PSILOTALES.
The two recent genera _Psilotum_ and _Tmesipteris_ are usually spoken
of as members of the family Psilotaceae which is included as one of the
subdivisions of the Lycopodiales. It is probable, as Scott[36] first
suggested, that these two plants are more nearly allied than are any
other existing types to the Palaeozoic genus _Sphenophyllum_.
We may give expression to the undoubted resemblances between
_Tmesipteris_ and _Psilotum_ and the Sphenophyllales by including the
recent genera as members of that group, originally founded on the
extinct genus _Sphenophyllum_; this is the course adopted by Thomas[37]
and by Bower[38]: or we may emphasise the fact that these two recent
genera differ in certain important respects from _Lycopodium_ and
_Selaginella_ by removing them to a separate group, the Psilotales.
The latter course is preferred on the ground that the inclusion of
_Psilotum_ and _Tmesipteris_ in a group founded on an extinct and
necessarily imperfectly known type, is based on insufficient evidence
and carries with it an assumption of closer relationship than has been
satisfactorily established.
The genus _Tmesipteris_ (fig. 120, A) is represented by a single
species _T. tannensis_ Bertr.[39] which usually occurs as an epiphyte
on the stems of tree-ferns in Australia, New Zealand, and Polynesia.
_Psilotum_, with two species _P. triquetrum_ Sw. (fig. 118) and
_P. complanatum_ Sw., flourishes in moist tropical regions of both
hemispheres, growing either on soil rich in organic substances or as an
epiphyte. Both genera are considered to be more or less saprophytic.
[Illustration: FIG. 118. _Psilotum triquetrum_ (½ natural size).
A. Synangium.
B. Sporophyll after removal of the synangium. (M.S.)]
_Psilotum._ The common tropical species _P. triquetrum_ (fig. 118) is
characterised by an underground rhizome which forms a confused mass
of dark brown branches covered with filamentous hairs as substitutes
for roots and gives off erect repeatedly forked aerial shoots. In _P.
complanatum_[40] the habit is similar to that of the more abundant
and better-known species, but the pendulous shoots are characterised
by their broader and flatter form. In both species the function of
carbon-assimilation is performed by the outer cortex of the green
branches, as the small size of the widely-separated foliage leaves
renders them practically useless as assimilating organs.
The sporophylls consist of a short axis terminating in two small
divergent forks and bearing on its adaxial surface a trilocular or in
rare cases a bilocular synangium (fig. 118, A and B). The walls of the
loculi are composed of several layers of cells and dehiscence takes
place along three lines radiating from the centre of the synangium.
Professor Thomas[41] has recorded “fairly numerous instances in
_Psilotum_ of a second dichotomy of one branch of the first fork, or,
less frequently, of both branches”: instead of one synangium subtended
by the two slender leaflets of the forked sporophyll-axis, there
may be two synangia and three leaf-lobes or three synangia and four
leaf-lobes. The occurrence of both these abnormalities in _Psilotum_
and _Tmesipteris_ shows a decided tendency in the Psilotales to a
repeated dichotomy of the sporophylls[42].
A single stele[43] with a fluted surface occupies the axis of an
aerial shoot (fig. 119, A); the axial region is occupied by a core of
elongated mechanical elements (_s_), which may occasionally extend to
the periphery of the xylem and break the continuity of the band of
scalariform tracheae (fig. 119, A, _a_). The tracheae form the arms of
an irregularly stellate stele and each arm is terminated by protoxylem
elements (fig. 119, B, _px_). The rays of the xylem cylinder, which
may be as many as six or eight in the upper part of the aerial shoots,
become reduced in number as the rhizome is approached, assuming a
diarch structure near the junction. In the rhizome the xylem forms
an approximately triangular group of tracheae without any core of
mechanical elements. Three to four layers of parenchyma succeeded
externally by an ill-defined phloem (fig. 119, A, _p_) surround the
xylem and a fairly distinct endodermis (fig. 119, A and B, _e_)
encloses the whole. To Mr Boodle[44] is due the interesting discovery
that in some parts of the rhizome the parenchymatous zone surrounding
the scalariform tracheae may become the seat of meristematic
activity which results in the production of secondary tracheae
often characterised by a sinuous longitudinal course. There is no
definite cambium, but the radially disposed tracheae and the adjacent
parenchymatous elements clearly demonstrate the secondary nature of the
tissue immediately external to the group of primary xylem. Fig. 119, C,
drawn from a section kindly supplied by Mr Boodle, shows the secondary
xylem elements at _x_² associated with radially disposed thin-walled
cells abutting on the primary xylem, _x_¹. It is probable that this
added tissue may be a remnant of a more extensive secondary thickening
characteristic of the ancestors of the recent species. In their manner
of occurrence and sinuous course these secondary tracheids bear a
resemblance to the secondary xylem of _Lepidodendron fuliginosum_[45].
The stele of the aerial shoot bears a fairly close resemblance to the
vascular axis of _Cheirostrobus_, and its three-rayed form in the lower
portions of the green branches recalls that of the _Sphenophyllum_
stele, except that the axial xylem elements of the Palaeozoic genus
are usually represented in _Psilotum_ by mechanical tissue. The
cortex consists of three regions (fig. 119, A), an outer zone of
chlorophyllous tissue (_a_) rich in intercellular spaces succeeded by
a band of mechanical tissue (_b_) which gradually passes into an inner
region of larger and thinner-walled cells (_c_).
[Illustration: FIG. 119.
A. Diagram of transverse section of aerial shoot of _Psilotum
triquetrum_. _a_—_c_, cortex; _p_, phloem; _e_, endodermis; _s_,
stereome; _x_, xylem; _a_, gap in xylem.
B. Enlarged view of one of the angles of the xylem shown in A.
_px_, protoxylem.
C. Part of transverse section of an approximately triangular
rhizome stele showing a portion of the metaxylem _x_¹; _px_,
protoxylem elements; _x_², secondary xylem.]
[Sidenote: TMESIPTERIS]
The genus _Tmesipteris_[46] agrees with _Psilotum_ in general habit
and in its epiphytic and probably in some degree saprophytic mode
of life. Its brown rootless rhizome, which grows among the roots of
tree-ferns or rarely in the ground, gives off pendulous or erect
shoots reaching a length of two feet and bearing lanceolate mucronate
leaves 2–3 cm. long (fig. 120, A) attached by decurrent leaf-bases.
The sporophylls, replacing the upper leaves or occurring in more or
less well-defined zones alternating with the foliage leaves, consist
of a short axis terminating in a pair of lanceolate lobes and bearing
on its adaxial surface an elongated bilocular synangium attached to a
very short stalk (fig. 120, B). Reference has already been made to the
divergent opinions as to the morphological nature of the sporophylls
or sporangiophores, but recent investigations distinctly favour the
view that a sporophyll is best interpreted as a stalked leaf with
two sterile laminae and an almost sessile, or in some cases a more
obviously stalked, synangium; the whole sporophyll is characterised
by the possession of a ventral and a dorsal lobe[47]. The drawings
reproduced in fig. 120, D and F, illustrate some of the frequent
variations described by Thomas in plants which he observed in the
New Zealand forests. The sporophyll shown in fig. 120, D and F, has
branched twice and bears three synangia.
[Illustration: FIG. 120. _Tmesipteris._
A. Foliage leaves.
B. Sporophyll and bilocular synangium.
C. Diagram of transverse section of stele. _px_, protoxylem.
D, F. Abnormal sporophylls. (From drawings made by Prof. Thomas and
generously placed at my disposal. A.C.S.)
E. Portion of C enlarged.]
The aerial branches of _Tmesipteris_ possess a central cylinder of
separate xylem groups in which the protoxylem occupies an internal
position (fig. 120, C and E, _px_) enclosing an axial parenchymatous
region. The cells of a few layers of the inner cortex immediately
outside the endodermis are rendered conspicuous by a dark brown
deposit. The cortex as a whole is composed of uniform parenchymatous
tissue. In the lower part of the aerial shoots and in the rhizome the
xylem forms a solid strand without protoxylem elements and conforms
more clearly to that of _Psilotum_.
In this short account of the anatomy of _Tmesipteris_ no mention
is made of the effect produced on the stele by the departure of
leaf-traces and of vascular stands to supply branches. Miss Sykes[48]
in a recently published paper on the genus has shown that the exit of
a leaf-trace does not break the continuity of the xylem of the stele,
while the exit of a sporophyll-trace is marked by an obvious gap.
Evidence is adduced in support of the conclusion that this difference,
which at first sight appears to be one of morphological importance,
is in reality merely a question of degree and “is due to the earlier
preparation for the formation of ‘sporophyll’ than leaf-traces.”
Miss Sykes gives her adherence to the view that the “sporophylls” of
_Tmesipteris_ are branches and not leaves, but despite the arguments
advanced this interpretation seems to me less probable than that which
recognises the sporophyll as a foliar organ. Prof. Lignier[49] has
pointed out that if Miss Sykes’s conclusion as to the axial nature
of the sporophyll in _Tmesipteris_ is accepted, it diminishes the
force of the comparison between the sporophylls of that genus and
_Sphenophyllum_ as those of the latter can hardly be regarded as other
than foliar organs.
Both members of the Psilotales may, as Boodle has suggested, be
regarded as descendants of a common parent in which the aerial
stems possessed a fluted or stellate cylinder of mesarch xylem.
There can be no doubt as to the significance of the morphological
resemblances between the Psilotales and the genera _Sphenophyllum_
and _Cheirostrobus_, but the position of _Tmesipteris_ and _Psilotum_
in the plant-kingdom may probably be best expressed by adopting the
group-name Psilotales rather than by transferring the recent genera
to the Sphenophyllales. One of the most striking differences between
the Psilotales and the genus _Lycopodium_ is in the form of the
sporophylls and sporangia; in _Lycopodium_ a single sporophyll bears
a unilocular sporangium, but in the Psilotales the sporophyll may
be described as a bilobed structure homologous with a foliage-leaf,
bearing a sporangiophore which consists of a short stalk terminating
in a bilocular or trilocular synangium; the short stalk receives a
special branch from the vascular bundle of the sterile portion of the
sporophyll[50].
_Fossils described by authors as being closely
allied to_ Psilotum.
A search through palaeobotanical literature reveals the existence
of a very small number of specimens which have been identified as
representatives of the Psilotales. An inspection of the material or
published drawings leads one to the conclusion that practically no
information of a satisfactory kind is available in regard to the past
history of the two southern genera _Psilotum_ and _Tmesipteris_, which
are regarded by some botanists as relics of an ancient branch[51] of
pteridophytes.
[Sidenote: PSILOTITES, ETC.]
In 1842 Münster[52] instituted the genus _Psilotites_ for a small
impression of a slender branched axis from Jurassic rocks near Mannheim
in Germany which he named _Psilotites filiformis_; Schimper[53]
spoke of the specimens as too doubtful for determination, an opinion
with which every botanist would cordially agree. Goldenberg’s species
_Psilotites lithanthracis_[54] from the Saarbrücken coal-field is
founded on impressions of axes: some of these are dichotomously
branched and bear small oval projections, which may be rudimentary
leaves or possibly leaf-scars. More recently Kidston[55] described
specimens of branched axes from the Lanarkshire coal-field bearing
a row of lateral thorn-like projections under the title _Psilotites
unilateralis_; but these fragments, as Dr Kidston himself admits, are
of no botanical value.
In a paper on fossil Salvinias, Hollick[56] mentions _Salvinia
reticulata_, originally described by Heer and by Ettingshausen and _S.
Alleni_ Lesq.[57] a Tertiary species, and calls attention to their very
close resemblance in form, nervation, and apex to the leaves of the
genus _Tmesipteris_: he refers both species to that genus. The drawings
reproduced by Hollick represent leaves with a midrib and numerous
anastomosing lateral veins, whereas in _Tmesipteris_ the lamina of the
leaf has a midrib without lateral branches. An enlarged drawing of the
outlines of the epidermal cells would correspond closely with the small
reticulations in the fossil leaves and it may be that there has been
some confusion between veins and cell-outlines. In any case there would
seem to be no reason for the use of the recent generic name[58].
Among other fossils assigned to the Psilotales we have Marion’s genus
_Gomphostrobus_ from the Permian of France and Germany[59]. Marion
placed this plant in the Coniferales on the strength of its resemblance
to _Walchia_ and _Araucaria_, but Potonié[60] is inclined to recognise
in the leaves and monospermic sporophylls characters suggestive of
Lycopodiaceous affinity.
The latter author in 1891[61], in ignorance of Marion’s proposal to
adopt the name _Gomphostrobus_, instituted a genus _Psilotiphyllum_
for the sporophylls of a species originally described by Geinitz[62]
as _Sigillariostrobus bifidus_, but he subsequently adopted Marion’s
designation and with some hesitation included the French and German
specimens in the Psilotales. As stated elsewhere[63], Potonié’s
arguments in favour of his view hardly carry conviction, and it is
probably more in accordance with truth to deal with _Gomphostrobus_ in
the chapter devoted to the Coniferales.
_Psilophyton._
The generic title _Psilophyton_, instituted by the late Sir
William Dawson[64], has become familiar to geologists as that of
a Pre-Carboniferous plant characteristic of Devonian and Silurian
rocks in Canada, the United States of America, and Europe. From the
botanist’s point of view the name stands for miscellaneous remains
of plants of different types and in many cases unworthy of record.
The genus was founded on impressions of branched axes from the
Devonian strata of New Brunswick resembling the rachis and portions of
lateral pinnae of ferns or the forked slender twigs of a Lycopod. The
type-species _Psilophyton princeps_ Daws. as represented on somewhat
slender evidence in Dawson’s restoration, which accompanies the
original description of the genus and has since been copied by several
authors, is characterised by the possession of a horizontal rhizome
bearing numerous rootlets and giving off dichotomously branched aerial
shoots with spinous appendages, compared with rudimentary leaves, and
terminating in slender branchlets bearing pendulous oval “spore-cases”
from their tips. Some of the branchlets exhibit a fern-like vernation.
The plant is spoken of by Dawson as apparently a generalised type[65],
resembling in habit and in its rudimentary leaves the recent genus
_Psilotum_ and presenting points of contact with ferns. Specimens were
found in an imperfectly petrified state showing a central cylinder of
scalariform tracheae surrounded by a broad cortical zone of parenchyma
and fibrous tissue.
Among other species described by the author of the genus we need only
mention _Psilophyton robustius_, characterised by vegetative shoots
and “spore-cases” similar to those of the type-species; but, as
Solms-Laubach[66] has pointed out, the petrified sections referred by
Dawson to _P. robustius_ are of an entirely different anatomical type
from that of _P. princeps_[67].
British fossils from the Old Red Sandstone from the north of Scotland,
Orkney and Caithness, originally figured by Hugh Miller and compared
by him with algae but more especially with recent Lycopods, were
subsequently placed by Carruthers[68] in the genus _Psilophyton_ as _P.
Dechianum_, the specific designation being chosen on the ground that
the Scotch specimens are specifically identical with fossils described
by Goeppert[69] as _Haliserites Dechianus_.
Various opinions have been expressed in regard to the nature of
the Devonian species _Haliserites Dechianus_ Goepp. with which
Carruthers[70] identified Miller’s Old Red Sandstone plant: reference
may be made to a paper by White[71] containing figures of dichotomously
branched impressions described as species of _Thamnocladus_ which he
includes among the algae.
In describing some Belgian impressions of Devonian age as
_Lepidodendron gaspianum_ Daws. Crépin[72] states that Carruthers has
come to regard the specimens named by him _Psilophyton Dechianum_
as branches of a Lepidodendron; he also quotes Carruthers as having
expressed the opinion that the name _Psilophyton_ had been employed by
Dawson for two kinds of fossils, some being twigs of _Lepidodendron_
while others, identified by Dawson as the reproductive branches of
species of _Psilophyton_, represent the spore-cases of ferns comparable
with Stur’s genus _Rhodea_[73]. One of the examples figured by
Carruthers[74] as _P. Dechianum_ from Thurso (preserved in the British
Museum, no. 52636), measuring 34 cm. in length and 8 mm. broad, bears
a close resemblance to a fern rhizome covered with ramental scales such
as that of a species of _Davallia_. Other Belgian specimens described
by Gilkinet[75] as _Lepidodendron burnotense_, like Crépin’s species,
are no doubt generically identical with some of the Scotch and Canadian
fossils placed in the genus _Psilophyton_, though Penhallow[76]
considers that the species _Lycopodites Milleri_ is more correctly
referred to _Lycopodites_ than to _Psilophyton_.
A more recent paper on the Geology of the Perry basin in South-eastern
Maine by Smith and White[77] contains a critical summary of the
literature on _Psilophyton_ and drawings of specimens. The latter
afford good examples of Pre-Carboniferous plant fragments, such as are
often met with in various parts of the world, which conform in habit to
the New Brunswick specimens made by Dawson the type of his genus.
An examination of material in the Montreal Museum and of Hugh Miller’s
specimens in the Edinburgh collection leads me to share the opinion
of Count Solms-Laubach that the name _Psilophyton_ has been applied
to plants which should not be included under one generic title. As
Kidston[78] pointed out, the Canadian species _Psilophyton robustius_
is not generically distinct from British and Belgian specimens
referred to _Lepidodendron_; it may possibly be identical with the
Bohemian plants on which Stur founded his genus _Hostinella_[79]. The
Devonian plants described by Stur have since been examined by Jahn[80]
who regards them as vascular plants, and not as algae to which Stur
referred them; he mentions two species of _Psilophyton_ but gives no
figures.
The “spore-cases” of Dawson may be found to be the microsporangia
or perhaps the small seeds of some pteridosperm; the forked axes
with a smooth surface and others figured by Miller and by Dawson,
with the surface covered with scales suggesting the ramenta of a
fern, may be the rachises or rhizomes of filicinean plants. Other
specimens may be Lepidodendron twigs, as for example the petrified
fragments figured by Dawson as _Psilophyton princeps_; while the stem
identified as _P. robustius_ is most probably that of a Gymnosperm.
It is doubtful whether a useful purpose is served by retaining the
genus _Psilophyton_. It was in the first instance instituted on the
assumption, which cannot be upheld, that the abundant material in the
New Brunswick beds bore a sufficiently close resemblance to the rhizome
and aerial branches of _Psilotum_. _Psilophyton_ has served as a name
for miscellaneous plant fragments, many of which are indeterminable. Dr
White concludes his account of the genus with the following words[81]:
“The examination of such so-called Psilophyton material as I have seen
shows the existence in America of two or more groups, represented
by several fairly well-marked species which possess stratigraphical
value, and which should be carefully diagnosed and illustrated. It is
probable also that additional material throwing light on the structure
and relationships of these very remarkable early types of land-plants
will be discovered at some locality. The inspection of the material in
hand emphasises the need, as was pointed out by Solms-Laubach, for the
revision of the material referred by various authors to _Psilophyton_,
together with a thorough re-examination and re-publication of the
types.”
Until a thorough re-examination has been made of the Canadian material,
with a view to determine whether there exist substantial reasons for
the retention of Dawson’s genus, it is undesirable to continue to make
use of this name for Pre-Carboniferous fossils which are too incomplete
to be assigned with certainty to a definite group of plants. Dr White
draws attention to the similarity of some of the Perry basin specimens
to Nathorst’s genus _Cephalotheca_[82] from Devonian rocks of Bear
Island in the Arctic regions, a comparison which might be extended to
other genera and which serves to illustrate the possibility that many
of the specimens labelled _Psilophyton_ may eventually be recognised as
examples of well defined generic types belonging to more than one group
of plants.