga-like type of plant to the type of the simpler
Vascular Cryptogams." Arber had grasped the position at a
time when only a portion of the evidence was before him.

Chapter VI, "The Procormophyta and the Origin of the Cor-
mophyta," completes the exposition of the author's theory. He
holds that there were three distinct main lines of descent among
vascular plants — the Sphenopsida, Pteropsida and Lycopsida,
derived severally from distinct Algal types. The Psilotales he
regards as another entirely separate group, also of Algal origin,
but of geologically very late appearance. This highly polyphy-
letic hypothesis has something in common with the brilliant
speculations of Dr A. H. Church, whose essay on "Thalassio-
phyta and the Subaerial Transmigration" would have interested
Arber immensely, if he had lived to see it.

PREFACE ix

We stand at a new point of departure in our theories of the
evolution of the higher plants. Arber was one of the first to
realise this, and his memoir represents a bold and vigorous
effort to grapple with the problems as they presented them-
selves to him, at the dawn of a new epoch.

It is fair to mention that the last chapter, "On the Origin of
the Stele in the Earlier Cormophyta," was regarded by the
author himself as quite unfinished; it could hardly have been
otherwise with the data at his disposal.

It has been a pleasure to me to write these few lines of appre-
ciation of mj^ old friend's latest work,

D. H. SCOTT.
October 2, 1920.

CONTENTS

CHAP. PAGE

INTRODUCTION 1

I. THE GEOLOGICAL AGE OF THE FLORAS . . 3

II. THE TWO DEVONIAN FLORAS 8

III. RECENT ADVANCES IN OUR KNOWLEDGE OF THE

MORPHOLOGY AND ANATOINIY OF MEMBERS OF

THE PSILOPHYTON FLORA .... 14

IV. A DISCUSSION OF THE NATURE AND AFFINITIES

OF THE PSILOPHYTON FLORA .... 46

V. RECENT ADVANCES IN OUR KNOWLEDGE OF THE

MORPHOLOGY OF THE ARCHAEOPTERIS FLORA 52

VI. THE PROCORMOPHYTA AND THE ORIGIN OF THE

CORMOPHYTA (EXTERNAL MORPHOLOGY) . 70

VII. THE ORIGIN OF THE STELE IN THE EARLIER COR-
MOPHYTA 89

BIBLIOGRAPHY 92

INDEX 97

LIST OF ILLUSTRATIONS

Portrait of the Author (1916)

Frnnli

FIG.

1. Psilophyton. [Dawson (1859)J ....

2. Psilophyton. [Reid and Macnair (1896)]

3. Psilophyion princeps, Dawson. [Dawson (1871)J

4. Psilophyton princeps, Dawson. [Halle (1916)]

5. Psilophyton princeps, Dawson. [Dawson (1859)]

6. Psilophyton princeps, Dawson. [Halle (1916)]

7. The emergences ot Psilophyton and Rhynia. [Kidston and Lang

(1917) and Halle (1916)] . . . ,

8. Arihrostigma gracile, Dawson. [Kidston (1893)]

9. Arthrostigma gracile, Dawson. [Halle (1916)]

10. Thursophyton Milleri, (Salt.). [Nathorst (1915)]

11. Thvrsophyton Milleri, (Salt.). [Reid and Macnair (1899)] and

T. hostimense, (P. & B.). [Potonie and Bernard (1904)]

12. Piilophyton Thomsoni, Dp.ws. [Salter (1859)]

13. Ptilophyton Thomsoni. Daws. [Carruthers (1873)] .

14. Ptilophyton C^) hostimense, (P. &B.). [Stur (1881)]

15. Pseudosporochnus Krejcii, (Stur). [Potonie and Bernard (1904)]

16. Pseudosporochnus Krejcii, (Stur). [Potonie and Bernard (1904)]

17. Broggeria norvegica, Nath. [Nathorst (1915)]

18. Barrandeina Diisliana, {Kv.). [Potonie and Bernard (1904)]

19. BarinophytonRichardsoni, {Daws.). [White (1905)]

20. Parka decipiens, Fleni. [Don and Hickling (1917)]

21. Lithophyllum lichenoides, (E. & S.) and Melobesia membranacea

(Esper) Lamour. [Rabenhorst, Kryptogamen-Flora (1885)]

22. Zosterophyllum niyreionianum, Penh. [Reid and Macnair (1899)]

23. Ilostimella hostimensis, P. & B. [Potonie and Bernard (1904)]

24. Sphenophyllum subfenerrimum, Nath. [Nathorst (1902)]

25. Ilyenia sphenophylloides , Nath. [Nathorst (1915)] .

26. Pseudobornia ursina, Nath. [Nathorst (1902)]

27. Psygmophyllum Kolderupi, Nath. [Nathorst (1915)]

28. Archaeopteris hibernica, (Forbes). [Carruthers (1872)]

29. Archaeopteris Archetypus, Schmalh. [Nathorst (1904)]

spiece

PAGE

15
16

17
18
19
21

25

27
27
28

30
31
32
33
33
34
35
36
38
39

40
41
43
52
53
54
55
56
57

LIST OF ILLUSTRATIONS

FIO.

30. Archaeopter is fimbriata,N ath. [Nathorst (1902)]

31. Archaeopteris fissilis, Schmalh. [Nathorst (1904)] .

32. Archaeopteris Hiichcocki, (Daws.). [White (1905)] .

33. Rhacopter is furciUat a, (LuAw.). [Potonie (1901)]

34. Sphenopteridium rigidum, (Ludw.). [Potonie (1901)]

35. Sphenopteridium Keilhaui, Nath. [Nathorst (1902)]

36. Cephalopteris mirabilis, Nath. [Nathorst (1902)]

37. Bothrodendron KiUorkense, (Haiigh.). [Original]

38. Archaeosigillaria Vanuxemi, (Goepp.). [Original]

39. Archaeosigillaria primaeva, Wh\te. [White (1907)]

40. Leptophloeum rhombicum, Daws. [White (1905)]

41. Protannularia laxa, (Daws.). [Dawson (1871)]

42. Rhacopteris paniculif era, Stur. [Stur (1875)]

43. Adiantitesfertilis, (White). [White (1905)] .

44. Sphenopteridium moravicum,{Eitt.). [Stur (1875)]

45. Sphenopteris affinis, L. & H. [Original] .

46. Sphenopteris bifida, L. & H. [Original] .

47. Aphlebiae of Pecopteris {Dactylotheca) plumosa, (Art.). [Zeiller

(1900)]

58
59
60
61
61
62
63
64
66
66
67
75
79
80
81
82

83

[ 1 ]

INTRODUCTION

It is now clear that in Devonian times, two terrestrial floras,
quite distinct as regards affinity, existed, one in the earlier part,
and one in the later portion of the Devonian period. The former
Avill here be termed the Psilopliyton flora; it consisted largely,
as we hope to show, of Thallophyta belonging, for the most
part, to a group now quite extinct which we propose to term
the Procormophyta or Propteridophyta. The later flora con-
sisted chiefly, but not entirely, of plants which were obviously
Pteridophyta. This assemblage we propose to term the Archaeo-
2)teris flora.

Our knowledge of both these floras, though still far from
complete, has been entirely revolutionised during the last few
years by the publication, both at home and abroad, of a series
of memoirs, to which we shall presently refer more in detail.
These invaluable contributions have necessitated a complete
revision of the whole subject, and since no general account of
these floras, including these recent advances, at present exists,
we propose to commence by a brief enumeration of the characters
of their more important genera. We have purposely omitted
from our review all the more doubtful types about which little
or nothing is known beyond the existence of very obscure or
fragmentary examples. We have further in Chapter III of this
Ijook confined our attention to a critical summary of the essential
features of the morphology and anatomy of the genera belonging
to the Psilopliyton flora, reserving for a separate chapter (p. 46)
the entire discussion of the question of their affinities and
systematic position. Finally in yet a further Chapter we discuss
the very important bearing of these new discoveries on the
phylogeny of Cormophyta and in particular of the various
Pteridophytic lines of descent.

The researches to which we particularly refer are firstly
Kidston and Lang's^ memoir on the Scottish plant Rhynia

^ Kidston and Lang (1917).

2 INTRODUCTION

•

Gwynne-V aughani, the first example of the Psilophyton flora to
be known in the petrified state. The importance of this beautifully
illustrated memoir can hardly be exaggerated. As we hope to
show, it offers us the key to the chief mysteries which have
hitherto surrounded the question of the affinities of this type of
land vegetation.

Next we have a series of important memoirs by Nathorst ^ and
Halle 2 on Lower and Middle Devonian floras from Norway.
There is, further, the interesting paper by Don and Hickling^
on the British Lower Devonian land plant, Parka decipiens.

With these recent contributions we may associate others,
published earlier, especially the works on Upper Devonian floras
by Nathorst* from Bear Island and Ellesmereland, by David
White ^ from the United States, and last but not least, the very
interesting Middle Devonian flora of Bohemia^.

1 Nathorst (1913), (1915). - Halle (1916).

3 Don and Hickling (1917). * Nathorst (1902), (190-1).

5 White (1905), (1907). « Potonie and Bernard (1904).

[ 3 ]

CHAPTER I

THE GEOLOGICAL AGE OF THE FLORAS

The question of the Geological age of the floras known from
various parts of the world is so vital to the conclusions of this
inquiry, that this matter demands special consideration at the
outset. So far as we are aware, no one has ever disputed the age
of the various beds from which these fossils are derived, with
a view to proving them to be younger than Devonian times.
Attempts have, it is true, been made in some cases to establish
a pre-Devonian age, particularly in Bohemia and Germany.
These views are however, we believe, now almost entirely
abandoned, and so they need not detain us here. In the great
majority of cases, there is stratigraphical or zoological evidence,
from the associated sediments, of undoubted Devonian age.
Further, as Nathorst and Halle have recently pointed out, the
De\'onian flora is now so well known, that the age can usually
be established by a consideration of the plant remains alone.

Such doubt on the geological side as may exist, relates not to
the question of age, but of horizon. This is often a more serious
difficulty, but it is not one Avhich is of first importance from
the botanical standpoint. We propose, however, to review the
evidence as to the horizon, wdiether Lower, Middle, or Upper
Devonian, of our more important Devonian floras, beginning
with that of Scotland.

Scotland. The Old Red Sandstone flora of Scotland has now
become of particular importance in view of the recent researches
on Rhynia and Parka to which we have drawn attention. The
former comes either from the Middle Old Red or from a lower
horizon. The latter is only known from the Lower Old Red, so
far as Devonian rocks are concerned.

The sub-division of the Scottish Old Red Sandstone has been
a matter of some difference of opinion, but the modern view is
that the original threefold classification proposed by Murchison
in 1859 is correct. Murchison distinguished an Upper, Middle,
and Lower division, each characterised by a pecvdiar fish faima.
This classification is now accepted by the Geological Survey of

I — 2

4

AGE OF FLORAS

[CH.

Scotland^ and by those ^ who in recent years have paid particular
attention to this question.

In Wales and some jjarts of Scotland, the Middle Old Red is
wanting, and the higher series rests directly, but with marked
discordance, on the lower. This, however, is a complication which
now presents no geological difficulty.

The correlation of the three divisions of the Old Red with the
three sub-divisions of the Devon facies of the Devonian is still,
to some extent, uncertain. The key to its solution is to be sought
in Russia, where the Middle Devonian includes both the Old
Red and Devon facies and faunas. Lower Devonian rocks appear
to be absent from Russia, but there is little doubt from the
evidence of the Middle Devonian as there developed, that the
three horizons in the Old Red correspond at least roughly to the
three main divisions of the Devonian^.

The following correlation, slightly modified from that given
by Frech^, expresses modern views on this point.

Old Bed Facies

Upper Old Red

(Cheirolepis, Holoptychius

and Asterolepis fauna)

Devon Facies
Upper Devonian

Middle Devonian

Middle Old Red

(Ptcrichthys, Coccostcus

and Osteolepis fauna)

Lower Devonian

Lower Old Red
(Pteraspis, Cephalaspis
and Pterygotus fauna)

1 Hinxman and Grant Wilson (1902) (see especially Appendix, Part I,
Palaeontologieal, pp. 81-83, by R. H. Traquair); Crampton and Carruthers
(1914); Home and Hinxman (1914).

2 Hickling (1908), p. 396; Macnair and Reid (1896).

3 Hickling (1908) and the references there quoted.
« Freeh (1897), p. 123.

i| AGE OF FLORAS 5

The horizon of the plant-bearing beds in the Old Red of
Scotland can in most cases be determined by the associated
fish faunas. Important specimens, including Psilophyton in a
petrified condition^, recently discovered in the Dryden Shales at
Rhynie, Aberdeenshire, by Dr Mackie^ are, however, exceptional
in that the precise horizon has not yet been ascertained. These
beds are, however, regarded as not younger than Middle Old
Red^, but since this genus is known to range throughout
Devonian time, the precise horizon of these beds is immaterial
from this point of view.

Ireland. The plant-bearing sandstones of the Old Red of the
South of Ireland are referred to the Upper Old Red (Upper
Devonian) on the evidence of the associated fish remains.

England. The few plants known from the Devonian of England
come from the type beds of the Upper Devonian (Baggy or
Cucullaea beds) in North Devon.

Belgium. Two plant-bearing horizons occur in Belgium which
on stratigraphical grounds are assigned to the Lower and Upper
Devonian respectively. These are the "Poudingue de Burnot"
and the "Psammites du Condroz,"

Germany. A small but important flora from near Herborn
in Hesse-Nassau has been recently referred to the Silurian. On
the fossil plant evidence, however, there can be no doubt that
it is of Devonian age, and belongs, in all probability, to the
Upper Devonian.

Bohemia. The large flora from the horizon {hi) of Barrande's
system of classification of the Devonian rocks of Bohemia was
likewise originally referred to the Silurian. The Devonian age of
these beds is now, however, admitted. More recently it has been
found'* that Stringocephalus Burtini, a characteristic Middle
Devonian fossil, occurs on a yet higher horizon (hS) and thus
the plant-bearing beds clearly also belong to this same zone.

Norivay. The evidence as to the horizon of the plant-bearing
beds in Eastern and Western Norway is purely palaeobotanical.
There is no zoological evidence. Nathorst, as we think rightly,

* For the case in support of this identification, see pp. 2rt-26.

- Mackie (1914); Home and Mackie (1917).

3 Kidston and Lang (1917), p. 7G2, footnote. * Jahn (1903).

6 AGE OF FLORAS [ch.

referred the Western Norwegian flora to the Middle Devonian,
while Halle assigned that of Roragen to the Lower Devonian.

Russia. In the Devonian of the Donetz Basin in Russia,
plants occur associated with a fauna believed to be Upper
Devonian.

Bear Island and Ellesmereland. The Upper Devonian horizon
of these beds is determined by the occurrence of fish remains.

Spitzhergen. In Spitzbergen two floras on different horizons
occur associated with fish remains. That of Mimers-Thal is
probably Upper Devonian, that of Dickson Bay, Lower Devonian.

Canada. In New Brunswick, plants occur on at least two
horizons, but until these beds have been more carefully studied,
it will not be certain whether the higher plant-bearing beds
are of Middle or Upper Devonian age. The lower plant horizon
of Gaspe is, however, undoubtedly Lower Devonian, as the
associated fish remains clearly indicate.

United States. Plant-bearing beds occur on several horizons
in the United States. Of these, the best known at present is the
Upper Devonian flora of the Perry Basin (S.E. Maine) probably
referable to the Chemung series. In New York State and else-
where other plants occur, some of which may be older thau the
Upper Devonian.

Australia. In New South Wales and Victoria, Devonian plant-
bearing rocks occur, some of which are believed on strati-
graphical grounds to be of Upper Devonian age. Certain plants
may even occur on lower horizons, but naturally a strict
correlation between these far distant rocks and the Devonian
horizons of Europe is a difficult matter.

The above are here relied upon as the best known Devonian
floras of which the horizons have been more or less satisfactorily
determined. Isolated members of these floras, particularly
examples of Psilophyton, occur in many other countries, such as
France and China, but these are not taken into consideration
here.

I] AGE OF FLORAS

Summary of the horizons of the chief Devonian
Plant-bearing beds.

Upper Devonian:

Scotland (Upper Old Red)
Ireland „ „ ,,

England (Marwood beds)
Belgium (Psammites du Condroz)
Germany (Hesse-Nassau)
Russia
Bear Island
Ellesmereland
Spitzbergen
United States
? Canada
Australia

Middle Devonian :

Scotland (Middle Old-Red)
Western Norway
Bohemia (Etage hi)

Lower Devonian :

Scotland (Lower Old Red)
Belgium (Poudingue de Burnot)
Norway (Roragen)
Spitzbergen
Canada (Gaspe)

CHAPTER II

THE TWO DEVONIAN FLORAS

As we have already stated, it is clear that two distinct floras
existed during Devonian times, not side by side, but successively.
This is clear from a comparison of the dominant genera of the
three horizons in that series. Among the earlier types, such
genera as Psilophyton, Arthrostigma and Hostimella^ are pro-
minent and many of the plants of the Upper Devonian period
are entirely wanting. This we propose to term the Psilophyton
flora. It was a flora not by any means sharply marked off from
that which preceded, or that which succeeded it. In the earlier
Devonian rocks, we find not only the dominant members of
this type of flora, but also survivals of a still earlier flora of
which Cryptoxylon, Nematophycus and Pachytheca are examples.
We do not say that these types are ever found in the same beds
as Psilophyton — all we remark is, that they existed at the same
period.

The land flora of Silurian times is at present almost unknown,
but we are acquainted, in Parka, with at least one British genus
of the Psilophyton flora, Avhich goes back as far as the Silurian.
It has been also stated by Dawson that Psilophyton itself occurs
in the Upper Silurian of Canada. It would thus seem that the
flora of the close of Silurian times, whether marine or terrestrial,
had much in common with that of the earliest stage in Devonian
history. We know of at least four genera common to these two
formations.

As we pass upwards from the lowest sediments of Devonian
age, we find that the members of the Psilophyton flora begin to
die out and that their place is taken by new arrivals, which, as

^ [The spelling HostimeUa and not IlostineUa is used here, since Jahn
(1903), p. 74, shows that the former is correct. See also Potonie, H. and
Bernard, C. (1904), p. 11. A. A.]

CH. iij THE TWO FLORAS 9

we shall show here, are of an entirely different morphological
nature. By the time we reach the Upper Devonian, these newer
types, the Archaeopteris flora, have become dominant in their
turn. Archaeopteris, Sphenophyllum, Bothrodendron, among
many other genera, are all clearly Pteridophyta. The members
of the Psilophyton flora were Thallophyta, as we hope to show
here.

In Upper Devonian times, some members of the Psilophyton
flora were still in existence, though in greatly reduced number
and in a position of subordination to the dominant Archaeo-
pteris flora. By the time we reach the next higher series, the
Lower Carboniferous rocks, the Psilophyton facies has entirely
disappeared. On the other hand, during this period, the
Archaeopteris facies reaches its maximimi develo]:)ment, and it
persisted unchallenged as regards dominance until the close of
this epoch, while many survivals lingered on even into Coal
Measure times. We have then, in the Lower Devonian, a very
ancient land flora, which in Middle and Upper Devonian times
was gradually displaced by a new flora which only reached its
maximum development in the earlier part of the Carboniferous
epoch. Thus, while the flora of the Upper Devonian is essentially
of the Carboniferous facies, that of the Lower Devonian is of
a quite different archaic tyjie. The following table shows the
distribution in Devonian time of the two floras.

A Summary of the Chief Genera of the Archaeopteris and
Psilophyton Floras ivith their distribution in Devonian Time.

Archaeopteris Flora Psilophyton Flora

Sphenophyllum )

„ , , ■ y Sphenopsidg

Pseudoborina J *^ '

Z

Psilophyton
Ptilophyton

Psygmophyllum Palaeophyllales Thursophyton

"< Archaeopteris i Barrandeina

I Rhacopteris | Barinophyton

^ Sphenopteris \ Taeniocrada

Q Sphenopteridium \ ^

§ Cephalopteris
g ? Cordaites
'-' Bothrodendron \

Archaeosigillaria . Lycopsida

Leptophloeum J

10 THE TWO FLORAS [ch.

Archaeopteris Flora Psilophyton Flora

H

a?

Ilyenia, Sphenopsida
Psygmophyllimi , Palaeophyllales

Sphenopteridium, Pteropsida

Archaeosigillaria \
Leptophloeum Lycopsida
Protolepidodendron )

Psilophyton

Broggerin

Thursophyton

Pseudosporochnus

Ilostimella

Barrandeina

2 >

? Leptophloeum, Lycopsida

Psilophyton

Arthrostigma

Parka

Ilostimella

Q

{Cryptoxylon)

We now propose to review the distribution of the above genera
in the standard Devonian floras indicated in Chapter I.

Scotland.

A revised Smmnary of the Devonian Flora of Scotland^ occurring
in the Old Red Sandstone with the dates of their first record
from Scotland.

Upper Old Red Sandstone (Caithness)- :

Ptilophyton Thomsoni, Dawson 3, 1878 (includ. Caulopteris
Peachii, Salter*, 1859).

Thursophyton Milleri, (Salter) % 1858.

T. Reidi, (Penhallow)", 1892.

Archaeopteris hibernica, (Forbes)', 1857.
Middle Old Red Sandstone (Cromarty)*:

Psilophyton princeps, Dawson**, 1841.

^ This list chiefly differs from that given by Kidston in 1902 (Kidston
(1902)) in regard to the horizons to which the fossils are ascribed, which
we have given in accordance with the conclusions of the Scottish Geological
Survey. - Crampton and Carruthers (1914).

3 Carruthers (1873), PI. 137; Dawson (1878), p. 385 and PI. IV.

* Salter (1859), p. 408, Fig. 14a.

5 Salter (1858), p. 75; Penhallow (1892), p. 5; Nathorst (1915), p. 17.

6 Penhallow (1892), p. 8, PI. I, fig. 2; Reid and Macnair (1899), PI. XXII;
Nathorst (1915), p. 19.

7 Miller (1857), p. 454, Fig. 124. « Home and Hinxman (1914).

9 Miller (1841), p. 100, PI. VII, figs. 3-5; Miller (1857), Fig. 119, p. 429,
Fig. 120, p. 432; Dawson (1859), p. 481.

II] THE TWO FLORAS 11

Lower Old Red Sandstone (Forfarshire, Perthshire, etc.)' :
Psilophyton princeps, Dawson-, 1859.
P. ornatum, Dawson^, 1871.
P. robuslius, Dawson-, 1859.
^r//*ro,s7/om« ^rac/Ze, Dawson*, 1871.
ZostcrophyUnm mifretonianum, Penhallow*, 1892.
Parka dccipicns, Fleming^, 1831.
Cryptojcylon Forfarcnse, Kidston', 1897.

The great peculiarity of the Old Red Sandstone flora of
Scotland, so far as it is at present known, is its wealth of members
of the Psilophyton flora and its poverty in examples of the
Archacopteris flora in its highest stage.

Ireland. The Upper Devonian flora of the South of Ireland
includes several species of Archaeopteris {A. hibernica, Forbes ^'^^
and A.TscJiermaki, Stur ^), Bothrodendron Kiltorlxense, (Haught.^"),
and Sphenopteris Hookeri^^, (Bailey). No members of the older
archaic flora have yet been recognised.

England. The Upper Devonian of Devonshire^^ has yielded
Sphenopteridium rigidum, a fragmentary Sphenopteris, a unique
fructification, Xenotheca, a Telangium and a doubtful leaf of
Cordaites; there are no traces of archaic forms,

Belgium. The Upper Devonian rocks of Belgium^^ contain
Archaeopteris, Sphenopteridium condrusorum, and a species of
Sphenopteris, associated with Barinophyton, and possibly
Psilophyton. The Lower Devonian flora^^ of the same country is
very obscure. Arthrostigma (the so-called Lep)idodendron Gas-
pianum) and Psilophyton probably occur.

Germany. In the Upper Devonian of Hesse-Nassau ^*, Spheno-
pteridium rigidum, an Archaeopteris (the Sphenopteris densepin-
nata of Ludwig) and a JRhacopteris (the CydojJteris furcillata,
etc., of Ludwig and Triphyllopteris of Schimper) are possibly
1 HickHno (1908). - Dawson (1859).

^ Included hitherto as a variety of Dawson's P. princeps.
* Dawson (1871), p. 41; Kidston (1894), p. 102.

5 Penhallow (1892), p. 9, Pis. I, II.

6 Fleming (1831); Don and Hickling (1917).
' Kidston (1897).

« Carruthers (1872); Kidston (1888) and (1906).

9 Johnson (1911^). i" Johnson (1913).

" Kidston (190G). , i- Arber and Goode (1915).

'3 Crepin (1874) and (1875); Gilkinet (1875^), (18752).
'* Ludwig (1869); Potonie (1901).

12 THE TWO FLORAS [ch.

associated with Psilophyton (? the Palaeophycus gracilis or
Noeggerathia bifurca of Ludwig).

Bohemia. The Middle Devonian flora of Barrande's horizon
h 1 in Bohemia^ consists almost entirely of members of the
Psilophyton flora, including Arthrostigma, Hostimella and
probably Psilojjhyton, with Pseudosporochnus, Barrandeina,
Protolepidodendron and Thursophyton. The only more modern
types are a doubtful example of Sphenopteridium and Archaeo-
sigillaria (the Protolejndodendron Scharyanum of Krejci and
Potonie).

Norway (Roragen). The probably Lower Devonian flora of
Roragen ^ comprises Arthrostigma, Psilojjhyton, Hostimella,
Aphyllopteris, and Sporogonites, without any more modern types.

Norway (Western). The possibly Middle Devonian of Western
Norway ^ includes Spiropteris, Aphylloptej'is, Thursophyton,
Broggeria, and possibly Barrandeina, associated with the more
modern types, Psygmojjhyllum and Hyenia.

Russia. From the Upper Devonian of the Donetz Basin^ are
known two species of Archaeopteris, examples of an isolated
fructification (Dimeripteris) with a species of Sphenoptcris and
possibly a Lepidodendron. No members of the Psilophyton flora
occur.

Bear Island (Arctic Regions). We find here a very interesting
Upper Devonian flora of the Archaeopteris type, including
Archaeopteris Roemeriana, (Goepp.) and other species, Botliro-
dendron Kiltorkense, (Haught.), species of Sphenopteridium,
Sphenophyllum and Stigmaria associated with the rare types
Pseudobornia ursina, Nath., and Cephalopteris mirabilis, Nath. ^.
No archaic types have been recognised.

Ellesmereland (Arctic Regions). From the Upper Devonian
of Ellesmereland^ two species of Archaeopteris {A. Archaetypus
and A. fissilis which both occur in Russia) are associated with
a Sphenopteridium.

Spitzbergen (Arctic Regions). In rocks assigned to the Upper
(or possibly the Middle) Devonian of Spitzbergen', we find a

1 Potonie and Bernard (1904); Stur (1881).

2 Halle (1916); Nathorst (1913). » Nathorst (1915).
* Schmalhausen (1894). ^ Nathorst (1902).
6 Nathorst (1904). " Nathorst (1894).

iij THE TWO FLORAS 13

Psygmophyllum and a Leptophloeum and perhaps a Bothrodendron.
From Lower Devonian beds, a Psilophyton-\\\;.e plant is known,
associated also with a doubtful Psygnioplnjlluiu (the CyclojJteris
sp. of Nathorst).

United States and Canada. In the Perry Basin^, South-east
Maine, we find PsygmophyUiDn Brownianum, (Dawson), several
species of Archaeopteris {A. Jacksoni, Daws., A. Rogersi, Daws.,
etc.), species of Sphenopteridium and Dimeripteris, associated
with archaic types such as PsilopJiyton, Barrandeina, Barino-
phyton, Leptophloeum, etc. This flora is Upper Devonian, pro-
bably Chemung, in age. In New York State, fossil plants
{Archaeosigillaria) occur in the Portage group of the Upper
Devonian 2, and in the Middle Devonian of Ohio^ (Corniferous
Limestone) where Barrandeina (the CaulojJieris spp. of Newberry)
is associated with a Leptophloeum and petrified wood. Other
examples are also known, but are neglected here, since the
records are somewhat obscure. In Canada, a very similar flora
to that of Perry occurs in the Upper Devonian^; but this is in
urgent need of revision. With regard to the Lower Devonian
flora of Gaspe, it is undoubted that species of Psilophyton and
Arthrostigma, and probably other types occur associated with
Nematophycus. But until this flora also has been revised, the
list of genera occurring on this horizon must remain somewhat
uncertain.

Australia. In Devonian rocks in Australia, especially in
Victoria and New South Wales, Leptophloeum australe (usually
known as Lepidodendron australe) is frequent in beds assigned
with certainty to the Upper Devonian^, and possibly also in
others of Middle Devonian age. On the Genoa River, Auckland ^,
a fragmentary leaf like that of Cordaites occurs, associated with
Archaeopteris, and Sjjhenopteris. The only archaic type known is
Barinophyton. A similar flora of Archaeopteris {A. Howitti and
A. Wilkinsoni), Sphenopteris, and Cordaites is associated with
Leptopliloeum and possibly Bothrodendron in Victoria, but without
an}^ archaic forms so far as is known.

1 White (1905). 2 white (1907). » Newberry (1889).

* Dawson (18.59) and (1871), exchidintr the fossil plants from St John's,
N.B., which are of Upper Carboniferous age.

* David and Pittman (1893). « Dun (1897).

[ 14 ]

CHAPTER III

RECENT ADVANCES IN OUR KNOWLEDGE
OF THE MORPHOLOGY AND ANATOMY OF
MEMBERS OF THE PSILOPHYTON FLORA

It has been known since the days of Hugh Miller that in the
Old Red Sandstone of Scotland a number of simple if somewhat
obscure plant remains are to be foimd. A similar flora was first
described by the late Sir William Dawson, from the Lower
Devonian of Canada in 1859. Halle^ has recently pointed out
that these are "the remains of the very oldest land-flora at
present known ; and it may be stated at once that there is a far
greater difference between this flora and that of the Upper
Devonian than between the latter and the Lower Carboniferous."

These fossils, occurring as impressions with only slight traces
of their original anatomical structure, have luitil recently been
generally regarded as very doubtful objects and much scepticism
has been expressed by botanists as to the morphological inter-
pretation of the Scottish and Canadian plants given by Dawson,
Penhallow and others. As we now know, this scepticism has
been largely misplaced. Except in the matter of affinities, on
which point the evidence hitherto has ahvays been very slender,
the earlier accounts of these fossils were extremely accurate.
The fact that these plant remains are apparently of a simple
type of habit has often been explained by an appeal to the
imperfection of the record. Such fossils were commonly regarded
as mere petioles or rachises of fronds which had been so damaged
before preservation that no trace of the lamina now exists.

It is now, however, quite clear that this conception is funda-
mentally erroneous. We know now, in several cases, what was
practically the whole of the plant body, and it is clear that
instead of dealing with fragments of Cormophytes, as was
formerly supposed, we are in reality confronted with a vegetation
occvipying a lower place in the scale of plant evolution.

In order to make these matters clear, Ave propose in the present
1 Halle (1916), p. 4.

CH. Ill]

PSILOPHYTON

15

chapter of this memoir to pass in review the main facts relating
to the morphology and anatomy of these fossils. In each case
we begin with a summary of what appear to us to be the critical
features of each genus and we then
pass on to a discussion, also of a
critical nature, of the more recent
advances in our knowledge of each
type.

It should, however, be clearly
luiderstood that while some of the
genera here discussed are now placed
on a firm scientific footing, many
others remain extremely obscure,
and are as yet only of minor in-
terest. In the matter of the litera-
ture, we have contented ourselves
as a rule by quoting only the latest
of a series of memoirs dealing with
each subject. References to the
earlier literature will be found in the
papers quoted.

PsiLOPiiYTON (including Rhynia
and Dawsonites).

(Figs. 1-7.)

Psilophyton, Dawson, 1859. Ter-
restrial plants, consisting of a rhizome
from which dichotomously branched,
erect axes arise, the terminations
of which are circinately coiled in
the young state. Shoots leafless,
vascular, possessing stomata, and
emergences, the latter being either
macro- or microscopic. Fructifica-
tion consisting of sporangia borne ter-
minally on some of the erect shoots ;
wall of sporangium multi-layered.

Fig. 1. Psilophyton. Dawson's
restoration of the sterile thal-
lus, published in 1859. This
restoration is correct except
in the matter of the lateral
organs borne on the longest
(central) axis. After Dawson
(1859).

16

THE PSILOPHYTON FLORA

[CH.

Distribution. Devonian and perhaps Silurian. In Scotland
Psilophyton apiDarently occurs only in the Lower and Middle
Old Red. It however occurs in Upper Devonian rocks in the
United States and possibly in Belgium and Germany. It is thus
clear that the genus is distributed throughout Devonian times.
According to Dawson ^ it also occurs in Silurian rocks of Canada.

Fig. 2. Psilophyton. Fertile fragments restored
by Reid and Macnair in 189G. a, b, fertile axis;
c, circinate vernation; d, rhizomata. Considerably
reduced. After Reid and Macnair (1896).

Several species of Psilophyton variously regarded as quite, or
as probably, distinct, occurring in Scotland and Canada, were
first discriminated by Dawson in 1859 or in 1871. Unfortunately
on the subject of species of this genus there is at present con-
siderable confusion and a critical revision of Psilophyton is noAv,
in the light of Rhynia, more than ever needed. The difficulty
here lies in the fact that it can only be attempted by some

1 Dawson (1871).

Ill]

PSILOPHYTON

17

authority i:esident in America where the type specimens of
Dawson are located.

According to modern opinion, Dawson^ in 1871 included at
least two species under the term P. princeps. Confusion has also
arisen from the attribution by Carruthers^ in 1873 of some of
the Scottish examples to Goeppert's Haliserites Dechianus, as

Fig. 3. Psilophyton princeps, Dawson. Type
specimens of Dawson's variety " ornatum,^''
with macroscopic emergences. Tlie left-hand
figure sliows the circinate vernation. After
Dawson (1871).

P. Dechianum (Goepp.). In fact at the present time the Scottish
plant is probably better known under that name than by any
other. These difficulties may be overcome as follows. Carruthers'
determination should be completely ignored. In our opinion it
is erroneous. Further the specimens which he figiu'cs do not
belong to Psilophyton at all, but to a distinct genus Ptilophyton.

1 Dawson (1871).

Carrutliers (1873).

18

THE PSILOPHYTON FLORA

[CH.

The difficulty in regard to Dawson's species Avill, we believe,
vanish, if it can be shown, as we shall attempt to demonstrate

here, that the presence or ab-
sence of macroscopic emergences
or so called spines is a matter
of no systematic importance.
The species Psilophyton jyrinceps
should include those erect shoots
with fairly stout spine-like emer-
gences (Fig. 3), those on which
the emergences are small delicate
structures (Figs. 4 and 5) and,
further, those stems which are
apparently smooth and without
macroscopic emergences of any
kind. This it may be remarked
is exactly the view of the im-
portance of these structiu'cs
which Dawson himself lu'ged.

He states 1 explicitly that
most observers would separate
specifically the two types P.
princeps and P. ornatum, but he
believes they pass into one
another and cannot be clearly
separated on these grounds.
We may therefore enumerate
the species of impressions of
Psilophyton occurring in Scot-
land as follows:

(1) P. princeps, Daws.^ (in-
cluding the variety P. ornatuin,
Erect shoots, slender or of medium thick-
or laterally branched, forks wide, bark
macroscopically smooth or covered with scattered, or numerous

Fig. 4. Psilophyton princeps, Daw-
son, from the Lower Devonian of
Roragen, Norway. 1. Attributed
by Halle to the genus Arthro-
stigina, but described as a "narrow,
PsilophytonAike stem." 2. Part of
"1" twice enlarged to show nerves
of emergences. 3. Attributed by
Halle to Psilophyton princeps, or
possibly to Arthrosiigma. After
Halle (1916).

Daws.3) (Figs. 3-5).
ness, dichotomously

1 Dawson (1871), p. 39. '' Dawson (1871), PI. IX, figs. 102-108.

3 Dawson (1871), PI. IX, figs. 97-101, 104, 104 a, 109-110, PI. X, figs.
112-114, 118.

Ill] PSILOPHYTON 19

and crowded, small chaffy scales or larger spinc-likc processes.
Tips of branches when young circinately coiled. Erect shoots
proceeding from a horizontal rhizome bearing rhizoids. Shoots
vascular, bearing stomata. Sporangial wall multi-layered,
sporangia borne on the finer terminations of some of the younger
macroscopically smooth shoots, singly or in pairs (Fig. 6, p. 21).
(2) P. robustius, Daws.^. Stems rather stout, bark smooth or
slighth^ furrowed without macroscopic emergences, branching
chiefly lateral or pseudo-dichotomous when terminal. Termina-
tions of branches bearing sporangia in clusters.

Fig. 5. Psilophytou princeps, Dawson. Type speci-
mens (1859) with small scale-like emergences, two
of the specimens showing the circinate vernation.
After Dawson (1859).

(3) P. elegans, Daws. 2. Axes very slender, dichotomously
branched, produced in tufts from thin rhizomes. Surface smooth,
with very delicate wrinkles, but without macroscopic emergences.
Fructification (?) believed to consist of small oval bodies borne
below the bifurcations of the axes.

More recently important observations have been published on
Psilophyton by Solms Laubach^, and David White*. The former
recognises P. princeps alone as a good species. Dawson's other
types of the same genus are regarded as indefinable. White*
discriminates between a spiny type of Psilophyton [P. ornatum)

1 Dawson (1871), PI. XII.

2 Dawson (1871), p. 40, PI. X, figs. 122, 12:5.

* Solms Laubach (1895), p. 7G.

* AVhite (1905), p. 61.

2—2

20 THE PSILOPHYTON FLORA [ch.

and a smooth type with characteristic costatiou and no evi-
dence of spines or scales. The latter is a more lax type, and freely
branched.

The most recent work bearing on this genus is that of Halle
on Lower Devonian specimens from Norway, and of Kidston
and Lang on the Scottish Devonian Rhynia. We Avill now con-
sider these very important contributions in some detail beginning
with Halle's conclusions.

Halle ^, in an elaborate attempt to apply some definite
meaning to the term Psilophyton — an attempt with which at
the time it was written we were much in sympathy — would wish
to confine this term to those stems alone which, in whole or
part, bear spine or leaf-like organs. He says " in order to establish
an acceptable definition of the genus Psilophyton, it is necessary
to confine its use to stem-like structures bearing spines or small
leaves. Isolated branch-systems without spines... cannot be
regarded as belonging to Psilophyton unless they are found in
actual connection with spine-bearing Psilophyton-stems"." The
non-spinous stems bearing fructifications (Fig. 6), which Dawson
referred to P. jorinceps, are removed by Halle to a distinct genus
Daivsonites, as D. arcuotus n. spec. ; according to Halle the term
P. princeps should be used only for the spiny type of stem (Fig.
3), the fructification of which he asserts is not as yet known.

Throughout Halle's criticisms it is clear that he shared in no
small degree the doubt which others had long cast on the
correctness of Dawson's morphological and taxonomic con-
clusions. It is perfectly true of course that Dawson did not
prove, by means of incontestable figures, many of his state-
ments, in the manner which we have learnt to expect in modern
research. It has also to be borne in mind that when Halle wrote
his memoir, he did not know of the entirely new light which
Rhynia has since shed on these questions.

It must however be confessed that even at that time there
were no just grounds for discriminating species merely on the
presence orabsence of macroscopic scale or spine-like emergences.
Several species oi Psilophyton [P. robustius, P. elegans, etc.) were
already known in which no such emergences are foimd, and thus
1 Halle (1916). - Ibid. p. 22.

Ill]

PSILOPHYTON

21

the removal of the smooth axes bearing fnictifications to a separate
genus could hardly be justified. Hailed himself describes a new
s^oecies {P. GoUhdnnidtii) in which the axes below were spinous,
though without visible macroscopic emergences in the higher

Fig. G. Psilophyioii princeps, Dawson, from the
Lower Devonian of Roragen, Norway. Fer-
tile axes (the Daivsonites arcuatus of Halle)
without macroscopic emergences. After Halle
(1916).

parts. We shall return to this point a little later when we shall
endeavour to show that in P. j^^'i'i^cejJS, the axes ahvays bear
emergences, though sometimes they are of microscopic size. We
may agree with Halle that the spiny shoots {P. ornatum of
Dawson) are not known in the fertile state. These shoots are

1 Halle (1916), p. 21.

22 THE PSILOPHYTON FLORA [ch.

probably old shoots and the old shoots may have been sterile.
At any rate we think it probable that the degree to which the
emergences are visible depends partly on the age of the shoots,
though it may, as Dawson states, be also a very variable
character, depending perhaps on habitat. Our point, however,
is that a fossil is not justly excluded from the genus Psilophyton
merely on the ground that it has no visible emergences.

We may also agree that Halle ^ was the first to discover the
vascular nature of the spiny type of stem {P. ornatum), but it was
shown by Dawson and has been more recently confirmed by
Kidston and Lang that the apparently spineless stem is also
vascular. The fructifications of what we here term P. princeps
(figured by Halle ^ under the name Dazvsonites arcuatus, sp. et
gen. nov.) are the best examples we know in the form of im-
pressions (Fig. 6). They are described as "terminal capsules of
a narrowly obovoid or short fusiform shape and usually 3-5 mm.
long." Spores have not been recognised in them, a fact which
has become immaterial in view of the fuller evidence of the same
organs which we now possess in the petrified state (Rltj/nia).

We now reach the most recent contribution to the subject of
Psilophyton, and undoubtedly the most important yet made,
namely a recent account of a member of this genus published by
Kidston and Lang^ under the name Rliynia Gzoynne-V aughani.
For reasons which will be fully discussed a little later, we have no
hesitation in referring Rhynia to Psilopihyton and this species is in
all probability either P. prince2JS or P. elegans as here defined.
The great interest of these Scottish specimens, from a chert
bed, not younger than the Middle Old Red, at Rhynie in
Aberdeenshire, is that the plants are not only petrified but
complete. They occur in a most remarkable series of beds of
silicified peat, crowded with stems of this plant in situ. The
description of the habit and morphology of Rhynia given by
Kidston and Lang confirms in a remarkable manner the account

^ Halle (1916). The xylem elements are described by Halle, as by Dawson,
as scalariform, whereas in Rhynia they are annular, but from impressions
and macerated material it must be very difficult to distinguish between
these two types of thickening.

- Halle (1916), p. 25, PI. 3, figs. 1-9, PI. 4, figs. 18-21.

* Kidston and Lang (1917).

Ill] PSILOPHYTON 23

of Psil<yphyton given by Dawson as far back as 1859, on which so
much scepticism has since been expressed. There is the same habit
(Figs. 1, 2, pp. 15, 16) — branched underground rhizomes attached
to the peaty soil by numerous rhizoids, and bearing erect aerial
shoots, eight inches or more in length and ranging from 6 mm.
to under 1 mm. in diameter. These shoots bore no leaves. They
occasionally branched dichotomously. They bore "small hemi-
spherical projections Avhich were more or less closely placed
without apparent regularity. On some of these bulges tufts of
rhizoid-like hairs were borne, while in other cases the projections
developed into adventitious branches,... some aerial axes ended
in large, elongate-pointed sporangia^."

The most interesting matter in connection with these stems
is that the anatomy, which is very perfect^ preserved in silica,
is often but not always vascular. Here again we have an im-
portant confirmation of a fact which Dawson as early as 1859
had been able to ascertain from simple impressions of similar
stems. There is a small central strand of xylem (annular
tracheids), surrounded by phloem and externally by a wide
parenchymatous cortex and a well-marked epidermis. There is
no distinct endodermis or pericycle. Stomata^ occur on the
aerial stems, but they do not appear to have been frequent. Some
slender axes are also met with in which no vascular tissues are
developed. Sieve plates have not been recognised in the phloem
region. All the tracheids of the xylem are alike in size, there being-
no obvious protoxylem groups and no conjunctive parenchyma.

The branches were vascular, but apparently their steles were
not connected with that of the main axis.

The sporangia varied considerably in size, but seem to have
attained a length of at least 12 mm. Their walls were several
layers thick. They contained an enormous number of spores.

As Dawson originally pointed out, these plants were land

plants, and Kidston and Lang's^ account of the habitat of

Rhynia as "a peaty soil practically composed of the decaying

remains of the same species" agrees fundamentally with

1 Kidston and Lang (1917), pp. 765-6.

- Dawson (1871), p. 90, stated that stomata occurred but he did not
figure them clearly on PI. XI, fig. 129.
3 Kidston and Lang (1917), p. 771.

24 THE PSILOPHYTON FLORA [ch.

Dawson's earlier conclusions on this point. These authors also
all agree in finding in Psilotum the nearest existing type of habit.

We now pass on to consider the question of the identity of
Rhynia with Psilophyton. These authors, after discussing this
question, conclude that the two genera are distinct. They
compare Rhynia with P. princeps and conclude that the latter
differs from the former "in the presence of spines, in the more
profuse dichotomous branching, in the subordination of some
of the branches to a sympodial main axis, and in the absence,
so far as we know, of lateral adventitious branches^."

The weak point of this argument is that it does not take into
account species of Psilophyton other than P. princeps. As we
have seen, several other species of this genus, e.g. P. elegans and
P. ro&W5^iw5, have neither visible spines nor scales, while the latter
has " slender alternate branches " arising from a relatively robust
axis^. The agreement of/?//^«?'a with Dawson's account of P. prin-
ceps and P. elegans appears to us to be so close that we have no
doubt as to a generic identity, at least, existing between these types.

In support of this contention as to the generic identity of
Rhynia with Psilophyton, we have one constructive addition to
make to the discussion, and this is perhaps important. Rhynia
was not a sjaineless type, despite Kidstbn and Lang's assertion.
The small "lateral protuberances or bulges" of those authors
are the spine-like emergences. In order to realise this, it is only
necessary to compare the Figs. 7 and 8 on PI. Ill of the paper
of these authors, which show the surface of stems of Rhynia
enlarged fourteen times, with the similar figures of Halle's
Psilophyton princeps (i.e. the very spiny type, Dawson's P.
ornatum) on PI. 2, figs. 3 and 5 of Halle's paper, which are of
nearly equal magnification (x 16). These figures are reproduced
here in Fig. 7. This comparison is we think conclusive, and it
also settles once and for all the correlation of tlie external
morphology of the spines with their internal anatomy.

We may add that on microscopic examination, certain impres-

^ Kidston and Lang (1917), p. 779. [In a more recent paper, Trans. Roy. Soc.
Edinb.Yol. 52, 1920, p. 603, Kidston andLang have described a second species
of Rhynia, R. major which shows no trace of adventitious branches. A. A.J

2 Dawson (1859), Fig. 2 a, p. 481.

Ill]

PSILOPHYTON

25

sions of P. princeps^ (the nearly smooth type) from the Middle
Old Red of Cromarty in Scotland, collected by the present author
some years ago, appear to show indistinct indications of the
emergences comparable to those of Rhynia and Halle's specimens.

Fiff. 7. The proof of the identity of Psilophyton and
Rhynia. («) Enlarged surface view of a petrified axis of
Rliyiiia Givynne-V aughani showing surface sculpture and
several emergences ("the small projections or bulges" of
Kidston and Lang) ( x 14). After Kidston and Lang
(1917). {b) Enlarged surface view of an impression of
Psilophyton princeps showing surface sculpture and two
emergences (the "spines" of Halle) ( x 16). After Halle
(1916).

It is thus we think clear that all known examples of P.
princeps bore scales or spines, and the anatomy of these structures
shows clearly that they are morphologically emergences and
neither leaves nor branches. In many cases, however, especially
near the apex of the shoots, these emergences are so small and
^ No. 25 Devonian Plant Coll., Sedgwick Museum, Cambridge.

26 THE PSILOPHYTON FLORA [ch.

scattered that they are invisible macroscopically. As they
increase in size and in number, they gradually become more and
more macroscopic, at first resembling chaffy scales, later spinovis
outgrowths. These facts explain the different appearance of
shoots of the same plant to which we have already referred
(pp. 21, 22) when discussing Halle's conclusions.

In view of these facts, impressions oi Psilophyton which, when
withovit emergences, must appear to be very featureless fossils,
can no longer be regarded as doubtful objects or as unworthy
of serious consideration. The genus is now as important as any
from a phylogenetic standpoint, as we shall see later.

It may eventually prove convenient to retain the term Rhynia
as a type of internal structure and if this is necessary such would
seem to be the chief justification of its existence. As a generic
term Psilophyton has undoubted priority.

Arthrgstigma.
(Figs. 8, 9.)

Arthrostigma, Dawson, 1871 1. Axis very stout, bifurcating
and giving off lateral members, irregularly furrowed or ribbed
longitudinally, bearing numerous large and long scattered,
straight, sometimes falcate, spine-like organs. Axes possessing
a slender central strand of vascular tissue. Fructification un-
known.

Distribution. Lower Old Red, Scotland; Lower Devonian,
Canada, Norway, and (?) Belgium; Middle Devonian, Bohemia.

According to Halle 2, this plant displays considerable variations
in the shape of the spine-like organs and their manner of attach-
ment. There are first of all cylindrical impressions with a radial
arrangement of their leaf-like projections, though, in a few
instances, cases of a pseudo-verticillate arrangement occur.
"There can sometimes be noted a very fine but distinct vein-
like line running through the leaf, "...which "no doubt represents
a vein or vascular strand." No leaf scars on the stem can be
seen in the case of the Norwegian specimens.

Secondly, there are stems with " unusually densely and regularly
placed leaves." A third type consists of stouter specimens " with
1 Dawson (1871), p. 41. 2 Halle (1916), p. 7.

Ill]

ARTHROSTIGMA

27

/ ^

Fig. 8. Arthrostigmagracile, Dawson,
from the Lower Old Red Sandstone
of Scotland. (Nat. size.) After Kid-
ston (1893).

4 .. f

:/

Fig. 9. Arthrosiigma gracile, Dawson,
from the Lower Devonian of Roragen,
Norway. After Halle (1916).

28

THE PSILOPHYTON FLORA

[CH.

Fig. 10. Thursophyton Milleri, (Salt.)
from the Middle Devonian of Western
Norway. Sterile axes with scale-like
emergences. After Nathorst (1915).

1 Kidston (1893), p. 108.

thick, short (rudimentary or
broken) distant leaves." The
leaves here are pyramidal and
thorn -like.

\^'ith regard to the mor-
phology of these spine-like
processes, Kidston ^ has con-
cluded that these are not " the
bases of leaves, as has been
suggested, but are the leaves
themselves, though developed
in a very rudimentary form,
as in Psilophyton."

Halle 2 says "the append-
ages of these stems can hardly
be anything but leaves." To
this we may reply that, on
the present evidence, they
are quite as likely to be emer-
gences, especially in view of
the similar structures occur-
ring in certain species of
Psilophyton, to which we ha^•e
just drawn attention. On our
view the emergences of Ar-
throstigma and Psilophyton
differ chiefly in size and the
variation in this respect in
the former genus (which Halle
has pointed out), as we have
just said, is exactly what we
should exjject to find in view
of the corresponding varia-
tion met with in Psilophyton
(see pp. 21 and 22).

Halle also confirms Daw-
son's conclusion that the
stem was vascular, the vas-

2 Halle (1916), p. 13.

iiij THURSOPHYTON 29

cular tissue forming a solid column without any pith. This
much can be made out by macerating impressions.

Petrifactions of this genus are as yet unknown, nor do we
know anything of its fructifications.

We think it probable however that Arthrostigma and Psilo-
phyton are nearly related genera.

Thursophyton.
(Figs. 10, 11.)

Thursophyton, Nathorst^, 1915. Axes possibly herbaceous,
dichotomously branched, of uniform thickness. Axes covered
with crowded, imbricated, small (about 7 mm. long) scale-like
emergences, spirally arranged and lanceolate in form, swollen
at base, curving upwards. No leaf scars occur on thti stem.
Fertile shoots similar to the sterile axes bearing large (?) sporangia
in the axils of some of the scale-like emergences.

Distribution. Upper Old Red, Scotland; Middle Devonian,
Bohemia, Western Norway.

This fossil was originally described under the name Lycopodites
Milleri, Salter, but, as Nathorst has pointed out, it has little or
nothing in common with the Carboniferous plants referred to
that genus and it is therefore best transferred to a new genus,
Thursophyton. The first fertile shoots were described by Pen-
hallow- and later by Rcid and Macnair^ as a distinct species
T. Reidi, Penh. (Fig. 11, 1, p. 30). Here what appear to be
globular sporangia, 1 mm. in diameter, frequently occur in the
axils of some of the scale-like emergences. Nathorst^, who has not
seen actual specimens of these fertile shoots, regards Penhallow's
interpretation of these fossils as fertile as being extremely
doubtful. He suggests that it is not yet proved that the organs
in question are sporangia and that they might be foreign bodies.
While admitting the Justice of some doubt on these points and
the need of further evidence, we are inclined to think that
Penhallow's interpretation will eventually prove to be in the
main correct. For in the case of another species of the same genus,

1 Nathorst (1915), p. 17. ^ Penhallow (1892).

3 Reid and Macnair (1896). * Nathorst (1915), p. 19 footnote.

30

THE PSILOPHYTON FLORA

[CH.

Thursophijton'^ (the Lycopodites hostimensis of Potonie and
Bernard) (Fig. 11, 2) from the Middle Devonian of Bohemia,
what are clearly sporangia occur in much the same way as in
Penhallow's plant. The sporangia here are also large, 1-5 to

#

'4

§

>

: '

..:,.■ .- 2.

Fig. 11. (1) Thursnphyion M?7Zm.(Salt.) =T. Heidi,
Penh., from the U))])cr Old Red Sandstone of Scot-
land. A fertile shoot. After Reid and Macnair
(1899). (2) Thursopln/Um hostimeme, (P. & B.),
from tlie Middle Devonian of Bohemia. Fertile
shoots. After Potonie and Bernard (1904).

2 mm. in diameter, circular or perhaps a little elliptical. The
stem in this case is dichotomous and the leaves are apparently
broader and perhaps less crowded than in T. JReidi. We thus
have very little doubt that the fertile shoots of Thursophyton

1 Potonie and Bernard (1904), p. 45, Fig. 105 on p. 44.

Ill]

THURSOPHYTON AND PTILOPIIYTOX

31

are known and that their aspect is remarkably Lycopodian. The
lateral organs, in view of the evidence of Psilophyton, we shovild
be inclined to regard as scale-like emergences, though they may
have begun to function as leaves. At any rate they do not on
the present evidence appear to have been vascular.

Fig. 12. Ptilophyton Thomsoni, Daws. The lower
part of a main axis from the Upper Old Red of
Scotland. After Salter (1859).

32

THE PSILOPHYTON FLORA

[CH.

Ptilophyton.
(Figs. 12-14.)

Ptilophyton, Dawson^,
1878. Main axis (Fig. 12,
p. 31) very stout, striated,
(the Caulopteris ? Peachii
of Salter 2; cf. also the
genus Barrandeina) giving
off stout lateral decurrent
shoots almost at right
angles. Branches covered
with scale-like emergences.
At the apex the axis is
freely and closely alternate-
ly branched, producing a
tuft of shoots ^, the ends of
which are circinately coiled
(Fig. 18). The ramifications
of this tuft bear, apparently
on one side, a row of long
thin (? filamentous) obscure
organs, the nature of which
is unknown. They have
been described as " tufts of
linear bodies^."

Distribution. Middle De-
vonian, ? Bohemia; Upper
Old Red, Scotland.

This genus, markedly
different from Psilophyton
in habit, is still entirely ob-
scure. We may note how-
ever that what is probably

1 Dawson (1878).

2 Salter (1859), p. 407, Fig. 14
on p. 408; Kidston (1902) de-
finitely states that Caulopteris
Peachii, Salter, is the stem of
Ptilophyton Thomsoni, Dawson.

3 Carruthers (1878), PI. 137.

Fig. 13. Ptilophyton Thomsoni, Dawson.
The terminal portions of axes possessing
emergences. Type specimen from the
Upper Old Red of Scotland. (Reduced
about |.) After Carruthers (1873).

ml PTILOPHYTON AND PSEUDOSPOROCHNUS 33

\

Fig. 14. Plilophyion (?) hostimeiisc,
(P. & B.). Type specimen from
tlie Middle Devonian of Bohemia.
After Stur (1881).

Fig. \ 5. Pseu(losj)orocfinus
Krejcii, (Stur), from the
Middle Devonian of Bo-
hemia. Lower extremity
of axis showinji basal
bulb. (INIuch reduced.)
After Potonie and Ber-
nard (1904).

3

84

THE PSILOPHYTON FLORA

[CH.

Fig. 16. Pscuchspowchims Krejcii, (Stur). (o) Branching of thallus (re-
duced), (b) Terminal portion of thallus (much reduced). After Potonie
and Bernard (1904).

Ill]

PSEUDOSPOROCHNUS AND BROGGERIA

35

another representative of it (Fig. 14) occurs in the Middle Devon-
ian of Bohemia and was referred by Potonie and Bernard ^ to
tlie genus Spiropteris. These examples are again all equally
obscure.

PSEUDOSPOROCHNUS.

(Figs. 15, 16.)

Pseudosporochnus, Potonie & Bernard^, lOOi. Axis stout and
undivided below, bulbous? at base (Fig. 15), freely branched
above (Fig. 16) in a pedate manner, secondary branches further
dichotomised above, the slender branches of the third order
being repeatedly and frequently dichotomised so that the higher
parts of the secondary axes are clothed with fairly dense tufts
of delicate, dichotomous, very narrow branchlets. The stems are
known to be vascular.

Distribution. Middle Devonian, Bohemia.

This very remarkable plant is apparently only known from
Bohemia. No fructification is described.

Fig. 17. Broggeria norvegica, Nath., from the Middle Devonian of
Western Norway. (Somewhat reduced.) After Nathorst (1915).

1 Potonie and Bernard (1904), p. 11, Text-figs. 1-5 on p. 12.

2 Potonie and Bernard (1904); Stiir (1881).

3—2

36

THE PSILOPHYTON FLORA

[CH.

Fis. 18. Barrandeina Dusliana, (Kr.), from the Middle Devonian of Bohemia.
(About § nat. size.) After Potonie and Bernard (1904).

in J imOGGERIA, BARRANDEINA, BARINOPHYTON 37

Broggeria.

(Fig. 17.)

Broggeria, Nathorst^, 1915. Stout branched axes, of which
some terminate in large cyhndrical sporangial sjjikes or catkins,
lip to 50 mm. long and 15 mm. broad.

Distribution. Middle Devonian, Western Norwa}^; ? Upper
Devonian, United States.

The stems recall species of Psilophyton, but the fructifications
are entirely dissimilar to anything known in that genus.

Barrandeina.
(Fig.. 18.)

Barrandeina, Stur^, 1881. Ribbed dichotomous axes; the ribs,
which are formed by deeurrcnt bases of lateral axes spirally
arranged, are longitudinal, irregular, broad, low, flat, nearly
contiguous, somewhat obscure.

Distribution. Middle Devonian, Bohemia, ? Western Norway
and United States; Upper Devonian, United States.

No fructification is known in connection with these remarkable
axes which appear to be largely made up of decurrent leaf bases.
The lower portion of the axis of Ptilophyton (see Fig. 12, p. 31)
appears to be somewhat similar and no doubt several of the
specimens from the Devonian of America attributed by Dawson
and Newberry^ to the genus Caulopteris belong here.

Barinopiiyton.
(Fig. 19.)
Barinophjiton, D. White^, 1905. Axes thick, smooth or
irregularly ribbed, bearing alternate stout compact boat-shaped,
fertile branches, usually lanceolate, consisting of a very thick
llcshy keel, bearing on either side on its ventral surface, a row
of alternating small thick oblong or oblong-lanceolate scales or
bracts. Bracts fleshy at the base, more or less distinctly carinate,
provided with a small ventral pit or pocket, probably the seat
of a sporangium.

1 Nathorst (1915). - Stur (1881); Potonie and Bernard (1904).

3 Newberry (1889). * White (1905).

38

THE PSILOPHYTON FLORA

[CH.

Distribution. Upper Devonian of Belgium, United States,
Canada and Australia.

This fertile shoot is at present
wholly obscure, but it is very un-
like any other organ known from
more recent rocks. It is widely
distributed in Devonian rocks.

Parka.
(Fig. 20.)
Parka, Fleming i, 1831. Body
small, of variable size, rarely ex-
ceeding one or two inches across,
more or less circular, lenticular,
of very small thickness, containing
many disc-like oval or circular
masses, which in their turn contain
spores.

Distribution. Silurian and Lower
Old Red, Scotland and England.
Not known outside Britain.

Don and Hickling^, in an im-
portant and quite recent paper on
this mysterious fossil, have shown
conclusively that the disc-like masses of Parka undoubtedly
contain spores and that the fossil is thus clearly of vegetable
origin. In this matter they confirm the conclusions of Dawson
and of Penhallow^ and more recently of Reid, Graham and
Macnair^. Don and Hickling show that Parka consists of a flat,
dorsiventral, multi-cellular and multi-layered thallus, develop-
ing by marginal growth (Fig. 20, 1). The shape is roughly
circular or oval ; but lobate, reniform and even irregular forms
occur. In size the thalli measure 5 mm. to 7 cm. across. The
margin, Avhich is distinctly frilled, is usually less than 1 mm.
broad. The rest of the thallus is composed of small oval

1 Fleming (1831). " Don and Hickling (1917).

3 Dawson and Penhallow (1891); Penliallow (1892).
* Reid, etc. (1897) and (1899).

Fig. 19. Bdriiiojiii/jton Bichard-
soni, (Daws.), from the Upper
Devonian of the United States.
After White (1905).

Ill

PARKA

39

thickened discs of nearly constant size, visually 2 mm. in
diameter, and of variable number. The discs are isolated spore
masses, containing numerous cuticularised spores (Fig. 20, 3) of
wliieh there is no evidence that they were formed in tetrads, or
that they ^vere heterosporous. These discs ncAcr overlap, though
they coalesce occasionally. A grooved lamina (Fig. 20, 2) occiu-s
on one side of the thallus and is probably ventral.

In general habit Don and Hickling compare Parka with the

Fig. 20. Parka decipiens, Flem., from the Lower Old Red of Scotland:
(1) a large thallus (natural size); (2) the folded lamina (x2); (3) spores
( X 150). After Don and Hickling (1917).

Coralline Alga Melohesia (Lithophyllum) lichenoides, Ag. of the
Rhodophyceae (Fig. 21) and they conclude that the thallus
grew probabh^ attached to or on the surface of mud or sand.

With regard to the all -important question of affinity, Don
and Hickling dissent entirely from the Dawsonian view that
Parka was a sporocarp of "a somewhat generalised plant,
shadowing forth the recent rhizocarps^," They regard it as a
very low spore-bearing plant, belonging to a group which

^ Reid, Graham and jNIacnair (1897).

40

THE PSILOPHYTON FLORA

[CH.

possibly no longer exists, in fact as a " Thallophy te with Algal
affinities^."

As to whether Parka, as here described, represents the whole
plant, there must always remain a slight doubt, until petrified
specimens are known. This uncertainty however appears to us
to be very slight, and Don and Hickling find no evidence that
Parka represented "aquatic plants with creeping stems, linear
leaves" as had been asserted by Dawson and Penhallow^ as late
as 1891. They have also failed to discover any evidence of the
prothalli and heterospores which the earlier workers believed
they could recognise.

lO.'lt'v--

Lq?.^

nf\nAi\Anr>nnononofiAOf^f^nnnnnnnnononnn

Fip. 21. Liviiifj Coralline Alffae for comparison with Parka. (1) Litho-
phifUum licbenokles, (E. & S.), external morpholooy (nat. size). (2) Melobesia
membranacea (Esper) Lamour, vertical section through a conceptacle contain-
ing tetraspores( x 350). Both after Rabenhorst's Kryx>togamen-Flora (1885).

There can be no doubt that Don and Hickling's work on
Parka has removed a cloud of doubt and suspicion in regard
to this fossil, much of which appears to have been ill-founded.
Several points will remain uncertain until petrified material is
discovered. All that has so far been made out of the structure
of the thallus has been accomplished by means of macerating
carbonised and ctiticularised impressions. At the same time the
important conclusion that we are dealing here with a lowly
Thallophyte, not far removed from the Algae, and even com-
parable in habit to certain living Coralline Algae, is one which
is not likely to be displaced, but rather to be confirmed, by a
knowledge of structure material.

1 Don and Hickling (1917), p. 661.

2 Dawson and Penhallow (1891), p. 16.

Ill]

ZOSTEROPHYLLUM

41

Fig. 22. ZosteroplnilliiDi mijret()nianio)i,Y'enh.,fron^ the Lower
Old Red of Scotland. (Slisjhtly reduced.) After Keid and
Macnair (1899).

42 THE PSILOPHYTON FLORA [ch.

Taeniocrada.

Taeniocrada, D. White^, 1905, This name is applied to a very
fragmentary and obscure palmate "frond," deeply dissected
into dichotomous lobes, with an indistinct central strand or axis.
It is regarded as an "alga" by David White.

Distribution. Upper Devonian, United States.

ZOSTEROPIIYLLUM.

(Fig. 22.)

ZosterophyUuin, Penhallow^, 1892, Obscure elongate ? axes
aggregated in the form of a tuft, arising from a common horizontal
?axis; erect? axes longitudinally finely striated; striations equal,
parallel ; axes ? ribbon-like, linear, simple or dichotomously
branched. Some of the ? axes bore small, ? lateral, rounded or
oval sporangium-like bodies.

Distribution. Lower Old Red, Scotland.

This fossil is at present wholly obscure.

Protolepidodendron.

Protolepidodendron, Krejci^, 1879. Small leafy twigs with
small, oval-lanceolate leaves; leafless stems with spirally
arranged leaf bases; leaf bases small, fusiform; leaf scar absent
or indistinct (? decorticated).

Distribution. Middle Devonian, Bohemia.

A very obscure type, P. karlsteini, in Bohemia.

LIOSTIMELLA.

(Fig. 23.)

Hostimella, Potonie and Bernard^, 1904. Repeatedly bifur-
cating slender branch systems without emergences, or other
distinctive characters, though sometimes possessing circinate
vernation. Typical example, H. hostimensis, P. & B. var.
rhodeaeforniis, P. & B.

1 White (1905).

2 Penhallow (1892); Reid and Macnair (1899).

3 Krejti (1879); Potonie and Bernard (1904).
" Potonie and Bernard (1904).

Ill]

HOSTIMELLA AND APHYLLOPTERIS

43

Distribution. Lower Devonian, Norway (Roragen); Middle
Devonian, Bohemia.

The examples of this oenus are still entirely obscure objects.
They appear to stand nearest to Psihrphyton.

Fig. 28. Ilostimdla liostiinensis, P. & B. var. rliodene-
formia, P. & B., from the Middle Devonian of Bohemia.
(About half nat. size.) After Potonie and Bernard
(1904).

Aphyllopteris.

Aphyllo'pteris,- Nathorst, 1915 =-'i Pteridorackis, Nathorst,
1902. This gemis, as emended by Halle^, consists of stout rachis-
like branch systems, without emergences, leaves or fructifica-
tions, and not dichotomously branched. As used originally bj'
Nathorst, it included more slender dichotomously branched types
such as Hostimella.

Distribution. Lower Devonian, Norway (Roragen); Middle

Devonian, ^Vestern Norway; LTpper Devonian, Bear Island and

other countries.

1 Halle (1916), p. 24.

44 THE PSILOPHYTON FLORA [ch.

This type is again wholly obscure and many examples of it
no doubt represent small fragments of some of the preceding
eenera above discussed.

Sporogonites.

Sporogonites, Halle, 1916. Isolated stalked sporangia; spor-
angia obovoid or clavate, G-9 mm. long and 2-4 mm. broad;
apex roimded, base attenuated.

Distribution. Lower Devonian, Roragen, Norway.

This type has been recently described by Halle ^ from impres-
sions from Roragen as examples of a Bryophytic sporogonium.
He claims to have made out by maceration methods that the
lower part of the capsule was "sterile throughout, the upper
part consisting of three different zones : a wall of several layers
of cells, a thick sporogenous tract and a sterile central columella^."
The spores are tetrahedral, globular, 0-020-0 -025 mm. in diameter,
with cutinised walls.

An attentive examination of the description of these specimens
given by Halle, has left us entirely unconvinced that any valid
grounds exist for regarding these sporangia as sporogonia. In
the absence of well-petrified material, it appears to us that the
present distribution of the spores Jnay well be secondary and
not original. The fact that the wall of the sporangium, as we
prefer to call it, is several layers in thickness has no bearing on
the matter. The walls of the sporangia oiPsilophyton (= Rhynia),
as Kidston and Lang have shown, are also multi-layered, and
to our eyes there is nothing about that genus w^hich suggests
affinities with the Bryophyta. The presence of a cohuiiclla we
regard as entirely unproven, and we doubt very much if the
presence of such an organ, even if it imdoubtedly existed, could
be established from material preserved in the manner of the
Norwegian specimens. That Sporogonites may be something more
complicated than a sporangiimi with a simple uni-layered wall
is quite possible, but even admitting this, it appears to us that,
on the present evidence, its relationships are to be sought for

1 Halle (1910).

2 Ibid. p. 27.

Ill] SPOROGONITES 45

among the Thallophyta rather than the Bryophyta. If the
sporanoium Avere sej^tate, the longitudinal septa might be
easily mistaken for a eolumella in material so imperfectly
petrified 1.

^ [Tlicse criticisms seemed justified at the time they were written, but
the discovery by Kidston and Lang of a Middle Devonian plant, Ilornea
Lignieri, in which the si)orangium undoubtedly possesses a columella, puts
the whole question in a different light. The specimens of Hornea are
petrified and the whole organisation of the plant is shown; the columellate
sporangia are borne terminally on the dichotomous branches of a stem of
the Rlii/Nia type. While Hornea and Sporogouites are evidently quite
distinct, there is now every reason to believe that Halle's interpretation
of the structure of his fossil was essentially correct; the importance of his
discovery is manifest, whatever view may be taken of the affinities of the
plants in question. (See Kidston and Lang, On Old Red Sandstone Plants
showing Structure, from the Khynie Chert Bed, Aberdeenshire. Part ii.
Additional Notes on Rfiynia Gwynne-Vaugfiani, Kidston and Lang; with
Descriptions of Rfiynia major, n.sp., and Hornea Lignieri, n.g., n.sp. Trans.
Roy. Soc. Edinb. Vol. 52, Part ni. 1920, p. 603.) D. H. Scott.]

[ 46

CHAPTER IV

A DISCUSSION OF THE NATURE AND
AFFINITIES OF THE PSILOPHYTON FLORA

The earlier conclusions as to the affinities of Psilophyton and
other members of that flora, advocated enthusiastically and
primarily by Dawson, and followed by some other workers,
need not detain us here. Dawson's ^ frequently repeated assertion
that Psiloijhyton and Parka in particular were related to the
Hydropterideae, or so-called Rhizocarps, was regarded by many
with grave suspicion, even at the time when no rival theory of
affinity was in the field. It is only necessary to add that all the
more recent work, especially the most recent studies of all
relating to these genera, has not produced, at any rate in our
opinion, one particle of evidence in favour of Dawson's con-
clusions as to affinity. In fact it may now be said that, whatever
views one may hold on this question, it is at any rate certain
that these plants were not related to the Water Ferns.

In any discussion of the affinities of these plants, the evidence
of Psilophyton must stand first. We know now the entire plant,
both in the form of impressions and petrifactions, and we are
thus in a singularly fortunate position , where questions of
affinity are involved. We may first, however, state the views
of those who have quite recently contributed so greatly to our
knowledge in regard to this genus.

Halle 2 has no hesitation in regarding Arthrostigma gracile as
a microphyllous Pteridophytc, and he extends this conclusion
to Psilophyton princeps. He appears to base his conclusions
largely on the presence of a true vascular strand in these
plants.

Kidston and Lang have no doubt that " Rhynia and Psilophyton
belong to the Vascular Cryptogams or Pteridophyta^" and they

1 Dawson (1888) and in many other places.

2 Halle (1916). ' Kidston and Lang (1917), p. 779.

CH. IV J PROCORMOPHYTES 47

propose to place them in a new class "the Psilophytales^,"
"characterised by the sporangia being borne at the ends of
certain branches of the stem without any relation to leaves or
leaf-like organs 2." Among existing Pteridophyta the authors
find in the living genera of the Psilotales the closest parallel to
Psilojjiiyton.

These conclusions, however, should not be accepted without
some reservation. For our part we find ourselves unable to
adopt them, for it appears to us that Psilophyton has been,
misinterpreted and that this and all the other genera belonging
to what has here been called the Psilophyton flora were much
more probably Thallophyta than Pteridophyta, This however is
likely to be a matter of prolonged controversy, involving a dis-
cussion of what we mean exactly by the former term.

In using this term here we recognise that the real problem is— -
was Psilojjlujton simply a Thallophyte or was it a very reduced
Pteridophyte ? In supporting the former view, as opposed to
recent workers on these fossils, we do not urge that, because this
or other genera were Thallophytes. they were necessarily Algae
in the sense in which that group is usually defined from a know-
ledge of its living members. On the contrary we think that
Psilophyton and some though perhaps not all the other genera,
belonged to a now obsolete race of Thallophyta, higher in the
scale of complexity than any living Algae. These plants we
propose to term the Procormophytes, and they will be further
discussed in a later chapter (p. 70).

Several members of the Psilophyton flora appear to have been
Algae pure and simple. This is the case with Taeniocrada
according to ^^'hite3 and Parka according to Don and Hickling^.
On the other hand in Psilojihyton, Arthrostigma and Pseudo-
sporochnus, we meet with other genera which appear to occupy
a somewhat higher position in the scale of morj^hological com-

1 This term is open to considerable objections on the grounds that it is
too simihu- in form to another already in general use, i.e. the Psilotales.
If it is maintained, confusion is certain to arise from the similarity between
these names.

2 Kidston and Lang (1917), p. 779.

3 White (1905).

* Don and Hickling (1917).

48 THE PSILOPHYTON FLORA [ch.

plexity, particularly in the possession of a vascular strand in the
main axis.

This anatomical fact, of the greatest interest and importance,
seems to have mesmerized both Halle and also Kidston and
Lang^ to such an extent that the thought that Psilojjhytoti might
be a Thallophyte, does not appear to have ever occurred to them.
The position is much the same as when Brongniart argued that
Sigillaria, because it possessed secondary wood, must be a
Gymnosperm ! The possession of a vascular strand in the main
axis does not appear to us to be a necessary sign of Pteridophytic
affinity. Thallophyta are living to-day which possess a well-
marked phloem. Thallophyta may have existed in the past
Avhich possessed a xylem strand. If they were terrestrial and
not hydrophytes, it is highly probable that this was the case.
Psilophyton was undoubtedly terrestrial. It is an immensely old
type taking us back to days when terrestrial Algae of a high
grade may have existed, though now long since extinct. At any
rate it would be a very rash conclusion to deny that such ])lants
have ever existed.

The Thallophyta are a race of plants which can by no means
be kept within such narrow boimcls. A race which among at
least some of its members had evolved alternation of generations,
a cormophytic habit, and true phloem, would if necessity arose
be quite capable of evolving a lignified conducting tissue. If
some of its members were at one time land plants, such a
necessity would be obvious.

We have next to enquire whether Psilophyton j^resents any

evidence of being an extreme case of Pteridophytic reduction

rather than a Thallophyte. Here it appears to us that both the

morphological and anatomical evidence is emphatically against

the former view. If Psilophyton is such a reduced type, how is it

that the xylem of the axis is reduced to a single protoxylem

strand, a state of affairs unknown in any other, however highly

reduced plant, whether living or fossil ? How is it that a lack of

1 [It should be noted tliat the present memoir was completed before the
ajjpearance of the following 'papers by Kidston and Lang : On Old Red
Sandstone Plants showing Structure, from the Rhynie Chert Bed, Aberdeen-
shire, Parts II and iii. Trans. Roy. Soc. Edinb. Vol. 52, 1920, pp. G03, 643.

A. A.]

IV] PROCORMOPHYTA 49

vascular connexion exists between the main axis and its
branches ? Surely here the evidence is emphatically on the side
of primitiveness ?

The morphological data a]:)pear to us to be equally emphatic.
In the first place the fructification is wholly Thallophytic. The
sporangia are quite unlike those of any fern borne on a highly
reduced frond, and they find their nearest homologues among
the Red Algae and those forms which possess a simple type of
carjiogonium. We have further no evidence at all of more than
one type of reproductive organ, although the complete plant is
undoubtedly known.

Further there is the evidence of the emergences, for such we
believe to be the real nature of the lateral organs on the erect
axes. These are anatomically non-vascular and histologically
emergences and not branches, as their anatomy clearly shows.
As we have pointed out here, these structures, in Psilophyton,
are of varied size, micro- or macroscopic, and in the latter case
scale-like. So far as we can see there are no grounds, either
anatomical or morphological, for regarding these structures as
leaves, however reduced. Yet this is the interpretation which
must be put upon them if these genera are to be regarded as
very reduced Pteridophytes. Finally the habit of PsilopJ/ytori, a
rhizome giving off rhizoids, and erect naked axes, some termi-
nating in sporangia, is much more typically Thallophytic than
Pteridophytic.

We fail to find any groimd of comparison except in habit
(which taken alone is a perfectly valueless character) to the
living plant Psilotum. The fructification and the vascular
structure of the two are quite distinct. Further if, as we believe,
Psilotum is rightly placed in close relation to Tmesipteris, it is
obvious that any affinity between these plants and Psilophyton
must be very remote.

We thus regard Psilophyton as first and foremost a Thallophyte,
which, while still Thallophytic in habit, may occupy anatomically
a place half-way between the Thallophyta and Pteridophyta.
We propose to term such plants Procormophyta.

With regard to the other genera of the Psilophyton flora, the
evidence is less emphatic, since they are less completely known.

50 THE PSILOPHYTON FLORA [ch.

But in many cases there is a strong family resemblance. Both
Arthrostigma and TJiursojjhyton possess abundant emergences,
and also in habit are obviously not far removed from Psilophyton.
The former is also known to be vascular. The habit of Pseudo-
sporochnus (Figs. 15, 16, pp. 33, 34.) is very remarkable and again
we are dealing with a vascular type. The bulb at the base of
the main axis (Fig. 15, p. 33) is an exceedingly algal feature, but
in the upper part of the plant the finer branches assimie a more
or less Pteridophytic form (Fig. 16 b, p. 34), whereas the lower
axes (Fig. 16a) resemble Thallophytes. Stur^ originally regarded
this genus as an Alga, pure and simple and even placed it in a
living genus {SporocJinus-, Ag.) of that group, but Potonie and
Bernard^ rightly point out that this genus is not a mere Alga,
as its vascular structure clearly shows.

With regard to the other genera of the Psilophyton flora,
Ptilo])hyton^, Hostimella, Broggeria, etc., avc make no remark in
this connection, save to point out that each is thalloid, and
appears to have some features in common with Psilophyton, and
that none of them are obvious Pteridophytes. They are however
still far too imperfectly known to afford any secure evidence of
the affinities of the flora to which they belong. Zosferophyllum
is perhaps the most obscure of all, though as regards hal)it. it
may be compared with species of the living Alga, Nemalion.

We shall discuss the bearing of the conclusions expressed here
in regard to P.silo2)hyto)i in a later chapter (p. 70),

In connection with the Psilophyton flora there remain one or
two matters which require consideration. It might be urged
that Psilophyton is a highly reduced ty})e. The stomata are few
and it might be thought that it is a reduced xerophyte. Against
this view is to be set the clear anatomical evidence that the
lateral organs are emergences and not reduced leaves. It is very
unlikely that the latter would be microsco})ic, as we have seen
they frequently are in Psilophyton. The absence of any leaf
traces is again not what we should expect in the case of a leaf-
less xerophyte. But probably the most convincing argument of

1 Stur (1881), 1). 342. 2 Potonie and Bernard (1904).

^ Tlie terminal portion of this thallus is distinctly al<)al in appearance.
Cf. Procarnium spp.

iv] LOWER AND UPPER DEVONIAN FLORAS 51

all is that, if we regard Psilophyton as a reduced Cormophyte,
all the other Procorniophytes such as ArtJirostigma, TImrsnjyJiijton,
etc. must be likewise regarded as reduced Cormophytes. This is
clearly not the case and there is thus every reason to regard the
Psilophyton flora as primitive and not reduced, especially as it
can be shown that more highly evolved types sprang from them
(see Chapter VI of this memoir).

Further, we know of no geological reasons which would lead
us to suppose that the conditions of existence of plant life were
at all different in Lower and Upper De^'onian times. In the
later epoch, members of the Psilophyton and Archaeopteris
floras, as we have seen (pp. 9, 10), existed side by side, and were
the latter in existence in Lower Devonian times there would
seem to have been nothing to have prevented them from
flourishing equally well at that period. As a matter of fact they
did not then exist, for the incoming of the Archaeopteris flora
is plainly indicated in IMiddle De\'onian times, and it was clearly
not established until the latest epoch of that period.

52

CHAPTER V

RECENT ADVANCES IN OUR KNOWLEDGE

OF THE MORPHOLOGY OF THE

ARCHAEOPTERIS FLORA

Before discussing the phylogenetic deductions which, as it
appears to us, may be drawn from a consideration of the Psilo-
phyton flora, it maybe well to review our present knowledge of
the later or Archaeopteris flora of the younger Devonian rocks
(see table, p. 9),

This flora is clearly Coriiiophijtic and Pteridophytic but it
sprang from the Psilophyton flora which as we have seen was
Tltallo phytic.

In this case again we shall only notice the less obscure types,
and in the case of genera which are well known and described
in every text-book, we have not added any diagnoses.

Sphenopsida.

Sphen ophyllu m , Brongn. ,
1828 (Fig. 24). Only a single
species of this Well-known
genus is recorded from the
Upper Devonian of Bear Is-
land^. Like all the earlier
representatives of the genus
it has small, very narrow,
highly divided leaves. The
species with entire or nearly
entire Avedge - shaped leaves
are not known earlier than
the Upper Carboniferous. The
Devonian type is very similar
to, perhaps even identical with,
a - species occurring in the
Lower Carboniferous.
Nathorst (1902).

Fig-. 24. SpheiiophijUum sitbtcucrri-
mum, Nath., from the Upper De-
vonian of Bear Island. Considerably
reduced. After Nathorst (1902).

CII. v]

HYENIA

53-

\ v^

y-^

\>u

Vi/

Fi'j. "-i"*. Uijciiid sjiheno/ilnjUoifles, Nath., from
the Middle Devonian of Western Norway. (Nat.
size.) After Nathorst (1915).

54

THE ARCHAEOPTERIS FLORA

[CH.

Hyenia, Nathorst^, 1915 (Fig. 25), Leafy shoots radiating in
tufts from a stem or rhizome, shoots coalescent at the base.
Shoots bearing leaves in whorls at nodes, which are either not
prominent or very indistinct. Leaves at least four, perhaps to
six in a whorl, successive whorls superposed. Leaves small.

Fii>'. 26. Pseudobornia ursiiia, Xath., from the Upper
Devonian of Bear Island. (1) Leafy shoots (much reduced).
(2) Fertile shoots (greatly reduced). Both after Nathorst
(1902).

10-15 mm. long, rarely 20 nmi. long and 1 mm. broad, uni-
nervcd, forked once or more rarely tAvice at apex. Fructification
unknown.

Distribution. Middle Devonian, Western Norway.

This type appears to be near to SpJienopInjllKtu but perhaps
differs in the habit and in the fact that the nodal lines are not
evident.

Psendohornia, Nathorst^, 1894 (Fig. 26). Stems large, seg-

1 Nathorst (1915). - ifo/f/. (1894).

V]

PSYGMOPHYLLUM

55

Fi<y. 27. Pstjgmojt/iijllum Kolderu/ii, Nath., from the Middle Devonian of
Western Norway. Leaves and leaf}- shoots. (Nearly nat. size.) After
Nathorst(1915).

56

THE ARCHAEOPTERIS FLORA

[en.

mented, nodes curved, irregularly branched. There may be 1-2
branches on each node. Leaves shortly stalked, borne in whorls
of four at the node, dichotomously divided, margins finely
toothed, veins fan-like. Fructification a catkin-like body, up to
32 cm. long, with short internodes; sporophylls whorled,? forked
dichotomously, ? all fertile in the lower parts; sporangia con-
taining megaspores.

Distribution. Upper Devonian, Bear Island.

This very interesting type is a now well-acknowledged member
of the Sphenopsida. It is a very rare plant.

AW

Fig. 28. Archaeopteris hibcrmca,
(Forbes), from the Upj)er Old Red
of Kiltorkan, Ireland. (1) Complete
frond (greatly reduced). (2) Fertile
portion of a frond (much reduced).
(8) Sterile pinnules (reduced). After
Carruthers (1872).

Palaeophyllales 1,

Psygmophyllwn, Schimper,
1870 (Fig. 27, p. 55). Leaves
large, flabellate or cuneiform,
arranged spirally on an axis,
sheathing at the base, which
is fairly broad and not con-
tracted to a slender petiole.
Apex broad, rounded or trun-
cated, entire or lobed, or
slightly divided. Nerves flabel-
late. Fructification entirely
unknown.

Distribution. ? Lower De^'o-
nian, Spitzbergen; ? Middle
Devoniaii, Western Norway;
Upper Devonian, Ellesmere-
land, Spitzbergen, Canada and
United States; Lower and
Upper Carboniferous.

This is a ^'ery striking and
Avidely distributed genus in
Devonian rocks. A monograph
of it was published by the

1 Arber (1912), p. 405.

vj

ARCHAEOPTERIS

57

Fig. 29. Archaeopteris Archettfpus, Schniiilli., from the Upper Devonian of
Ellesmercland. (Nat. size.) After Nathorst (1904).

58

THE ARCHAEOPTERIS FLORA

[en.

writer 1 some years ago. The large wedge-shaped leaves are in
most cases detached, except in specimens from the Middle
Devonian of \Vestern Norway, and much younger examples from
the Coal Measures of the Newcastle coal field, England, and the
Permo-carboniferous of South Africa, where they are borne
spirally on a shoot. The Devonian and the British (Carboniferous)
examples agree remarkably in habit in this respect.

Although several species of the genus are known, we remain
entirely unacquainted with the fructification.

Fig. 30. Archaeopteris fiuibriata , Natli., from the
Upjier Devonian of Bear Island, (f nat. size.)
After Nathorst (1902).

It has been thought jDrobable that PsygmophijUuin belongs to

a distinct group of plants, for which the name Palaeophy Hales

was suggested by the writer some years ago. Whether this view

will prove to be correct or not, depends primarily on the nature

of the unknown fructification. Until this has been discovered it

is probably wiser to assimie that it represents a distinct tj^pe

than to include it with any other Palaeozoic genus, for among

such plants the habit of Psygmophyllum is quite unique,

1 Ar])er (1912). To complete the lists contained in this monograph the
figures of Plaly]>hyllii)ti Brozviiifniiim of D. White (1905), of P. obtusa by
Prosser (1894), PI. II, and those of Nathorst from Western Norway, Nathorst
(191.5), should be added. It is now admitted that the Permian types from
Russia at one time included in this genus are in reality quite distinct.

vj

ARCHAEOPTERIS

Pteropsida.

59

Archaeopteris, Dawson, 1871^ (Figs. 28-32). Fronds of large
size, bipinnate with a stipiilar base, stipules in pairs, adnatc, and
a ranicntum on the lower part of the petiole. Sterile pinnules typi-
cally obo\'ate or ovatc-cuneate, entire ortoothed, with a flabellate

Fig. .31 . Ardtaeopleris fissilis, Schmalli., from tlic Upper Devonian of EUes-
mereland. (Nat. size except fig. marked 9, which is x|.) After Nathorst
(1904).

1 See Carruthers (1872); Kidston (1888); Nathorst (1902) and (1904);
Wliitc (190.5); Kidston (1906), p. 4.34; .Johnson (1911-).

60

THE ARCHAEOPTERIS FLORA

[CH.

nervation, or less typically the pinnules are lobed, fimbriated or
divided longitudinally into numerous narrow spreading seg-
ments. Sterile pinnules occur on the main axis between the
insertion of the pinnae. Fertile pinnules ^ very reduced, occurring
on the same frond as the sterile leaflets, usually the whole or
part of the lower pinnae of the frond being fertile. Fertile

Fig. 32. ArcJiaenpteris IlUchcocki, (Daws.),
from the Upper Devonian of United States.
A fertile frond. The type specimen (nat. size)
after White (1905).

pinnules bearing single or grouped (2-3) sporangia, sessile or
shortly stalked, sporangia fairly large, fusiform or oval, ex-
annulate.

Distribution. Upper Devonian to INIiddle Coal Measures. The
affinities of this genus will be discussed at a later stage (p. 81).

^ Sporangia may also occur on the margins of pinnules similar to sterile
leaflets, Kidston (190G), p. 434, and Johnson (191 1-).

V]

RHACOPTERIS AND SPHENOPTERIDIUM

61

Rhacopteris, Schimper, 1872 (Fig. 33). Fronds pinnate or
dichotomously branched. Pinnules large, unsymmetrically
wedge-shaped, rhoniboidal, typieally entire or more or less
deeply lobed or divided longitudinally, Avith a radiating un-
symmetrical nervation. Higher part of the frond sometimes
fertile, si^orangia tnfted, small, exannulate, globular.

This genus is very rare in the Devonian, though it appears
to occiu- on that horizon in German3\ It is more characteristic
of the Lower Carboniferous.

Distribution. Upper Devonian to Middle Coal Measures.

Fig. 83. RJtacopleris fiircillata,
(Ludvv.), from the Upper Devo-
nian of Germany. (About J nat.
size.) After Potonic (1901).

Fig. 34o Sphenopteridiinn rigi-
dum, (Luaw.), from the Upper
Devonian of Enaland and Ger-
many. (About I nut. size.)
After Potonie (.1901).

Sphenopteridium^, Schimper, 1874 (Figs. 34, 35). An indefin-
able generic name applied to a particular type of Spheno2:)terid
frond in which the pinnules are highly divided into very narrow

1 This term is to be preferred to Rhodea, Presl, 1838, since that term is
preoccupied for Angiosperms {Rhodea, Endlicher, 1837; Rvhdea, Roth,
1821).

62

THE ARCHAEOPTERIS FLORA

[cir.

linear or filiform forked lobes as in the type species S. rigidum,
Lndw. (Fig. 34).

Distribution. Middle Devo-
nian to Upper Carboniferous.

Cephalopteris, Nathorst^,
1910 (Fig. 36). Axes branched,
branches opposite in distichous
pairs, each pair connate on one
side of the axis and decurrent.
Lower portions of branches
fertile. Sporangia in dense
spherical heads arising from
the decurrent base of each
lateral branch. Sporangia long,
])ointed, dehiscing ? longi-
tudinally. Sterile foliage of
the ? Sphenopteridium type,
upper portions of branches
bearing small dichotomised
leafy segments.

Distribution. Upper Devo-
nian, Bear Island.

If Nathorst is correct in
correlating certain sterile leaf
segments (Fig. 36 (5)) with the
fertile main axes — a point on
which he expresses no doubt,
though to us there seems to be
no proof beyond mere associa-
tion— then this type is simply
a fertile Siihenopteridium, \ni-
less in its fructification it is
distinct from other members
of that genus, the fructifica-
tions of which are at present
quite unknown. For the

Fig. 35. Spheuopteridium KeiUiaui,
Nath., from tlie Upper Devonian of
Bear Island. (Nat. size.) After Na-
thorst (1902).

1 Nathorst (1902), first described under the name Cephalotbeca in tliat year.

V]

CEPHALOPTERIS

63

Md

Fig. 80. Cephalopieris mirabilis, Nath., from the Upper Devonian of Bear
Island. Figures 1— 1, fertile shoots (1-3 nat. size, 4- x k). Figure 5, sterile foliage
(nat. size). After Nathorst (1902).

64

THE ARCIIAEOPTERIS FLORA

[CH.

moment it may be retained as a distinct tyj^e in a position close
to Sphenopteridium.

Sphenopteris, Brongniart, 1822. This well-known but well-
nigh indefinable type of compound frond with rounded pinnules,
more or less deeply lobed and contracted at the base, appears to
be rarer in Devonian rocks than Sphenopteridium. It, however,
occurs in England, Ireland, Belgium and several other regions
in Upjjer Devonian rocks.

Distribidion. From Upper Devonian onwards.

Lycopsida.

Bnthrodendron, L. et H., 1885
(Fig. 37). This well-known genus is
of frequent occurrence in Devonian
rocks. There has been some ten-
dency to include the Devonian species
in a distinct genus Cyclostigma^, as
originally suggested by Haughton in
1859. This, however; can now hardly
be justified^. There is little doubt
that the best known of the Devo-
nian species, B. Kiltorkense, occurring
in Ireland and Bear Island, is a
thoroughly typical representative of
the genus, as the recent studies
of Nathorst and Johnson clearly
show.

Practically all the organs of B.
Kiltorkense are now known. The
lower part of the trunk consists of a
Stigmarian rhizophore, the features
of which agree closely with the
Stigmarias of the Coal Measures.

Fig. 37. Bothrodendron Kil-
torkense, (Haugh.), from the
Upper Old Red of South Ire-
land. Stem with leaf scars.
(Reduced |- nat. size.) Speci-
men No. 20 Devonian Plant
Coll., Sedgwick Museum,
Cambridge. (W.Tams photo.)

According to Johnson^, " the
leaves are cleai-ly arranged in whorls at first, but become
distant and quincuncially arranged in older stems, owing to

1 This term is in any case several times preoccupied by recent Angiosperms .

2 Johnson (1913), especially p. 505. " Ibid. (1913), pp. 523-4.

v] BOTHRODENDRON AND ARCHAEOSIGILLARIA 65

the unequal extension of the stem surface. . . .The [decorticated]^
stem may show a marked fluting or ribbing which is connected
with the parichnos and bundle-strands, but ])ossibly also with
internal sclerotic bands. The calamitoid appearance of such
stems is increased by the presence of horizontal or trans-
verse ridges or zones which are, unlike the longitudinal ridges,
coincident with the surface leaf-scars and suggestive of nodal
diaphragms.... The [heterosporous]^ cone is terminal, and carried
on its broad (hollow?) axis numerous whorls of sporophylls,
of which the megasporophylls are the ones at present best
known." The sporophylls appear to be of a leafy type well
known in Lepidostrohus, bearing sporangia on the upper surface
of the basal portion. The distribution of mega- and microsporo-
phylls in the cone is at present unknown.

With regard to the calamitoid appearance of certain decorti-
cated stems of this genus, we differ from Johnson who is inclined
to see in this feature some signs of affinity to the Sphenopsida.
These specimens appear to vis to represent sub-epidermal surfaces
which are not comparable with Calamite pith casts. Further in
many, but not all, Calamites, the external or sub-external
surface of the stem was not ribbed longitudinally 2.

Neither the leaves, which are uninerved, long, linear structures
of the usual Lycopod type, nor the cones attributed to B.
Kiltorkense have as yet been found attached to the stems. The
former are believed to have been cadvicous.

Distribution. Upper Devonian to Upper Carboniferous.

Archaeosigillaria, Kidston, 1901 ^ (Figs. 38, 39, p. 66). Plants
with stems attaining a diameter exceeding 2-5 cm., dichotomously
branched. Stem covered with spirally arranged persistent leaf
bases; leaf bases contiguous, fusiform in younger branches,
hexagonal in older stems, bearing a single print situated slightly
above the centre of the leaf base. Leaves small, deltoid, markedly

^ Inserted by the present author.

^ At the same time there is no doubt that the occurrence of these ribbed
stems has eiven rise to the assertions of Heer and others that such genera
as Calamites or Archaeocalaniites occur in Devonian rocks, of wiiich, however,
there is no real evidence.

3 For typical figures see Kidston (1885) and White (1907).

A.D. F. 5

66

THE ARCHAEOPTERIS FLORA

[CH.

falcate. Detailed structure of cone unknown. This genus is now
fairly well known from a species occurring both in the Lower
Carboniferous of England and in the Upper Devonian of America.

Fig. 39. ArchaeosigiUario priniaeva,
White, from the ftliddle Devonian
of the United States. Stem with
leaf scars (reduced). After Wliite
(1907).

Fig. 38. Archaeosigillaria Vatiuxemi,
(Goepp.), from the Upper Devonian of
the United States, and Lower Carboni-
ferous of England. (1) Leaf scars en-
larged #. (2) Leafy tvyigs (nat. size).
Specimens Nos. 1099 and 1106, Car-
boniferous Plant Collections, Sedgwick
Museum, Cambridge. (W.Tams photo.)

In the latter country it also occurs in Middle Devonian beds'^.
It is also probable that, as White ^ has suggested, certain
North American fossils ascribed to Lepidodendron really belong
here.

1 White (1907). 2 /^jV/. (1907), p. 339,

V]

LEPTOPHLOEUM AND FRUCTIFICATIONS

67

Distribution. Middle Devonian to Lower Carboniferous.

In the specimens from the Middle
Devonian of the United States, some of
the decorticated stems are markedly ribbed,
just as we have seen is the case in Devonian
Bothrodendrons.

Leptophloeum, Dawson, 1862^ (Fig- 40).
Stems subarborescent, dichotomously
branched, with a spirally arranged armour
of leaf bases ; leaf bases of relatively large
size as compared with the diameter of the
stem, nearly contiguous, rhomboidal, ar-
ranged in periods of large rhomboidal
bases alternating with periods of much
smaller, more transversely elongated bases.
Leaf scar very small, situated a little above
the middle of the base, oval or ovate, with
a single print situated a little above the
middle of the scar. Ligular ? pit at apex
of base.

We agree with White ^ in referring the
so-called Lepidodendron australe, McCoy,
and L. nothum, Unger, of Australia, to this
genus.

Distribution. Devonian, only in Canada,
United States, Spitzbergen and Australia.

Fig. 40. Leptophloeum
rhombicuni,Y)a,ws., from
the Upper Devonian of
the United States. Type
specimen (nat. size).
After White (1905).

Isolated Fructifications of Upper Devonian Age.

The most common types of isolated fructifications occiu-ring
in Upper Devonian rocks are those which are clearly similar to,
or even generically identical with, the fructifications of species of
Archaeopteris. The few other types found, including Xenotheca^
in England and Dimeripteris'^ of Russia and the United

1 White (1905).

-Ibid. (1905), pp. 72-3.

3 Arber and Goode (1915), p. 96, PI. IV, figs. 1-7, 10, 11, Text-fig. 2.

< White (1905), p. 53.

68 THE ARCHAEOPTERIS FLORA fcH.

States, the latter having something in common — though perhaps
remotely, with the Crossothecas of the Coal Measures, are at
present wholly obscure.

The absence of seeds associated with the undoubted Devonian
floras is very remarkable, A revision of this flora has not pro-
duced a single undoubted specimen of a seed. The three supposed
examples attributed to the genus Carpolithes by Dawson in 1863
and derived from the Devonian of America, have Jio claim
whatever to be regarded as seeds ^, One of these more recently
figured by White is a small object of a doubtful nature which
he thinks may be merely a scale, possibly comparable to
those of Bariiwphyton^.

The present author has seen from the Kiltorkan beds of South
Ireland, one or two small bodies which bear some slight resem-
blance to seeds, but it is quite possible that they may be capable
of an entirely different explanation. If seeds do occur at
Kiltorkan, they are undoubtedly exceedingly rare, and since the
number of species of all groups there represented is very small,
probably not more than four, it is unlikely that Archaeopteris,
which is there by far the commonest type represented, is a seed-
bearing plant. This is a point to which Ave shall return later
(p. 81). For the present it is best to assume that undoubted seed
impressions are unknoAvn from Devonian rocks.

Genera of very Doubtful Occurrence in
Devonian Rocks.

Lepidodendron, Sternberg, 1820. There is still no really con-
vincing evidence of the occurrence of this Avell-known genus in
Devonian rocks, from which, however, it has been reported from
many parts of the world. That is to say, no examples have been
figured which show a typical Lepidodendroid leaf-scar and its
prints. If the genus does occur, the specimens so far known
appear to be all more or less decorticated. This is obviously the
case in some examples figured from Australia, Canada, the
Arctic Regions, Russia, etc. It seems probable, however, that
some of the best preserved of these fossils belong to distinct
genera such as Archaeosigillaria and Leptophloeiun . The type

1 Cf. White (1905), p 78. ^ white (1905), p. 78, PI. IV, fig. II.

v] DOUBTFUL AND UNKNOWN GENERA 69

Avhich occurs widely and frequently in Australia, the so-called
Lepidodendron australe, is best referred to Lejitophloeum , which
is wholly Devonian. ^Vhite^ has also recently concluded that by
far the greater number of the so-called American liCpidodendreae
belong in reality to Arcliacosigillaria, occurring both in the Upper
Devonian and Lower Carboniferous though not in later rocks.
Lepidodendron appears to be wholly Carboniferous and Permian.

Cordaites, Unger, 1<S5(). The occurrence of Cordaites-like
leaves in Devonian rocks is very rare. One such is known from
England 2, though in a fragmentary condition, and others have
been recorded from Australia, but it cannot be said that the
evidence for the existence of Cordaites in Devonian times is at
present at all trustworthy. The evidence from the Devonian of
America is even less satisfactory.

Distribution. ? Upper Devonian, Lower Carboniferous to
Permian,

Genera unknown in the Devonian Rocks.

Despite many assertions to the contrary in the older literature,
the following genera do not appear to be known in Devonian
rocks, or rather there are no trustworthy records of their occur-
rence in those beds.

These genera were mostly well developed in Lower Carboni-
ferous times, as was also the genus Rhacopteris which is exceed-
ingly rare in the Devonian. Thus the Lower Carboniferous flora
is distinguished from that of the Devonian, by the presence of
the following genera, in addition to others common to the two

formations.

i Archaeocalamites
Equisetales j Calamites (rare)

Pteridospermeae ]^aiantites
... . I Cardiopteris

i Lepidodendron
Lepidophloios
Sigillaria (rare)
Cordaitales Cordaites (rare)

1 White (1907).

2 Arber and Goode (1915), PI. V, fig. 5.

70

CHAPTER VI

THE PROCORMOPHYTA AND TH'E ORIGIN OF
THE CORMOPHYTA (EXTERNAL MORPHOLOGY)

Historical.

That these recent discoveries affecting the Psilophyton flora
have a very important theoretical bearing has been apparent to
all workers on the subject. Halle^ has recently entered into these
matters at some length and a further discussion is promised by
KidstonandLang^. The origin of Cormophyta is a problem which
has already attracted considerable attention, chiefly on the basis
of a study of recent plants. Bower's Origin of a Land Flora
(1908) contains an excellent epitome of these conclusions. In
addition there have been studies in which fossil plants of
Devonian age, and especially Psilophyton, have played some
part. The earliest of these was apparently Potonie's^ theory of
the descent of all Pteridophyta from Algal ancestors^, a view
which we are inclined to support enthusiastically.

Potonie^ supplemented this conception by his pericaulome
theory, to which we shall refer again shortly. Both these
explanations appear to us to be on the right lines, although we
suggest a more limited application in regard to the pericaulome
theory than that which Potonie himself advocated.

Almost simultaneously with Potonie's publications, a quite
independent series of conclusions were put forward by the late
Prof. Lignier. In 1903 Lignier^ postulated a primitive terrestrial
type, the Prohepatics, as having given rise to the Vascular
Cryptogams on the one hand and to Bryophyta on the other.

1 Halle (1916), p. 35. ~ [See footnote, p. 48. A. A.] ^ Potonie (1898).

* For a recent discussion of the same subject from the point of view of
a study of recent Algae see Fritsch (1916). [In this connexion see also
Church, A. H., Thalassiophyta, Oxford University Press, 1919 — a memoir
which had not appeared when the present book was written. A. A.]

5 Potonie (1898), p. 19, (1902^) and (1902-).

« Lionier (1903) and (1908).

CH. vi] LIGNIER'S THEORIES . 71

On this view the Equisetales and Sphenophyllales are descended
from a common fern-hke ancestor^. Lignier recognised Psilo-
pliyton as a primitive Vascular Cryptogam 2. In his chief contri^
hution to this subject^, which appeared in 1909, he elaborates
his views so as to cover the whole ^•egetable kingdom. From
aquatic Algal ancestors, he derives terrestrial Prohepatics,
giving rise, to Bryophyta on the one hand and Prolycopods on
the other. From the latter arc derived the Primofilices and
Lycopods. In the development of these two races*, two distinct
morphological tendencies are recognised by Lignier as at work:

1. Phylloideae : leaves originating from emergences of the thallus —

Prolycopods, and Lycopods alone.

2. Phyllineae: leaves originating from modified thalloid branches —

Primofilices, and all Vascular Cryptogams except Lycopods, and
all Spermophyta.

The former were ab initio microphyllous, the latter mega-
phyllous. The Spheno2:)sida or Articulatae are regarded as
derived from megaphyllous fern-like ancestors. The author also
proceeds to consider certain Spermophytes such as the Coniferae
which are microphyllous. but as we are not here concerned with
any group higher than the Vascular Cryptogams, these matters
need not detain us.

From this view we should be inclined to dissent in several
particulars. We regard it as extremely unlikely that the Bryo-
phyta, using that term in the widest sense, had any connection
with the origin of Pteridophyta. We should agree rather with
Fritsch^, in deriving the Bryophyta independently from the
Algae but at a much later period than that with which we are
here concerned. Halle's Sporogonites (see p. 44), which we have
discussed, has not shaken our conclusions in this respect.

On the other hand we are strongly in favoiu* of accepting
Lignier's conceptions of the Phylloideae and Phyllineae, though
on somewhat different lines, but at the same time we dissent
from any notion of either a primitive fern or prolycopod ancestry
for the Sphenopsida and Pteropsida.

1 Lignier (190.3), p. 1.32. 2 jj^ifi (1903), p. 95.
3 Ibid. (1909); see also Scott (1910).

* Cf. Janchen (1911) who also separates the Lycopods from other

Pteridophyta on phyletic grounds. '" Fritsch (1916).

72 ORIGIN OF CORMOPHYTA fcH,

Our next historical reference, founded chiefly on the palaeo-
botanical evidence, is to the view of Dr Scott ^ pubhshed
some years ago in regard to the descent of Pteridophyta along
three main lines : the Sphenopsida, Pteropsida and Lycopsida.

Lastly we have Halle's ^ recently expressed conclusions founded
on the evidence of the Psilophyton flora. Apart from his views
on the antiquity of the sporogonium and Brj^ophyta to which
we have already referred (p. 44), Halle regards Arthrostigma as
a microphyllous Pteridophyte possessing leaf-bearing stems and
Psilophyton as possibly of the same nature. Our interpretation
is that they are Thallophytes possessing emergences. We, how-
ever, agree with Halle in regarding the vascular structure as
primitive in these genera.

Plalle also discusses the question whether megaphyllous
Pteridophyta occiu'red in Lower Devonian times.

With this brief review of previous opinion, we proceed to
point out how recent work on the Devonian floras appears to
us to establish even more securely the conception of the early
existence of three distinct lines of descent, the Sphenopsida,
Pteropsida and Lycopsida. In the present chapter we shall
confine our attention to questions concerning the origin of the
external morphology of these groups.

The Cormophytic Habit.

We regard it as probable that the Psilophyton habit (Figs. 1
and 2, pp. 15 and 16) was primitive for all three lines of Cormo-
phytic descent. That is to say, there was an epiterraneous or subter-
raneous limited, erect or horizontal axis, fixed by rhizoids or roots,
and giving off one or more vertical erect branches. This type of
habit persists throughout the earlier Sphenopsida (e.g. Calamites),
Pteropsida (Pteridosperms) and Lycopsida (Lepidodendron).

These three groups, however, differed essentially as regards
the morphology of the erect shoots and leaves, and it was the
evolution of these distinct lines of modification of types met
with among the Psilophyton flora — as is already foreshadowed
in the Archaeopteris flora — that originally called these three
groups into being. In some cases the main erect axis was trans-

1 Scott (1909), p. 616, and (1910). - Halle (1916), pp. 35-40.

VI] THREE LINES OF EVOLUTION 73

formed into the aerial shoot directly, and was not built up
partly of decurrent branches. This was the case in the Spheno-
psida. On the other hand in the Pteropsida, and to a less general
extent in the Lycopsida, the erect axes possessed a pericaulome,
as Potonie^ has shown, the external tissues being morphologically
lateral organs fused to the original axis. The same thing of covirse
occiu's in many Algae, e.g. Polysiphonia.

The leaves also vary in origin. In the Sphenopsida they are
metamorphosed small lateral branch systems, and, in the
Pteropsida, large lateral branch systems. In Lycopsida they are
metamorphosed emergences.

We should define these lines of evolution as follows :

(1) Sphenopsida, descended from Thallophytic Algae bearing
whorled branches. Limited, whorled branches typically small,
converted into leaves which were originally and always micro-
phyllous. Stem not built up of foliar decurrences. Sporangia and
sporangiophores, modifications of branches or segments of the
same.

(2) Pteropsida, descended from Thallophytic Algae in which
the branches were large, munerous, scattered and not whorled.
Branches compound, eventually metamorphosed to mega-
phyllous leaves. Stem largely built up of foliar decurrences
(pericaulome). Sporangia and sporangiophores, modifications of
segments of branches.

(3) Lycopsida, descended from Thallophytic Algae in which
the aerial axes were rarely branched and then usually in a
dichotomous manner. The branches bore microscopic or macro-
scopic emergences which were metamorphosed to microphyllous
leaves. Stem partly built up of foliar decurrences in some cases.
Sporangia and sporangiophores, modifications of emergences or
segments of emergences.

The above outline agrees exactly with Potonie as regards
Algal ancestry, but to a limited extent only as regards peri-
caulome characters. It agrees with Lignier as regards the

^ Potonie (1898), p. 19, (1902i), (19022). The pericaulome theory has more
recently been disputed by Kubart (1918) on anatomical grounds. In the
present paper, however, it is used solely as a morpholooical conception
without reference to the physiolooical functions which the fused organs
may be supposed to have performed originally.

74 ORIGIN OF CORMOPHYTA [ch.

Lycopsida — his Phylloideae — but in place of his PhylHneae, two
distinct hnes of descent are recognised, and the. Sphenopsida
are regarded as primitively microphyllous and not megaphyllous.
In this and other features these conclusions differ from any
previously expressed, and since our interpretation of PsilojjJiyton
and Arthrostigma differs from that to which Halle has inclined
(though in some cases he admits that we may here be dealing
with emergences and not leaves) so our conclusions as regards
phylogeny are quite distinct from his.

We now propose to trace the early stages of the evolution of
each group in some detail.

The Evolution of the Sphenopsida.

It seems probable that the Sphenopsida took their origin from
Algae possessing a whorled habit like that of the living fresh
water Red Alga — Batrachosjyermum. In this ancestor we should
expect to find some form of primitive vascular system, at least
as far advanced as in Psilophyton, and a distinct alternation of
generations.

As regards the vegetative habit, we should expect to find that
the whorled branches gradually took on the leaf function,
without any very radical change in external morphology. They
remained either simple or dichotomously forked microphyllous
elements, the branching of the primitive lamina being the most
general method of increasing ths lamina area, in view of the
new functions which these organs had assumed. As regards the
cones, the primitive whorled habit was again retained. These
were originally made up of whorled branches, the branches being
sometimes simple and sometimes forked several times. Some of
the branches of the whorl or their segments took the path of
remaining protective organs (i.e. the bracts) and in some cases
became coalescent to a considerable degree in order to fulfil this
function more effectually. Other segments were metamorphosed
partly into sporangiophores, partly into sporangia.

Such represents our conception of the main course of evolution
in this group. We now have to enquire what fossil evidence we
have for the support of these contentions.

With regard to the primitive whorled algal type there is little

VI]

SPHENOPSIDA

75

evidence — perhaps no certain evidence — at present. Attention
may however be drawn to some very obscure fossils which may
conceivably have some bearing on this point. At the same time
it must be freely admitted that these bodies are extremely
obscure and that no emphasis can be laid on them from this
point of view. There do, however, occur in the Ordovician,
Silurian and Devonian rocks, carbonaceous impressions which
in some respects recall the Ammlarian k'a\('s of Cahiinitcs. They

Fig. 41. Prolan mdaria laxa, (Daws.), from the Upper Devonian
of Canada (f nat. size). After Dawson (1871).

have in fact more than once been referred to Annularia itself.
Perhaps the best specimens of this nature are those described
by Dawson^ from the Devonian of Canada as Annularia laxa.
Daws. (Fig. 41).

Dawson^ states that "the ends of the leaves are curled in a
circinate manner" and he figures them as each possessing a vein.
It may be also recalled here that many years ago Nicholson ^

1 Dawson (1871), p. 31, PI. VI, fig. 64; cf. also figs. 65-69.
- Ibid. (1871), p. 31.

3 Nicholson (1869), p. 495, PI. 18, fig. B; Nicholson and Lydekker (1889),
p. 1514.

76 ORIGIN OF CORMOPHYTA [ch.

also figured from rocks as ancient as the Lower Ordovician
(Arenig) of the Lake District, similar specimens under the
name Protannularia {Buthotrephis) radiata, (Nich,), Examples of
these are in the Sedgwick Museum, Cambridge^. Nicholson ^

says " it is. . .difficult. . .to imagine what this can be if not a plant

It seems, however, pretty certain that if its vegetable nature be
conceded, it can hardly be referred to the Algae." With this
conclusion we entirely agree, though we find no trace of any
vascular structure in the "leaves." It is quite possible that
Protannularia radiata may be the oldest, in a geological sense,
British land plant, and Procormophyte. Most authorities will
we think agree that these specimens have no real claim to a place
in the genus Annularia. We may transfer them to the non-
committal place of Protannularia. It may be objected that
these specimens are inere lusus naturae, mineral aggregates of
inorganic origin, but this does not appear to us to be the case,
so far as Ave have examined such fossils.

We think it probable that the earlier examples may have
been simply Algae ^, while the later were Procormophytes — still
thalloid although vascular — and that it is conceivable that they
may have been among the ancestors of the Sphenopsida, though
as we have said they are at present still too obscure for any
weight to be laid on this suggestion.

From some such ancestors as these, two lines of descent sprang
at slightly different periods, both inheriting a nodose arrange-
ment of microscopic branches. The branches became vascular
gradually, in exactly the same way as those of Psilophyton
became vascular.

In the older line of descent, the Sphenophylls, the branches
were characteristically divided longitudinally into forked seg-
ments. This is well seen in the leaves of all members of this group
known in the Archaeopteris flora, i.e. Sphenophyllum (Fig. 24,
p. 52), Hyenia (Fig, 25, p, 53), and Pseudobornia (Fig. 26, p. 54).
These organs remain morphologically thalloid. The wedge-shaped

1 The type No. 51 (also No. 1), Ordovician Plant Coll., Sedgwick Mus.,
Cambridge. ^ Nicholson (1869), p. 497.

* Such Algae still exist in the genus Crnuania, etc., in which morpho-
logically though not structurally the habit is distinctly Sphenophyllaceous.
The living Lomentaria, Gaill. is in habit very equisetaceous.

VI] SPHENOPSIDA 77

type of leaf which we associate more particuhirly with Spheno-
jjJiyUum is a nmch later developiiieut, unknown before the Upper
Carboniferous. It appears to have arisen by a broadening of the
narrow segments of the primitive type on exactly the same lines
as we shall see were so frequently followed among Pteropsida.

We know \ery little of the fructifications of the earlier members
of this race, but those of Pseudobornia (Fig. 26, 2, p. 54) appear to
have possessed sporophylls essentially of a divided leafy type.
At any rate in younger types, such as the Lower Carboniferous
Cheirostrobus and the Upper Carboniferous Sphenophyllimi
Dazvsoni, the sporophylls are divided, some lobes being fertile,
others sterile. In the typical Sphenophyllaceous cone, one
segment is sterile and protective and two segments (the sporangio-
phores) are fertile. The sporangia, as in all those groups, are
simply metamorphoses of parts of a fertile leaf-branch, just as
they are in living ferns of to-day. Sometimes the whole segment
is thus metamorphosed (when the sporangia are sessile), some-
times only a part, while one portion remains as a sporangiophore.

As regards the stem, neither in the Sphenophyllales nor the
Equisetales is there any trace of a pericaulome origin. The
leaves being typically small in both these phyla, the stem appears
to have acquired sufficient inherent mechanical strength without
any such adaptation.

In the Equisetales, a group, which on the present evidence
appears to be a little later in time than the Sphenophyllales, and
unknown before the Lower Carboniferous, the leaves were
primitively thalloid and forked. Those of Archaeocalamites come
very near those oi Sphenophyllum in this respect. The prevailing
tendency in this phylum since then has been the reduction of
such compound structures to a single segment (i.e. the leaves of
Calamites), and at later periods to the almost complete union of
these reduced leaf bases (e.g. Equisetites and Equisetum).

As regards the cones the tendency has been to sterilize certain
leaf branches (the so-called bracts) alternating with fertile leaf
branches. In Archaeocalamites these structures are absent, but
in later cones, such as Calamites, they are always present and
remain simple, undivided structures with the solitary exception
of Cingularia.

78 ORIGIN OF CORMOPHYTA [ch.

The Evolution of the Pteropsida and Palaeopliyllales.

The next group which we may consider is the Palaeophyllales,
still entirely enigmatical. We may take these with the Pteropsida,
because some Psygmophyllums so closely resemble some species
of Archaeopteris that dispute has arisen as to the genus in which
they should be included^. In Psygtnophyllum (Fig. 27, p. 55) the
leaf appears to have arisen by the flattening oiit of a large branch,
in a wedge-shaped manner. The morphological unit here appears
to be an axis bearing alternated metamorphosed branches, each
branch being completely metamorphosed into a large cuneiform
leaf, with the radiating nervation so common among primitive
leaves. As a later development each leaf tends to become lobed
or divided longitudinally.

Further the leaves are decurrent and thus the stem is to some
extent pericaulomic.

Be5^ond this our knowledge of Psyg)nophyllum does not at
present extend, and, in our ignorance of its fructification, it is
for the moment maintained as the type of a distinct race.

Nearly all, if not all, known Pteropsida from Palaeozoic rocks
are markedly pericaulomic and we agree with Potonie that in
this group the stem is largely built up of leaf bases (cf. Medullosa,
Calamopitys, and many other genera). Exact homologues of
such stems are common among the Red Algae, e.g. Polysiphonia.
It is clear also that this type of structure was common among
members of the Psilophyton flora, for it is well seen in Barrandeina
(Fig. 18, p. 36) and Ptilophyton (Fig. 12, p. 31) which appear to
be Thallophytes.

The stem, in cases where large leaves are being evolved, would
naturally require some accession to its mechanical strength, and
this advantage would be gained by a pericaulome, a truly algal
feature.

The differentiation of megaphylly from large branch structures
appears to have taken place in several different directions. There

^ This is the case with Archaeopteris obtusa, (Dawson), inchided in that
genus by American Palaeobotanists (see Prosser (1894), Pi. II, j). 49) but
transferred more recently by the present writer to Psygmophyllum (Arber
(1912), p. 398) on the ground that in tliis type we appear to be deahng with
leaves spirally arranged and not a pinnate type of frond.

VI]

PTEROPSIDA

79

is first the Archaeopteris type of leaf. This arose from the meta-
morphosis of the branches of the nth. order in a system which
was at least bipinnate, if not more compound still. Each
alternate branchlet was flattened out either into a sterile wedge-
shaped leaf, with a radiating nervation, or metamorphosed into
one or more sporangia. Every stage is clearly seen in Devonian
species of Archaeopteris (Figs. 28-31, pp. 56-59). Further
modifications of the wedge-shaped entire, primitive type of leaf
soon set in. As in Psygmophyllum, these leaves tend to become
lobed or segmented longitudinally.

In A. Archetypus, Schmalh. (Fig. 29, p. 57)
and A. Rogersi, Daws., the leaflet is primi-
tive, large and undivided. We next pass on
to types such as A. hihernica, (Forbes) (Fig. 28,
p. 5Q) and A. Roemeriana, (Goepp.) in which
the leaf tends to become unsymmetrical and
more or less toothed or lobed, and finally we
reach such types as A. fimbriata, Natli. (Fig. 30,
p. 58) and A. fissilis, Schmalh. (Fig. 31, p. 59)
in which it is divided nearly to the base, longi-
tudinally, into very narrow segments. A still
further elaboration of this type by splitting-
would be indistinguishable from Spheno-
pteris.

Further, other later genera, especially cha-
racteristic of the Lower Carboniferous, such
as Rhacopteris^ (Figs. 42, and 33, p. 61),
Adiantites (Fig. 43, p. 80) and Cardiopteris, have Fig. 42
leaves essentially similar to Archaeopteris and P'^"*
with a radiating nervation. Further, here also "j.j^g ' lq^^^ ^^r
the tendency to longitudinal splitting is marked
at least in the two genera first named. Such
types are in fact chiefly distinguished from
Archaeopteris by the shape of the leaf and its
segments — by some small peculiarity of its
symmetry. They are all, however, obviously derived from the
Archaeopteris type of leaflet.

1 In this genus the frond also dichotomises.

lihoco-
panicuU-
om

boniferous of

Austria. Fertile
frond (reduced to
\ nat. size). After
Stiu'. Culm-Flora
(1875).

.80

ORIGIN OF CORMOPHYTA

[CH.

iS

I

7« J- 2

Fig. 43. Adiantites fertilifi . (White), Irom the Lower
Carboniferous of the United States. (1) Sterile
fronds (reduced). (2) Fertile fronds and seeds (all
natural size except where magnification is stated).
After White, " Fossil Plants of the Group Cycado-
filices," Smith's Misc. Coll., Vol. 47, 1905.

VI]

PTEROPSIDA

81

Several attempts have been recently made to determine
whether Archaeopteris was a member of the Primofilices or a
Pteridosperm, Kidston^ and others think the latter the more
probable. We, however, agree with Johnson ^ that there is
absolutely no evidence that this very completely known type
(at any rate in the case of A. hibernica) bore seeds. It is much
more probably a Primofilix. The evidence of Rhacopteris, so far
as it goes, agrees with this conclusion. On the other hand at
least one Adiantites (Fig. 43) was a seed-bearing plant, but here
the seeds clearly arose by the metamor-
phosis of a segment of the frond. No
fructifications are yet known in the case
of Cardiopteris.

Another very common type of frond
evolution is seen in Sphenojjteridium (Figs.
34, p. 61, 35, p. 62 and 44), beautifully fore-
shadowed in the Psilophyton flora by Pseudo-
sporochnus (Fig. 16, p. 34) and perhaps P/i7o-
phyton (Fig. 13, p. 32), and fully developed
in the Archaeopteris flora.

Here the ends of large branches frequently
divide, usually dichotomously, to form a tuft
of very narrow forked, nerved segments^.
This type of frond persisted long after the
Devonian period. It is very common in
Lower Carboniferous rocks, e.g. Sphenopteris
qffinis (Fig. 45, p. 82) and S. bifida (Fig.
46, p. 82) among many others, and it is
also met with in Upper Carboniferous times.
The genus Eremopteris simply represents a
somewhat peculiar modification of this tjq^e.

Between Sphenopteridium and Sphenopteris it is not jiossible
to draw any really satisfactory line.' The latter type possesses
broader and more rounded segments as a rule, with a pinnate
nervation. The term Sphenopteridiujn is in fact chiefly retained

1 Kidston (1906), p. 435. 2 Johnson (1911i).

* A similar type of leaf also occurs where the division of an Archaeopteris
or Rhacopteris type of pinnule is at its maximum.

Fig. 44. Sphenopterid-
ium moravicum , ( Ett . ) ,
from the Lower Car-
boniferous of Austria,
Thalloid foliage (re-
duced to I nat. size).
After Stur, Culm-
Flora (1875).

82

ORIGIN OF CORMOPHYTA

[CH.

because this particular type of leaf appears to be much more
common than SpJienopteris in the older rocks. Sphenopterids
however do occur there, and they appear to have been derived
from Procormophytes by the division of the ends of flattened
branch systems into more rounded and broader segments.

Fig. 45. Splienopteris affinis,
L. & H., from the Lower
Carboniferous of Scotland,
showing thalloid foHage.
(Reduced |nat.size.) Speci-
men No. 667, Carbonifer-
ous Plant Coll., Sedgwick
Museum, Cambridge. (W.
Tams jjhoto.)

Fig. 46. Splienopteris bifida, L. & H.,
from the Lower Carboniferous of Scot-
land, showing thalloid foliage ( x f ).
Specimen V. 162, British Mus. (Nat.
Hist.).

We know very little of the fnictifications associated with
either of these types of fronds in Devonian rocks. If Nathorst
is right in attributing to Sphenopteridium-like fronds the tufted
sporangia of CephaJopteris (Fig. 3G, p. 63), then here at any rate

VI

PTEROPSIDA

83

we have something different in the way of a fructification to
anything known among Carboniferous plants. On the other
hand, that of the Psilophytic genus Broggeria (Fig. 17, p. 35)
has at least some superficial resemblance to that of Cephalopteris.
In both cases these fructifications may well arise from the
metamorphosis of a tuft of finely divided branclilets, and may

Fig. 47. Aphlebiae oi Pecopleris (Dacfy-
loiheca) plutnosa, (Art.), from the Coal
Measures, showing persistence of thalloid
fohage. After Zeiller's t^lem. Paleobot.
(1900).

be compared with those of Archaeopteris (Figs. 28, 2, p. 50 and
especially Fig. 32, p. GO).

It is thus clear that many of the earlier Pteropsida, like the
Sphenopsida, retained a sub-thalloid type of foliage. This type
further persisted among aphlebiae well into the Upper Carboni-
ferous period. The aphlebiae of Pecopteris (Fig. 47) are a case
in point and are of the Sphenopteridium type, whereas many

6—2

84 ORIGIN OF CORMOPHYTA [ch.

other aphlebiae referred to the genus Rhacopkyllwn are extra-
ordinarily algal in aspect. Even ArcJweopteris itself retained not
only scaly emergences at the base of the frond but a large pair
of connate stipules (Fig. 28, 1, p. 56), no doubt originally of
branch origin like the fused branches of Cephalopteris. The
Archaeopteris type of leaflet also survived in the form of aphlebiae
well into UpiDcr Carboniferous times. Many of the smaller
Cyclopterid pinnules of Neuropteris are more or less Avedge-
shaped with a radiating nervation. In the larger and ? older
pinnules the shape changes to reniform or even orbicular, but
the same type of nervation persists.

These facts afford another illustration of the law to which the
present writer^ in conjunction with Mr Parkin, called attention
some years ago, namely that corresponding stages in the evolu-
tion of the various members of a plant are not contemporaneous
in point of time. Here the foliar members clearly lag behind
other organs and are of more primitive form.

It may be also pointed out that many genera of Carboniferous
Pteropsida exhibit in the dichotomies of their fronds traces of
their algal ancestry, as Potonie^ long ago pointed out. Such
genera as Rhacojyteris, Fig. 33, p. 61 (Lower Carboniferous),
and Mariopteris and many SphenoiDterids (Upper Carboniferous)
are cases in point.

Primofilices^ . So far as we can see, all the fern-like plants of
the Devonian period were probably members of the Primofilices
and not Pteridosperms. We have failed to find any evidence of
the latter group and the remarkable absence of fossils obviously
of a seed nature has already been remarked upon here. However,

1 Arber and Parkin (1907), p. 35. - Potonie (1895).

' Prof., Seward (1910, p. 433) has taken exception to the term Primo-
fiUces on the ground that, according to his view, the name imphes "primary
or primitive ferns." This is an entire misconception for which we think no
justification will be found in the original paper by the writer (Arber, 1906)
in which the term was first used. So far as we know, the adjective jrritnus
has never been interpreted as 'primitive' whereas it may imply 'early,'
e.g. prima nox. The term Primofilices was and is intended simply to imply
"early ferns," and also to suggest the "Primary or Palaeozoic Age of the
race" (Arber (1906), p. 222). We fail to see any reason why this term should
be abandoned for the name Coenopterideae which Seward adopts, especially
as these plants do not appear to be generalised types.

VI] LYCOPSIDA 85

until we know something of the structure of the fern-Hke plants
of the Upper Devonian, it is impossible to feel confident as to
their precise affinity. As things stand at present we have no
certain evidence of the Pteridosperms before the Lower Carboni-
ferous period. They certainly appear to have occurred then and
in the case of the well-known Adiantitesfertilis, (White) (Fig. 43,
p. SO), it would appear that the seeds took their origin in much
the same circumstances as the sporangia of the Primofiliccs, i.e.
by the metamorphosis of part of a fertile branch, as in the Algae.
No doubt the chief modifications in this case were the reduction
of the number of megaspores in the sporangium to one and the
development of an integument to the same. In Adiantites
^ertilis these features appear to have been arrived at very simply.
Cordaitales. There is one class of Pteropsida on the origin of
which the Devonian floras as yet have thrown no light, namely
the Cordaitales. A few obscure and fragmentary leaves, some-
thing like those of Cordaites, occur in Upper Devonian beds, but
in the earlier floras we meet with nothing, so far as we can see,
which appears to belong to this group. There is, it is true, the
Zostemphijllutn of the Lower Devonian (Fig. 22, p. 41), with long
parallel-sided leaves or axes, but the fossil is so obscure at present
that no conclusion of any value can be drawn from it. Even
from Lower Carboniferous rocks evidence of the Cordaitales is
at present extremely scanty, and until we learn more about the
group at this period, we are not likely to trace it back to
Devonian times.

The Evolution of the Lycopsida.

The evolution of the Lycopsida along the lines first perceived
by Lignier, is particularly well seen among members of the
Psilophyton flora. From such a type eiS P silophy ton (Figs. S, p. 17,
and 5, p. 19) with its chaffy scattered scales, it is but a short step
to Thursophyton (Fig. 10, p. 28), which is to all intents and pur-
poses a Lycopod. Even if this type was not vascular, either in
axis or leaf, it is quite easy now, in the light of Psilophyton, to
understand how similar vascular types may have arisen. From
Thursophyton it is a short step to Protolepidodendron.

86 ORIGIN OF CORMOPHYTA [ch.

In this group a pericaulome was not always evolved. Bothro-
dendron, one of the earliest Devonian Lycopods (Fig, 37, p. 64),
does not possess this featiu-e, which however occurs in Archaeo-
sigillaria (Fig. 38, p. 66) and Leptophloeum (Fig, 40, p. 67) of
the same period. Most of the later Palaeozoic Lycopods, such as
LejDidodendron, as well as many Sigillarias, possess a pericaulome,
but we see how in the later types of the latter genus (section
Clathraria) the pericaulome was abandoned.

In the Lycopsida the leaves, arising from emergences, are
always unbranched and independent. The sporangia were
originally borne at the base of the upper (adaxial) surface of
the leaf, as we see them in Thursophyton (Fig. 11, p. 30) and as
indeed has been the rule among Lycopods ever since. Among
Lycopods the Selago condition is thus primitive as regards the
place of origin of the sporangia.

We are able to throw no light on the original morphology and
function of the ligule. Such an organ is universally present in
Palaeozoic Lycopods, but it is there nlways in much the same
condition as in the living Selaginello.

The pseudodichotomous branching of many Palaeozoic
Lycopods is another indication of algal ancestry.

The Psilotales.

There remains for consideration one further group, the
Psilotales, which may be dealt Avith here, since by some
authorities these plants are regarded as more nearly related to
the Lycopsida than to any other group. There are also further
grounds for a discussion of this race arising from the fact that
Kidston and Lang^ have recently instituted a comparison
between Psilophyton and members of the Psilotales, and have
drawn the conclusion that some phylogenetic connexion may
exist between them.

On the much debated question of the general affinity of the
Psilotales, we agree with Mrs Thoday^ and others that they are
best regarded as an entirely distinct group from either the
Lycopods or the Sphenophylls. We should not include them in
either the Lycopsida or Sphenopsida, At the same time we do
1 Kidston and Lang (1917), p. 776. ^ gykes (1908).

VI ] PSILOTALES 87

not deny that there arc })oints of similarity between the Psilotales
and both those oroups. These resemblances however appear to
ns to be far outweighed by the differences, and the latter seem
to be of a more fundamental nature than the former.

In comparison with the Lycopsida, we would ])oint to the
microphyllous habit and the presence of a perieaulome, as
features in common. On the other hand in the anatomy of their
sporophyte and its fructifications the Psilotales are quite imlike
living or fossil Lycopods.

In recent years the view that this group may be related to
the Sphenopsida rather than to the Lycopsida has undoubtedly
gained ground, despite the \ ast differences in habit. In certain
points in the anatomy of the shoot and in the type of sporophyll
it is jiossible to institute some comparison between the Psilotales
and the SphenojDhy Hales i. We are now inclined to regard these,
however, as cases of parallelism of development and not to
attach any special phyletic arguments to such resemblances
which are far outweighed by more important dissimilarities^.
We should be inclined to regard the Psilotales as a quite inde-
pendent race, also of algal origin, which appeared on the scene
long after the other races which we are discussing here, possibly
in Mesozoic times or even later. We know of no evidence (not-
withstanding Kidston and Lang's views of Psilotum to which
we shall shortly refer again) of the existence of any plant in
Palaeozoic times which has any real claim to inclusion in this
group. Certainly none occurs among the known members of the
Psilophyton or Archaeopteris floras. The Psilotales, like the
Bryophyta, are thus a much later group in point of age than the
Sphenopsida, Lycopsida and Pteropsida. That the Psilotales
retain certain features which may be primitive we should agree,
though certainly in Psilotum and probably also in Tmesipteris
other characters, which might be regarded at first sight as

1 Scott (1909), Vol. II, p. fi26.

- The most recent advances of our knowledge of the Psilotales which we
owe to Lawson (1917) do not help us here since they are concerned with
the gametoph\'tes. The latter do not appear to be very remote from those
of Lycopsida, but since the corresponding structures of the Sphenophyllales
are not known and probably never will be known, we have no grounds for
any comparison in this respect.

88 ORIGIN OF CORMOPHYTA [ch. vi

primitive, are, in our opinion, reduction features, the reduction
being correlated with the semi-saprophytic habit of these plants.

As primitive features we would point to the microphyllous
habit and pericaulome, characters which as we have already
seen, appear to have arisen in different groups quite independ-
ently. The forking of the sporophylls is another such feature,
originating as in other groups from dichotomous thalloid branches.
In this feature the Psilotales may resemble Sphenophylls rather
than Lycopods as has often been pointed out, and it may be
that in this group the leaf was originally a branch and not an
emergence as in the Lycopods. But even if this is the case, it
is more than likely, in our view, that these features were quite
independent in origin from the similar organs of Sphenophylls,
especially since the latter race was in all probability entirely
defunct long before the Psilotales appeared on the scene.

The vascular structure of the Psilotales represents a primitive
stage passed through in the history of the evolution of the stele
in many, if not all groups, and here either not developed further
or regained by reduction in correlation with the semi-saprophytic
habit.

Finally as regards Psilophyton we see no real point of contact
with the Psilotales except in habit, a character which of course
is worse than useless as a guide to affinity, if considered by itself.
In the first place the fructification and the manner in which it
is borne are entirely distinct. Tmesipteris is a Cormophyte
without emergences, Psilophyton is a Thallophyte possessing
emergences, and Psilotum is in all probability a reduced semi-
leafless Cormophyte. Finally the only comparison as regards the
stele of Psilo2Dhyton is confined to the tip of the shoot in the
Psilotales, near the growing point. In the mature stems of the
latter the stele has well-developed primary wood, arising from
several protoxylem groups (imknown in Psilophyton) and has
altogether reached a higher stage of development than any thing-
known in Psilophyton. At the same time, as we shall show in the
following chapter, this being a stage passed through in the
evolution of the steles of most groups, has no obvious phyletic
bearing.

[ 89

CHAPTER VII

THE ORIGIN OF THE STELE IN THE
EARLIER CORMOPHYTA

The discovery, or rather the confirmation of the discovery, of
an extremely primitive stele in species of Psilophyton and also
of Arthrostigma has naturally a very important bearing on our
notions of the origin of the stele in Pteridophyta.

Among the oldest fossil plants, of far greater antiquity than
the very ancient land plants imdcr consideration here, there are
many multi-cellular types without any trace of a specialised
conducting tissue. These fossils commonly occur in marine rocks
and they so closely resemble types of living Algae in their
general structure that we are justified in regarding them as
marine Algae.

Somewhat later in point of time, in the marine Silurian rocks
and extending into the early Devonian, we find another type of
anatomical habit in which the whole thallus is tubular. This
type is represented in Nematophycus where the tubes are some-
times of two different calibres, the smaller forming a dense
packing between the larger tubes. This is however a feature also
common to some living Algae and needs no further remark here.

Among living Algae no more advanced type of water con-
ducting tissue is met with, though some Algae have evolved
what we regard as a quite typical phloem. The fact that nearly
all the higher living Algae, other than the symbiotic types
(Lichens) are hydrophytes and not terrestrial plants, furnishes
a ready explanation of the absence of a lignified conducting
tissue. There is no need of such a tissue.

When however we come to Devonian land plants we find in
Psilophyton and Arthrostigma and other types an extremely
early stage in the evolution of the stele. What we have is a
single protoxylem groiip alone, formed by the simultaneous
modification of a set of procambial elements. That modification

90 ORIGIN OF STELE [ch.

was not continuous or progressive. A certain small set of
procambial elements were converted together and then the
process ended. There is not the slightest sign of a leaf trace
origin of the caiiline stele. In fact we know from Psilophyton
that the conversion of the procambial elements took place in the
main axis and in the branches quite independently (probably at
different periods) and further that these foci of lignification were
not in continuity except by means of parenchymatous tissues.
Thus here at any rate the axes existed first before their steles.
Continuity between the steles of members of the different orders
of axes was a later modification, the usefulness of which soon
became obvious.

We may picture Psilophyton as a terrestrial plant growing
under damp conditions in a bed of peat, and no doubt in such
a habitat a partial, little-developed and discontinuous con-
ducting system met the case.

The next stage in the evolution of the stele apjaears to have
been the substitution of a continuous for a jDiu-ely initial trans-
formation of procambial elements. When the first set of elements
had been converted (protoxylem) the process continued and
primary wood, focused on the single protoxylem, was evolved.
This stage is thus the evolution of a monarch strand. Such a
stage exists for instance in the rootlet {not the rhizophore) of
Stigmaria.

The next step appears to have been different in different cases.
The procambial activity having ceased, a secondary cambium
was sometimes (but probably rarely) initiated and secondary
wood was added to the framework of a single monarch strand.
This state of affairs is also perceived in certain Stigmarian
rootlets.

A more common and perhaps more successful course of stelar
evolution in stems was the substitution of a number of proto-
xylems with prolonged cambial activity in the central region
of the stem in place of a single such group. These groups may
have been centric or excentric, in the latter case the groups being
concentric.

In the case of centric groups of protoxylem, space was natm-ally
limited, and even where the groups were excentric but con-

vii] STELE OF DEVONIAN PLANTS 91

centric, the room for the development of primary tissues was
limited. In either case a stage was eventually reached when
further vascular development could only be provided for by
peripheral additions, i.e. secondary wood. This new type of
structure, as is well known, was often accompanied by a reduc-
tion in the primary scaffolding.

In drawing attention to the existence in Devonian times of
a primitive type of stele, it must not be imagined that all
Devonian plants possessed such anatomy. On the contrary we
know of several arborescent types possessing secondary wood of
the modern type found among the higher plants, occurring in
the Middle Devonian of Scotland^ and the Dc\onian of Russia^
and the United States^ and Canada*. It is thus clear that along-
side of Psilo2Jhijtnn there existed other plants which had already
reached a far higher stage of evolution as regards the stele than
we meet with in that genus. In fact Psilophyton was probably
one of the latest survivals of the primitive Devonian types 5.

1 Palaeopihjs MiUerii, M'Nab (1870).

- CaUixijlon Trifilievi (^Dadoscylon Trifilievi), Zalessky (1909) and (1911),
p. 28, PI. IV, figs. 1-3.

' Callixylon Ouieni, Elkins and Wieland (1914).

4 Dawson (1871).

5 [The Author left this chapter unfinislied, and regarded it merely as a
preliminary draft. A. A.]

92 ]

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96 BIBLIOGRAPHY

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97 ]

INDEX

The principal references are in black type

Aberdeenshire, o

Adiantitcs, (il), 70, 80 (Fig. 4;}), 81

AdiuniHes fertilis, 80 (Fig. 43), 85

Afriea, South, 58

Age, geological, of Devonian Floras,

3-7
Algae, 39, 40, 42, 47, 49, 70, 71 , 73,

74, 7(i, 78, 84, 89
.America, 17, 37, 66, 68, 69
Annularia, 75, 7()
Aphlebiae, 83 (Fig. 47), 84
Apfiyllopleris, 12, 43, 44
Archaeocalamites, 69, 77
Arcfiaeopteris, 9, 11, 12, 13, 56 (Fig.

28), 57 (Fig. 29). 58 (Fig. 30),

59 (Fig. 31), 60 (Fig. 32), 67,

68, 79, 81, 83, 84
Archaeopteris Flora, 1, 9-10, 11,

12, 51, 52-68, 87
Archaeopteris Archctypus, 12, 57

(Fig. 29), 79
A.fimbriata, 58 (Fig. 30), 79
A.fissilis, 12, 59 (Fig. 31), 79
A. hibernica, 10, 11, 56 (Fig. 28).

79, 81
A. Ilitchcncki, 60 (Fig. 32)
A. Ilozcitti, 13
A. Jackson i, 13
A. Hoemcriaua, 12, 79
A. Roger si, 13, 79
A. Tschermaki, 11
A. Wilkinsoni, 13
Archaeosinillaria, 9, 10, 12, 13, 65,

66 (Figs. 38, 39), 67, 68, 69, 86
Archaeosigillaria primaeva, 66 (Fig.

39)
A. Vanuxemi, 66 (Fig. 38)
Arctic Regions, 12, 68
Arenig, 76
Arthrostigma,8, 10, 11, 12, 13, 26,

27 (Figs. 8 and 9), 28 (Fig. 10),

29, 47, 50, 51, 72, 74, 89
Arthrosligma gracile, 11, 27 (Figs. 8

and 9), 46
Asterolepis, 4
Auckland, 13
Australia, 6, 7, 13, 38, 67, 68, 69

Raggv Beds, 5

Bari))ophiit(m, 9, 11, 13, 37, 38
(Fig. 19), 68

Barinopliyton liichardsoni, 38 (Fig.

19)
Barrande, J., 5, 12
Barrandeina, 9, 10, 12, 13, 36 (Fig.

18), 37, 78
Barrandeina Dusliana, 36 (Fig. 18)
Batrach'jspermum, 74
Bear Island, 2, 6, 7, 12, 43, 52, 62, 64
Belgium, 5, 7, 11, 16, 26, 38, 64
Bernard, C, 30, 35, 50
Bohemia, 2, 3, 5, 7, 12, 26, 29, 30,

35, 37, 42, 43
Bothrodendron, 9, 13, 64 (Fig. 37),

65, 86

Botlirodcndron Kiltorkense, 11, 12,

64 (Fig. 37), 65
Bower, F. O., 70
Broggeria, 10, 12, 35 (Fig. 17), 37,

50, 83
Broggeria norvegica, 35 (Fig. 17)
Brongniart, A., 48, 64
Bryophyta, 44, 70, 71, 72, 87
Buthotrephis radiata, 76

Caithness, 10

Calamites, 65, 69, 72, 77

Calamopitys. 78

Canada, 6, 7, 8, 13, 14, 16, 26, 38,

56, 67, 68, 75, 91
Carboniferous, Lower, 9, 14, 52, 56,

66, 67, 69, 77, 81, 84, 85
Carboniferous, Upper, 52, 56, 62,

65, 77, 81, 83, 84
Cardiopteris, 69, 79, 81
Carpolithes, 68
Carruthers, W., 17
Caulopteris, 13, 37
Caulopteris Peachii, 10
Cephalaspis, 4
Cephalopteris, 9, 62, 63 (Fig. 36),'

64, 83, 84
Cephalopteris mirabilis, 12, 63 (Fig.

36)
Cheirolepis, 4
Cheirostrobus, 77
Chemung series, 6, 13
China, 6
Cingularia, 77
Clathraria, 86

Coal Measures, 9, 58, 60, 61, 64, 68
Coccosteus, 4

98

INDEX

Coniferae, 71
Coidiiitales, 69, 85
Cordaites, 9, 11, 13, 69, 85
Corniferous Limestone, 13
Cromarty, 25
Crossotheca, 68
Cryptoxylon, 8, 10
Cryploxylo)} Forfarense, 11
Cucullaea Beds, 5

Cyclopterid pinnules of Neuropieris,
, 84

Cyclopteris, 13
Cyclopteris furcillata, 11
Cydostigma, 64

Dactylotheca plumosa, 83 (Fig. 47)

Dawson, J. W., 14, 16, 17, 18, 20,
21, 22, 23, 24, 26, 37, 46,67, 68,75

Dazvsonites, 15, 20

Dawsonitcs arcuatus, 20, 21 (Fig. 6),
22

Devon facies, correlation with Old
Red Sandstone, 4

Devon, North, 5

Devonshire, 11

Dickson Bay, 6

DimerijJieris, 12, 13, 67

Don, A. W. R., 2, 38-40, 47

Donetz Basin, 6, 12

Doubtful genera in Devonian rocks,
68, 69

Dryden Shales, Rhynie, Aberdeen-
shire, 5

Ellesmereland, 2, 6, 7, 12, .",6
Emergences on axis of Psilophyton,

18-26
England, 5, 7, 11, 38, 58, 64, 66,

67
Equisetales, 69, 71, 77
E<p(iselHes, 77
Equiscturn, 77
Eremoptcris, 81

Filicales, 69
France, 6
Freeh, F., 4
Fritsch, F. E., 71

Fructifications, Isolated, of Upper
Devonian age, 67—68

Gaspe, 6, 7, 13

Genera, Doubtful, in Devonian

rocks, 69
Genera, Unknown, in Devonian

rocks, 69
Genoa River, Auckland, 13
Germany, 3, 5, 7, 11, 16
Goeppert, H. R.. 17

Habit of primitive Cormophyta, 72

IlaUserites Dechianus, 17

Halle, T. G., 2, 3, 14, 20, 21, 24, 26,

43, 44, 46, 48, 70, 71, 72, 74
Haughton, S., 64
Herborn, 5

Hesse-Nassau, 5, 7, 11
Hickling, G., 2, 38-40, 47
Iloloptjjcldus, 4
Horizons of Devonian plant-bearing

beds. Summary of, 7
Ilostimella, 8, 10, 12. 42, 43 (Fig'.

23), .50
Ilostimella hostimensis var. rhodeae-

formls, 42, 43 (Fig. 23)
IlosthieUa, see Ho^timella
Hydropterideae (Rhizocaros), 39, 46
Ilyenia, 10, 12, 53 (Fig. 25), 54, 76
Ili/em'a sphenophylloidrs. 53 (P^ig.
' 25)

Ireland, 5, 7, 11, 64, 68

Johnson, T., 64, 65, 81

Kidston, R., 1, 20, 22, 46, 48, 65, 70,

81. 86, 87
Kiltorkan, 68
Krejci, J., 12

Lake District, 76

Lang, W. H., 1, 20, 22, 46, 48, 70,

86, 87
Lepidodendreae, 69
Lepidodendron, 12, 66, 68, 69, 72, 86
Lepidodeitdron australe, 13, 67, 69
L. Gdspianum, 11
L. itotlium, 67
Lepidopldoios, 69
Lepidostrobus, 65
Leptopldoeum, 9, 10, 13, 67 (Fig.

40), 68, 69, 86
Lepiophloeuni australe, 13, 67, 69
L. rhomhicum, 67 (Fig. 40)
Lichens, 89
Lignier, O., 70, 71, 85
Ligule of Palaeozoic Lycopods, 86
Lithophyllum lichenoides, 39, 40

(Fig. 21)
Ludwig, R., 11, 12
Lycopodiales, 69
Lycopodites hostimensis, 30
L. Milleri, 29
Li/copodiiim Selago, 86
Lvcopods. 71, 85, 86, 88
Lyc()i)si(la, 9, 10, 64-67, 72, 73, 74,

85-86, 87

Mackie, W., 5

INDEX

99

Macnair, P., 29
Maine, 0, 13

3IariojjJeris, 84

Marwood Beds, 7

MeduUosa, 78

Meaapliyllous ancestry of Sphcno-

psiida, 71
Mejiapliylly of Pteropsida, 78, 78
Mclobesia lichenoides, 39, 40 (Fiii.

21)
M. membranacea, 40 (Fig. 21)
Mesozoic, 87
Microphylly, of Arthrofitigma, 72; of

Conifcrae, 71 ; of Lycopsida, 73;

of Psilotales, 87, 88; of Spheno-

psida. 73
Miller, Hugh, 14
Mimers-Thal, 6
Murchison, R. I., 3

Nathorst, A. G., 2, 3, 5, 13, 29, 37,

43, 64
Ncuialioii, ;")()
Nemalopliycus, 8, 13, 89
Neuropteris, 84
New Brunswick, 6
New South Wales, 6, 13
New York State, G, 13
Newberry, J. S., 37
Newcastle coal field. .58
Nicholson, H. A., 75, 76
Nocggernthia hi f urea, 12
Norway, 2, 5, 6, 7, 12, 26, 29, 37,

43, 44, 54, 56, 58

Ohio, 13

Old Ked Sandstone, classification in

Scotland, 3-5; correlation with

Devon facies, 4
Ordovician, 75, 76
Osteolepis, 4

Pdclnjtlivcd, 8
Pal<i('<i/)lii/c>is gracilis, 12
Palacophyllales, 9, 10, 56-58, 78-84
Par!>a {lecipicNs, 2, 3, 8, 10, 11, 38,

39 (Fig. 20), 40, 46, 47
Parkin, .T., 84
Pecopleris, 83

Pecopteris plumosn, 83 (Fig. 47)
Penhallow, I). P., 14, 29, 30
Perieaulome, 70, 73, 77, 78, 86, 87,

88
Permian, 69

Permo-carboniferous, 58
Perry Basin (S.E. Maine), 6, 13
Phyllineae, 71 , 74
Phylloideae, 71, 74
Polysiphonia, 73, 78

Portage group, 13

Potonie, H., 12, 30, 35, 50, 70, 73,

78, 84

Poudingue de Burnot. 5, 7

Primofilices, 71, 81, 84^85

Procormo{)hyta, 1, 47, 49, 51, 76, 82

Prohcpatics, 70, 71

Prolycopods, 71

Proj)teridophyta, 1

Protannularia laxa, 75 (Fig. 41)

P. radiata, 76

Proiolepidodcndmn , 10, 12, 42, 85

Protolepidodcndron knrlsteini, 42

P. Scharyuiiuui, 12

Psamniites du Condroz, 5, 7

Pscudohornia, 9, 54 (Fig. 26), 56,
76, 77

Pseudoboruia nrsitia, 12, 54 (Fig.
26)

Pseudo-dichotomy of Palaeozoic Ly-
copods, 86

Pseudosporochuus. 10, 12, 33 (Fig.
15), 34 (Fig. 16), 35, 17, 50. 81

Pseudosporocluius Krejcii, 33 (Fis.
15), 34 (Fig. 16)

Psilophytales, 47

Psilnphytniu 5, 6, 8, 9, 10, 1 1, 12 13,
15 (Fig. 1), 16 (Fig. 2), 17 (Fig.
3), 18 (Fig. 4), 19 (Fig. 5), 20,
21 (Fig. 6), 22-25 (Fig. 7), 26,
37, 43, 44, 46, 47, 48, 49, 70, 71.
74, 76, 85, 86, 88, 89, 91

Psilophyton Flora, adinities, 46-51;
distribution of clnef genera in
Devonian time, 9, 10; morpho-
logy and anatomy, 14—45; peri-
eaulome structure in, 78, 87. See
also 1, 2, 8, 9, 10, 11, 12

Psilophyton Deehianum, 17

P. elegans, 19, 20

P. Goldschmidtii. 21

P. ornaium, 11, 18, 22

P. princeps, 10, 11, 17 (Fig. 3), 18
(Fig. 4), 19 (Fig. 5), 20, 21
(Fig. 6), 22, 24, 25 (Fig. 7), 46

P. robusiius, 11, 19, 20

Psilotales, 47, 86-88

Psilotum, 49, 87, 88

Psygmophyllum, 9, 10. 12. 13, 55
(Fig. 27), 56, 58, 78, 79

Psygmophyllutn lirowniaimm, 13

P. Kolderupi, 55 (Fig. 27)

Pteraspis, 4

Pterichthys, 4

Pteridop'hyta, 1, 52, 71, 72, 89

Pteridorachis, 43

Pteridospermeae, 69, 72, 81, 85

Pteropsida. 9, 10, 71, 72, 73, 77,
78-85, 87

100

INDEX

Pierygotiis, 4

Ptilophijton, 9, 17, 31 (Fig. 12), 32

(Fig. 13), 33 (Fig. 14), 35, iV7 ,

50, 78, 81
Ptilopliyton Iiostimense. 33 (Fip-. 14)
P. Thomsoni, 10, 31 (Fig. 12), 32

(Fig. 13)

Reid, J., 29

Khacophyllum, 84

Rhacopteris, 9, 11, 61 (Fig. 33), 69,

79 (Fiij-. 42), 81, 84
lihaajji/eris fiircillaia. 01 (Fig. 33)
R. jxdiiciilijera, 79 (Fig. 42)
Hhiz()carj).s (Hydropterideae), 39, 46
Rhynia, 3, 15,' 10, 20, 24, 25 (Fig.

7), 26, 44, 46
Rhyiiia Clivynne-Vaughani, 1,2, 22-

25 (Fig. 7), 26
Rhynie, Aberdeenshire, 5
Roragen, 6, 7, 12, 43, 44
Russia, 4, 6, 7, 12, 67, 68, 91

Schimper, W. P., 11

Scotland, 3, 5, 7, 10, 11, 14, 16, 20,

29, 38, 42, 91
Scott, D. H., 72

Sedgwick ]Museum, Cambridge, 76
SelagineUa, 86
Sigillaria, 48, 09, 80
Silurian, 5, 8, 16, 38, 75, 89
Spermophyta, 71

Sphenophyllales, 71, 77, 86, 87, 88
Sphenophyllum, 9, 12, 52 (Fig. 24),

54, 70, 77
Sphoio/ihylluni Dmvsoni, 77
S. sublciicniniNni. 52 (Fig. 24)
Sphenopsida, 9, 10, 52-56, 05, 71,

72, 74-77, 80, 87
Sphenopteridmm, 9, 10, 12, 13, 61

(Fig. 34), 62 (Fig. 35), 81 (Fig.

44), 83
Sphenopleridium condmsorum, 11
S. moravicum, 81 (Fig. 44)
S. Tigidum, 11, 01 (Fig. 34), 02
Sphenopterids, 84
Sphenopteris, 9, 11. 12, 13, 64, 79,

81, 82 (Figs. 45 and 40)
Sphenopteris affinis, 81, 82 (Fig. 45)

,S'. bifida, 81, 82 (Fig. 40)

S. deiisepinuatu, 11

.S'. Ilook-eri, 11

"Spines" on axis of Psilophyton,

18-20
Spiraptcris, 12, 35
Si)itzlKTgen, 0, 7, 12, 50, 07
Sporocliitus, 50
Sporogoniies, 12, 44, 45, 71
Stele, comparison of that of Psilo-

tales with that of Psilophyton,

88; origin of in the earlier

Cormophyta, 89-91
Sternberg, C. von, 08
Stigmaria, 12, 04, 90
Stringoceplialus Burtini, 5
Stur, D., 37, 51
Sykes (Tlioday), M. G., 80

Taeniocrada, 9, 42, 47 *
Telangiion., 11
Thoday (Sykes), M. G., 80
Thursophyton, 9, 10. -12, 28 (Fig.

10), 29, 30 (Fig. 11), 31, 50,

51, 85, 80
Thursophyton Miller i, 10, 28 (Fig.

10), 29, 30 (Fig. 11), 31
T. Reidi, 10, 29, 30
Tmesipteris, 49, 87, 88
Triphyllopteris, 11

Unger, F., 09

United States, 2, 0, 7, 13, 10, 37,

38, 42, 50, 67, 91
Unknown genera in Devonian rocks,

69

Victoria, 6, 13

Wales 4

White' D., 2, 37, 42, 47, 66, 67,

68
Wood, Petrified, 13, 91

Xenoiheca, 11, 67

Zosterophyllum, 50, 85
Zosterophyllnm nupetonianurn; 11,
41 (Fig. 22), 42

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