PAST HISTORIES OF PLANT FAMILIES
V. Pteridosperms
This group consists entirely of plants which are extinct, and which were
in the height of their development in the Coal Measure period. As a group
they are the most recently discovered in the plant world, and but a few
years ago the name “Pteridosperm” was unknown. They form, however, both
one of the most interesting of plant families and one of the most
numerous of those which flourished in the Carboniferous period.
To mention first the vital point of interest in their structure, they
show _leaves which in all respects appear like ordinary foliage leaves,
and yet bear seeds_. These leaves, which we now know bore the seeds, had
long been considered as typical fern leaves, and had been named and
described as fern leaves. There are two extremely important results from
the discovery of this fossil group, viz. that leaves, to all appearance
like ordinary foliage, can directly bear seeds, and that the leaves,
though like fern leaves, bore seeds like those of a Cycad.
As the name _Pteridosperm_ indicates, the group is a link between the
ferns and the seed-bearing plants, and as such is of special interest and
value to botanists.
The gradual recognition of this group from among the numerous plant
fragments of Palæozoic age is one of the most interesting of the
accumulative discoveries of fossil botany. Ever since fossil remains
attracted the attention of enquiring minds many “ferns” have been
recognized among the rich impressions of the Coal Measures. Most of them,
however, were not connected with any structural material, and were given
many different names of specific value. So numerous were these fern
“species” that it was supposed that in the Coal Measure period the ferns
must have been the dominant class, and it is often spoken of even yet as
the “Age of Ferns”. From the rocks of the same age, preserved with their
microscopical structure perfect, were stems which were called
_Lyginodendron_. In the coal balls associated with these stems (which
were the commonest of the stems so preserved) were also roots, petioles,
and leaflets, but they were isolated, like the most of the fragments in a
coal ball, and to each was given its name, with no thought of the various
fragments having any connection with each other. Gradually, however,
various fragments from the coal balls had been recognized as belonging
together; one specimen of a petiole attached to a stem sufficed to prove
that all the scattered petioles of the same type belonged also to that
kind of stem, and when leaves were found attached to an isolated fragment
of the petiole, the chain of proof was complete that the leaves belonged
to the stem, and so on. By a series of lengthy and painstaking
investigations all the parts of the plant now called _Lyginodendron_ have
been brought together, and the impressions of its leaves have been
connected with it, these being of the fernlike type so long called
_Sphenopteris_, illustrated in fig. 77.
[Illustration: Fig. 77.—_Sphenopteris_ Leaf Impression, the fernlike
foliage of _Lyginodendron_]
[Illustration: Fig. 78A.—Diagram of the Transverse Section of Stem of
the _Lyginodendron_
_p_, Pith; P, primary wood groups; W, secondary wood; _l.t_, leaf
trace; _s_, sclerized bands in the cortex; S, longitudinal view of wood
elements to show the rows of bordered pits.]
The anatomy of the main stem is very suggestive of that of a Cycad. The
zones of secondary wood are loosely built, the quantity of soft tissue
between the radiating bands of wood, and the size of the pith being
large, while from the main axis double strands of wood run out to the
leaf base. The primary bundles, however, are not like those of a Cycad
stem, but have groups of _centripetal_ wood within the protoxylem, and
thus resemble the primary bundles of _Poroxylon_ (see p. 97), which are
more primitive in this respect than those of the Cycads.
The roots of _Lyginodendron_, when young, were like those of the
Marattiaceous ferns, their five-rayed mass of wood being characteristic
of that family, and different from the type of root found in most other
ferns (cf. fig. 78B with fig. 35 on p. 60). Unlike fern roots of any
kind, however, they have well-developed zones of secondary wood, in which
they approach the Gymnospermic roots (see fig. 78B, _s_).
[Illustration: Fig. 78B.—Transverse Section of Root of _Lyginodendron_
_w_, Five-rayed mass of primary wood; _s_, zone of secondary wood; _c_,
cortical and other soft tissues.]
A further mixture of characters is seen in the vascular bundles of the
petioles. A double strand, like that in the lower Gymnosperms, goes off
to the leaf base from the main axis, but in the petiole itself the bundle
is like a normal fern stele, and shows no characters in transverse
section which would separate it from the ferns. Such a petiole is
illustrated in fig. 79, with its V-shaped fernlike stele. On the petioles
and stems were certain rough, spiny structures of the nature of complex
hairs. In some cases they are glandular, as is seen in _g_ in fig. 79,
and as they seem to be unique in their appearance they have been of great
service in the identification of the various isolated organs of the
plant.
As is seen from fig. 77, the leaves were quite fern-like, but in
structural specimens they have been found with the characteristic
glandular hairs of the plant.
The seeds were so long known under the name of _Lagenostoma_ that they
are still called by it, though they have been identified as belonging to
_Lyginodendron_. They were small (about ¼ in. in maximum length) when
compared with those of most other plants of the group, or of the Cycads,
with which they show considerable affinity. They are too complex to
describe fully, and have been mentioned already (see p. 76), so that they
will not be described in much detail here. The diagrammatic figure (fig.
56) shows the essential characters of their longitudinal section, and
their transverse section, as illustrated in fig. 80, shows the complex
and elaborate mechanism of the apex.
[Illustration: Fig. 79.—Transverse Section through Petiole of
_Lyginodendron_
_v_, Fern-like stele; _c_, cortex; _g_, glandular hairlike
protuberances.]
Round the “seed” was a sheath, something like the husk round a hazel nut,
which appears to have had the function of a protective organ, though what
its real morphological nature may have been is as yet an unsolved
problem. On the sheath were glandular hairs like those found on the
petiole and leaves, which were, indeed, the first clues that led to the
discovery of the connection between the seed and the plant
_Lyginodendron_.
The pollen grains seem to have been produced in sacs very like fern
sporangia developed on normal foliage leaves, each grain entered the
cavity _pc_ in the seed (see fig. 56), but of the nature of the male cell
we are ignorant. In none of the fossils has any embryo been found in the
“seeds”, so that presumably they ripened, or at least matured their
tissues, before fertilization.
These, in a few words, are the essentials of the structures of
_Lyginodendron_. But this plant is only one of a group, and at least two
other of the Pteridosperms deserve notice, viz. _Medullosa_, which is
more complex, and _Heterangium_, which is simpler than the central type.
[Illustration: Fig. 80.—Diagram of Transverse Section of _Lagenostoma_
Seed near the Apex, showing the nine flutings _f_ of the coat _c_; _v_,
the vascular strand in each; _nc_, cone of nucellar tissue standing up
in the fluted apex of the nucellus _n_; _pc_, the pollen chamber with a
few pollen grains; _s_, space between nucellus and coat. Compare with
diagram 56.]
_Heterangium_ is found also in rocks rather older than the coal series of
England, though of Carboniferous age, viz. in the Calciferous sandstone
series of Scotland, it occurs also in the ordinary Coal Measure nodules.
It is in several respects more primitive than _Lyginodendron_, and in
particular in the structure of its stele comes nearer to that of ferns.
The stele is, in fact, a solid mass of primary wood and wood parenchyma,
corresponding in some degree to the protostele of a simple type (see p.
61, fig. 36), but it has towards the outside groups of protoxylem
surrounded by wood in both centripetal and centrifugal directions, which
are just like the primary bundles in _Lyginodendron_. Outside the primary
mass of wood is a zone of secondary wood, but the quantity is not large
in proportion to it (see fig. 81), as is common in Lyginodendron.
Though the primary mass is so fernlike in appearance the larger tracheids
show series of bordered pits, as do most of the tracheids of the
Pteridosperms, in which they show a Gymnosperm-like character.
[Illustration: Fig. 81.—_Heterangium_
A, Half of the stele of a stem, showing the central mass of wood S
mixed with parenchyma _p_. The protoxylem groups _p. x._ lie towards
the outside of the stele. Surrounding it is the narrow zone of
small-celled secondary wood W. B, A few of the wood cells in
longitudinal view: _p. x._, Protoxylem; _p_, parenchyma. S, Large
vessels with rows of bordered pits.]
The foliage of _Heterangium_ was fernlike, with much-divided leaves
similar to those of _Lyginodendron_. We have reason to suspect, though
actual proof is wanting as yet, that small Gymnosperm-like seeds were
borne directly on these leaves.
_Medullosa_ has been mentioned already (see p. 72) because of the
interesting and unusually complex type of its vascular anatomy. Each
individual stele of the group of three in the stem, however, is
essentially similar to the stele of a Heterangium.
Though the whole arrangement appears to differ so widely from other stems
in the plant world, careful comparison with young stages of recent Cycads
has indicated a possible remote connection with that group, while in the
primary arrangements of the protosteles a likeness may be traced to the
ferns. The roots, even in their primary tissues, were like those of
Gymnosperms, but the foliage with its compound leaves was quite fern-like
externally. A small part of a leaf is shown in fig. 83, and is clearly
like a fern in superficial appearance. The leaves were large, and the
leaf bases strong and well supplied with very numerous branching vascular
bundles.
[Illustration: Fig. 82.—Steles of _Medullosa_ in Cross Section of the
Stem
A, Primary solid wood; S, surrounding secondary wood.]
[Illustration: Fig. 83.—Part of a Leaf of _Medullosa_, known as
_Alethopteris_, for long supposed to be a Fern]
The connection between this plant and certain large three-ribbed seeds
known as _Trigonocarpus_ is strongly suspected, though actual continuity
is not yet established in any of the specimens hitherto discovered. These
seeds have been mentioned before (p. 76 and p. 82). They were larger than
the other fossil seeds which we have mentioned, and, with their fleshy
coat, were similar in general organization to the Cycads, though the fact
that the seed coat stood free from the inner tissues right down to the
base seems to mark them as being more primitive (cf. fig. 55, p. 76).
Of impressions of the Pteridosperms the most striking is, perhaps, the
foliage known as _Neuropteris_ (see fig. 6, p. 13), to which the large
seeds are found actually attached (cf. fig. 85).
[Illustration: Fig. 84.—Diagrammatic Section of a Transverse Section of
a Seed of _Trigonocarpus_
S, Stone of coat with three main ridges and six minor ones. F, Flesh of
coat: _i f_, inner flesh; _n_, nucellus, crushed and free from coat;
_s_, spore wall.]
[Illustration: Fig. 85.—Fragment of Foliage of _Neuropteris_ with Seed
attached, showing the manner in which the seeds grew on the normal
foliage leaves in the Pteridosperms]
Ever-increasing numbers of the “ferns” are being recognized as belonging
to the Pteridosperms, but _Heterangium_, _Lyginodendron_, and _Medullosa_
form the three principal genera, and are in themselves a series
indicating the connection between the fernlike and Cycadean characters.
Before the fructifications were suspected of being seeds the anatomy of
these plants was known, and their nature was partly recognized from it
alone, though at that time they were supposed to have only fernlike
spores.
The very numerous impressions of their fernlike foliage from the
Palæozoic rocks indicate that the plants which bore such leaves must have
existed at that time in great quantity. They must have been, in fact, one
of the dominant types of the vegetation of the period. The recent
discovery that so large a proportion of them were not ferns, but were
seed-bearing plants, alters the long-established belief that the ferns
reached their high-water mark of prosperity in the Coal Measure period.
Indeed, the fossils of this age which remain undoubtedly true ferns are
far from numerous. It is the seed-bearing Pteridosperms which had their
day in Palæozoic times. Whether they led directly on to the Cycads is as
yet uncertain, the probability being rather that they and the Cycads
sprang from a common stock which had in some measure the tendencies of
both groups.
That the Pteridosperms in themselves combined many of the most important
features of both Ferns and Gymnosperms is illustrated in the account of
them given above, which may be summarized as follows:—
Salient Characters of the Pteridosperms
_G_=_Gymnospermic_ _F_=_Fernlike_
_F_ Primary structure of root.
_G_ Secondary thickening of root.
_F_ In _Heterangium_ and _Medullosa_ the
_F_ solid centripetal primary wood of stele.
_G_ Pits on tracheæ of primary wood.
_G_ Secondary thickening of stem.
_G_ Double leaf trace.
_F_ Fernlike stele in petiole.
_F_ Fernlike leaves.
_F_ Sporangia pollen-sac-like.
_F_ Reproductive organs borne directly on ordinary foliage leaves.
_G_ General organization of the seed.
Thus it can be seen at a glance, without entering into minutiæ, that the
characters are divided between the two groups with approximate equality.
The connection with Ferns is clear, and the connection with Gymnosperms
is clear. The point which is not yet determined, and about which
discussion will probably long rage, is the position of this group in the
whole scheme of the plant world. Do they stand as a connecting link
between the ferns on one hand and the whole train of higher plants on the
other, or do they lead so far as the Cycads and there stop?