MINUTE STRUCTURE OF FOSSIL PLANTS—DIFFERENCES FROM LIVING ONES
We have seen in the last chapter that the main morphological divisions,
roots, stems, leaves, and fructifications, were as distinct in the Coal
Measure period as they are now. There is one structure, however, found in
the Coal Measure fossils, which is hardly paralleled by anything similar
in the living plants, and that is the fossil known as _Stigmaria_.
_Stigmaria_ is the name given, not to a distinct species of plant, but to
the large rootlike organs which we know to have belonged to all the
species of _Lepidodendron_ and of _Sigillaria_. In the frontispiece these
organs are well seen, and branch away at the foot of the trunk, spreading
horizontally, to all appearance merely large roots. They are especially
regularly developed, however, the main trunk giving rise always to four
primary branches, these each dividing into two equal branches, and so
on—in this they are unlike the usual roots of trees. They bore numerous
rootlets, of which we know the structure very well, as they are the
commonest of all fossils, but in their internal anatomy the main “roots”
had not the structure which is characteristic of roots, but were like
_stems_. In living plants there are many examples of stems which run
underground, but they always have at least the rudiments of leaves in the
form of scales, while the fossil structures have apparently no trace of
even the smallest scales, but bear only rootlets, thus resembling true
roots. The questions of morphology these structures raise are too complex
to be discussed here, and Stigmaria is only introduced as an example, one
of the very few available, of a palæozoic structure which seems to be of
a nature not clearly determinable as either root, stem, leaf, or
fructification. Among living plants the fine rootlike rhizophores of
Selaginella bear some resemblance to Stigmaria in essentials, though so
widely different from them in many ways, and they are probably the
closest analogy to be found among the plants of to-day.
The individual cells, we have already seen, are strikingly similar in the
case of fossil and living plants. There are, of course, specific
varieties peculiar to the fossils, of which perhaps the most striking
seem to be some forms of _hair_ cells. For example, in a species of fern
from the French rocks there were multicellular hairs which looked like
little stems of Equisetum owing to regular bands of teeth at the
junctions of the cells. These hairs were quite characteristic of the
species—but hairs of all sorts have always abounded in variety, so that
such distinction has but minor significance.
[Illustration: Fig. 48.—Stele of _Lepidodendron_ W, surrounded by a
small ring of secondary wood S]
As was noted in the table (p. 58) the only cell types of prime importance
which were not evolved by the Palæozoic plants were the wood vessels,
phloem and accompanying cells which are characteristic of the flowering
plants.
Among the fossils the vascular arrangements are most interesting, and, as
well as all the types of stele development noted in the previous chapter
as common to both living and fossil plants, there are further varieties
found only among the fossils (see fig. 50).
The simple protostele described (on p. 61) is still found, particularly
in the very young stages of living ferns, but it is a type of vascular
arrangement which is not common in the mature plants of the present day.
In the Coal Measure period, however, the protostele was characteristic of
one of the two main groups of ferns. In different species of these ferns,
the protostele assumed a large variety of shapes and forms as well as the
simple cylindrical type. The central mass of wood became five-rayed in
some, star-shaped, and even very deeply lobed, with slightly irregular
arms, but in all these cases it remained fundamentally monostelic.
Frequently secondary tissue developed round the protosteles of plants
whose living relatives have no such tissue. A case of this kind is
illustrated in fig. 48, which shows a simple circular stele surrounded by
a zone of secondary woody tissue in a species of _Lepidodendron_.
[Illustration: Fig. 49.—_Lepidodendron_, showing Part of the Hollow
Ring of Primary Wood W, with a relatively large amount of Secondary
Tissue S, surrounding it]
In many species of _Lepidodendron_ the quantity of secondary wood formed
round the primary stele was very great, so that (as is the case in higher
plants) the primary wood became relatively insignificant compared with
it. In most species of _Lepidodendron_ the primary stele is a hollow ring
of wood (cf. fig. 38, p. 62) round which the secondary wood developed, as
is seen in fig. 49. These two cases illustrate a peculiarity of fossil
plants. Among living ones the solid and the simple ring stele are almost
confined to the Pteridophytes, where secondary wood does not develop, but
the palæozoic Pteridophytes, while having the simple primary types of
steles, had quantities of secondary tissue, which was correlated with
their large size and dominant position.
[Illustration: Fig. 50.—Diagram of Steles of the English _Medullosa_,
showing three irregular, solid, steles A, with secondary thickenings S,
all round each. _a_, Small accessory steles]
Among _polystelic_ types (see p. 63) we find interesting examples in the
fossil group of the _Medulloseæ_, which are much more complex than any
known at present, both owing to their primary structure and also to the
peculiar fact that all the steles developed secondary tissue towards the
inner as well as the outer side. One of the simpler members of this
family found in the English Coal Measures is illustrated in fig. 50. Here
there are three principal protosteles (and several irregular minor ones)
each of which has a considerable quantity of secondary tissue all round
it, so that a portion of the secondary wood is growing in towards the
actual centre of the stem as a whole—a very anomalous state of affairs.
In the more complex Continental type of _Medullosa_ there are _very_
large numbers of steles. In the one figured from the Continent in fig. 51
but a few are represented. There is a large outer double-ring stele, with
secondary wood on both sides of it, and within these a number of small
steles, all scattered through the ground tissue, and each surrounded by
secondary wood. In actual specimens the number of these central steles is
much greater than that indicated in the diagram.
No plant exists to-day which has such an arrangement of its vascular
cylinder. It almost appears as though at the early period, when the
Medulloseæ flourished, steles were experimenting in various directions.
Such types as are illustrated in figs. 50 and 51 are obviously wasteful
(for secondary wood developing towards the centre of a stem is bound to
finally meet), and complex, but apparently inefficient, which may partly
account for the fact that this type of structure has not survived to the
present, though simpler and equally ancient types have done so.
[Illustration: Fig. 51.—Continental _Medullosa_, showing R, outer
double-ring stele with secondary wood all round it; S, inner stellate
steles, also surrounded in each case by secondary tissue]
Further details of the anatomy of fossils will be mentioned when we come
to consider the individual families; those now illustrated suffice to
show that in the Coal Measures very different arrangements of steles were
to be found, as well as those which were similar to those existing now.
The significance of these differences will become apparent when their
relation to the other characters of the plants is considered.
The fructifications, always the most important parts of the plant, offer
a wide field, and the divergence between the commoner palæozoic and
recent types seems at first to be very great. Indeed, when palæozoic
reproductive bodies have to be described, it is often necessary to use
the common descriptive terms in an altered and wider sense.
Among the plants of to-day there are many varieties of the simple
single-celled reproductive masses which are called _spores_, and which
are usually formed in large numbers inside a spore case or sporangium.
Among the higher plants _seeds_ are also known in endless variety, all of
which, compared with spores, are very complex, for they are many-celled
structures, consisting essentially of an embryo or young plant enclosed
in various protective coats. The distinction between the two is sharp and
well defined, and for the student of living plants there exists no
difficulty in separating and describing seeds and spores.
But when we look back through the past eras to palæozoic plants the
subject is not so easy, and the two main types of potentially
reproductive masses are not sharply distinct. The seed, as we know it
among recent plants, and as it is generally defined, had not fully
evolved; while the spores were of great variety and had evolved in
several directions, some of which seem to have been intermediate stages
between simple spores and true seeds. These seedlike spores served to
reproduce the plants of the period, but their type has since died out and
left but two main methods among living plants, namely the essentially
simple spores, the very simplicity of whose organization gives them a
secure position, and the complex seeds with their infinite variety of
methods for protecting and scattering the young embryos they contain.
Among the Coal Measure fossils we can pick up some of the early stages in
the evolution of the seed from the spore, or at least we can examine
intermediate stages between them which give some idea of the possible
course of events. Hence, though the differences from our modern
reproductive structures are so noticeable a feature of the palæozoic
ones, it will be seen that they are really such differences as exist
between the members at the two ends of a series, not such as exist
between unrelated objects.
Very few types can be mentioned here, and to make their relations clear a
short series of diagrams with explanations will be found more helpful
than a detailed account of the structures.
[Illustration: Fig. 52.—Spores
Each spore a single cell which develops with three others in tetrads
(groups of four). Very numerous tetrads enclosed in a spore case or
sporangium which develops on a leaflike segment called the sporophyll.
Each spore germinates independently of the others after being
scattered, all being of the same size. Common in fossils and living
Pteridophytes.]
[Illustration: Fig. 53.—Spores
Each a single cell like the preceding, but here only one tetrad in a
sporangium ripens, so that each contains only four spores. Compared
with the preceding types these spores are very large. Otherwise details
similar to above. Some fossils have such sporangia with eight spores,
or some other small number; living Selaginellas have four. In the same
cone sporangia with small spores are developed and give rise to the
male organs.]
[Illustration: Fig. 54.—“Spores” of Seedlike Structure
Out of a tetrad in each sporangium only one spore ripens, S in figure,
the others, _s_, abort. The wall of the sporangium, _w_, is more
massive than in the preceding cases, and from the sporophyll, flaps,
_sp f_, grow up on each side and enclose and protect the sporangium.
The one big spore appears to germinate inside these protective coats,
and not to be scattered separately from them. Only found in fossils,
one of the methods of reproduction in _Lepidodendron_. Other sporangia
with small spores were developed which gave rise to the male organs.]
[Illustration: Fig. 55.—“Seed”
In appearance this is like a seed, but differs from a true seed in
having no embryo, and is like the preceding structure in having a very
large spore, S, though there is no trace of the three aborting ones.
The spore develops in a special mass of tissue known as the nucellus,
_n_, which partly corresponds to the sporangium wall of the previous
types. In it a cavity, _p c_, the pollen chamber, receives the pollen
grains which enter at the apex of the “seed”. There is a complex coat,
C, which stands round the nucellus but is not joined to it, leaving the
space _l_ between them. Only in fossils; _Trigonocarpus_ (see p. 122)
is similarly organized. Small spores in fern-like sporangia, called
pollen grains.]
[Illustration: Fig. 56.—“Seed”
Very similarly organized to the above, but the coat is joined to the
nucellus about two-thirds of its extent, and up to the level _l_. In
the pollen chamber, _p c_, a cone of nucellar tissue projects, and the
upper part of the coat is fluted, but these complexities are not of
primary importance. The large spore S germinated and was fertilized
within the “seed”, but apparently produced no embryo before it ripened.
Small “spores” in fern-like sporangia form the pollen grains. Only in
fossils, _e.g._ Lagenostoma. (See p. 119.)]
[Illustration: Fig. 57.—Seed
Essentially similar to the preceding, except in the possession of an
embryo _e_, which is, however, small in comparison with the endosperm
which fills the spore S. The whole organization is simpler than in the
fossil _Lagenostoma_, but the coat is fused to the nucellus further up
(see _l_). Small “spores” form the pollen grains. Living and fossil
type, Cycads and Ginkgo.]
[Illustration: Fig. 58.—Seed
In the ripe seed the large embryo _e_ practically fills up all the
space within the two seed coats _c^1_ and _c^2_; endosperm, pollen
chamber, &c., have been eliminated, and the young ovule is very simple
and small as a result of the protection and active service of the
carpels in which it is enclosed. Small “spores” form the pollen grains.
Typical of living Dicotyledons.]
These few illustrations represent only the main divisions of an army of
structures with an almost unimaginable wealth of variety which must be
left out of consideration.
For the structures illustrated in figs. 54, 55, and 56 we have no name,
for their possible existence was not conceived of when our terminology
was invented, and no one has yet christened them anew with distinct
names. They are evidently too complex in organization and too similar to
seeds in several ways to be called spores, yet they lack the essential
element in a seed, namely, an embryo. The term “ovule” (usually given to
the young seed which has not yet developed an embryo) does not fit them
any better, for their tissues are ripened and hard, and they were of
large size and apparently fully grown and mature.
For the present a name is not essential; the one thing that is important
is to recognize their intermediate character and the light they throw on
the possible evolution of modern seeds.
A further point of great interest is the manner in which these “seeds”
were borne on the plant. To-day seeds are always developed (with the
exception of Cycas) in cones or flowers, or at least special
inflorescences. But the “seed” of _Lagenostoma_ (fig. 56), as well as a
number of others in the group it represents, were not borne on a special
structure, but directly on the green foliage leaves. They were in this on
a level with the simple sporangia of ferns which appear on the backs of
the fronds, a fact which is of great significance both for our views on
the evolution of seeds as such, and for the bearing it has on the
relationships of the various groups of allied plants. This will be
referred to subsequently (Chapter XI), and is mentioned now only as an
example of the difference between some of the characters of early fossils
and those of the present day.
It is true that botanists have long recognized the organ which bears
seeds as a modified leaf. The carpels of all the higher plants are looked
on as _homologous_ with leaves, although they do not appear to be like
them externally. Sometimes among living plants curious diseases cause the
carpels to become foliar, and when this happens the diseased carpel
reverts more or less to the supposed ancestral leaf-like condition. It is
only among the ancient (but recently discovered) fossils, however, that
seeds are known to be borne normally on foliage leaves.
From Mesozoic plants we shall learn new conceptions about flowers and
reproductive inflorescences in general, but these must be deferred to the
consideration of the family as a whole (Chapter XIII).
Enough has been illustrated to show that though the individual cells, the
bricks, so to speak, of plant construction, were so similar in the past
and present, yet the organs built up by them have been continually
varying, as a child builds increasingly ambitious palaces with the same
set of bricks.