MINUTE STRUCTURE OF FOSSIL PLANTS—LIKENESSES TO LIVING ONES
The individual plants of the Coal Measure period differed entirely from
those now living; they were more than merely distinct species, for in the
main even the families were largely different from the present ones.
Nevertheless, when we come to examine the minute anatomy of the fossils,
and the cells of which they are composed, we find that between the living
and the fossil cell types the closest similarity exists.
From the earliest times of which we have any knowledge the elements of
the plant body have been the same, though the types of structures which
they built have varied in plan. Individual _cells_ of nearly every type
from the Coal Measure period can be identically matched with those of
to-day. In the way the walls thickened, in the shapes of the wood,
strengthening or epidermal cells, in the form of the various tissues
adapted to specific purposes, there is a unity of organization which it
is reasonable to suppose depends on the fundamental qualities inherent in
plant life.
This will be illustrated best, perhaps, by tabulating the chief
modifications of cells which are found in plant tissues. The
illustrations of these types in the following table are taken from living
plants, because from them figures of more diagrammatic clearness can be
made, and the salient characters of the cells more easily recognized.
Comparison of these typical cells with those illustrated from the fossil
plants reveals their identity in essential structure, and most of them
will be found in the photos of fossils in these pages, though they are
better recognized in the actual fossils themselves.
Principal Types of Plant Cells
Epidermal.
[Illustration: Fig. 21
_Epidermis._—Protecting layer or skin. Cells with outer wall thickened
in many cases (fig. 21, _a_ and _b_). Compare fossil epidermis in fig.
34, _e_.]
[Illustration: Fig. 22 _Hairs._—Extensions of epidermis cells. Single
cells, or complex, as fig. 22, _h_, where _e_ is epidermis and _p_
parenchyma. Compare fossil hairs in figs. 79 and 120.]
[Illustration: Fig. 23
_Stomates._—Breathing pores in the epidermis. Seen in surface view as
two-lipped structures (fig. 23). _s_, Stomates; _e_, epidermis cells.
Compare fossil stomates in fig. 8.]
Ground Tissue
[Illustration: Fig. 24
_Parenchyma._—Simple soft cells, either closely packed, as in fig. 24,
or with air spaces between them. Compare 78, B, for fossil.]
[Illustration: Fig. 25
_Palisade._—Elongated, closely packed cells, _p_, chiefly in leaves,
lying below the epidermis, _e_, fig. 25. Compare fig. 34, _p_, for
fossil palisade.]
[Illustration: Fig. 26
_Endodermis._—Cells with specially thickened walls, _en_, lying as
sheath between the parenchyma, _c_, of ground tissue, and the vascular
tissue, _s_, fig. 26. Compare fig. 108 for fossil endodermis.]
[Illustration: Fig. 27
_Latex cells._—Large, often much elongated cells, _m_, lying in the
parenchyma, _p_, fig. 27, which are packed with contents. Compare fig.
107, _s_.]
[Illustration: Fig. 28
_Sclerenchyma._—Thick-walled cells among parenchyma for strengthening,
fig. 28. Compare fig. 34, _s_.]
[Illustration: Fig. 29
_Cork._—Layers of cells replacing the epidermis in old stems. Outer
cells, _o_, crushed; _k_, closely packed cork cells; stone cells, _s_,
fig. 29. Compare fig. 95, _k_.
_Cork cambium._—Narrow, actively dividing cells, _c_ in fig. 29, giving
rise to new cork cells in consecutive rows.]
[Illustration: Fig. 30
_Tracheides._—Specially thickened cells in the parenchyma, usually for
water storage, _t_, fig. 30. Compare fig. 95, _t_.]
Vascular Tissue
[Illustration: Fig. 31
_Wood._—_Protoxylem_, tracheids and vessels, long, narrow elements,
with spiral or ring-like thickenings, _s_^1 and _s_^2, fig. 31. Compare
fig. 81, A, _px_, for fossil.
_Metaxylem_, long elements, tracheids and vessels. Some with narrow
pits, as _t_ in fig. 31; others with various kinds of pits. In
transverse section seen in fig. 33, w, fossil in fig. 114, _w_.
_Wood parenchyma._—Soft cells associated with the wood, _p_ in fig. 31.
Fossil in fig. 81, B, _p_.
_Wood sclerenchyma._—Hard thickened cells in the wood.]
[Illustration: Fig. 32
_Bast._—_Sieve tubes_, long cells which carry foodstuffs, cross walls
pitted like sieves, _s_, fig. 32. In transverse section in fig. 33.
_Companion cells_, narrow cells with rich proteid contents, _c_, fig.
32. In transverse section at _c_, fig. 33.
_Bast parenchyma._—Soft unspecialized cells mixed with the sieve tubes,
_p_, fig. 32.
_Bast fibres._—Thick-walled sclerenchymatous cells mixed with, or
outside, the soft bast.]
[Illustration: Fig. 33
_Cambium._—Narrow cells, like those of the cork cambium, which lie
between the wood and bast, and give rise to new tissues of each kind,
_cb_, fig. 33. Compare fig. 114, fossil.]
There are, of course, many minor varieties of cells, but these illustrate
all the main types.
Among the early fossils, however, one type of wood cell and one type of
bast cell, so far as we know, are not present. These cells are the true
_vessels_ of the wood of flowering plants, and the long bast cells with
their companion proteid cells. The figure of a metaxylem wood cell, shown
in fig. 31, _t_, shows the more primitive type of wood cell, which has an
oblique cross wall. This type of wood cell is found in all the fossil
trees, and all the living plants except the flowering plants. The vessel
type, which is that in the big wood vessels of the flowering plants, and
has no cross wall, is seen in fig. 20, _x_.
The similarity between the living cells and those of the Coal Measure
fossils is sufficiently illustrated to need no further comment. This
similarity is an extremely helpful point when we come to an
interpretation of the fossils. In living plants we can study the
physiology of the various kinds of cells, and can deduce from experiment
exactly the part they play in the economy of the whole plant. From a
study of the tissues in any plant structure we know what function it
performed, and can very often estimate the nature of the surrounding
conditions under which the plant was growing. To take a single example,
the palisade tissue, illustrated in fig. 25, _p_, in living plants always
contains green colouring matter, and lies just below the epidermis,
usually of leaves, but sometimes also of green stems. These cells do most
of the starch manufacture for the plant, and are found best developed
when exposed to a good light. In very shady places the leaves seldom have
this type of cell. Now, when cells just like these are found in fossils
(as is illustrated in fig. 34), we can assume all the physiological facts
mentioned above, and rest assured that that leaf was growing under normal
conditions of light and was actively engaged in starch-building when it
was alive. From the physiological standpoint the fossil leaf is entirely
the same as a normal living one.
[Illustration: Fig. 34.—From a Photo of a Fossil Lea
_e_, Epidermis; _p_, palisade cells; _pr_, soft parenchyma cells
(poorly preserved); _s_, sclerenchyma above the vascular bundle.]
From the morphological standpoint, also, the features of the plant body
from the Coal Measure period fall into the same divisions as those of the
present. Roots, stems, leaves, and reproductive organs, the essentially
distinct parts of a plant, are to be found in a form entirely
recognizable, or sufficiently like that now in vogue to be interpreted
without great difficulty. In the detailed structure of the reproductive
organs more changes have taken place than in any others, both in internal
organization and external appearance.
Already, in the Early Palæozoic period, the distinction between leaves,
stems, roots, and reproductive organs was as clearly marked as it is
to-day, and, judging by their structure, they must each have performed
the physiological functions they now do. Roots have changed least in the
course of time, probably because, in the earth, they live under
comparatively uniform conditions in whatever period of the world’s
history they are growing. Naturally, between the roots of different
species there are slight differences; but the likeness between fern roots
from the Palæozoic and from a living fern is absolutely complete. This is
illustrated in fig. 35, which shows the microscopic structure of the two
roots when cut in transverse direction. The various tissues will be
recognized as coming into the table on p. 54, so that both in the details
of individual cells and in the general arrangement of the cell groups or
tissues the roots of these fossil and living ferns agree.
[Illustration: Fig. 35.—A, Root of Living Fern. B, Root of Palæozoic
Fossil Fern. _c_, Cortex; _px_, protoxylem in two groups; _m_,
metaxylem; _s_, space in fossil due to decay of soft cells.]
Among stems there has been at all periods more variety than among the
roots of the corresponding plants, and in the following chapter, when the
differences between living and fossil plants will be considered, there
will be several important structures to notice. Nevertheless, there are
very many characters in which the stems from such widely different epochs
agree. The plants in the palæozoic forests were of many kinds, and among
them were those with weak trailing stems which climbed over and supported
themselves on other plants, and also tall, sturdy shafts of woody trees,
many of which were covered with a corky bark. Leaves were attached to the
stems, either directly, as in the case of some living plants, or by leaf
stalks. In external appearance and in general function the stems then
were as stems are now. In the details of the individual cells also the
likeness is complete; it is in the grouping of the cells, the anatomy of
the tissues, that the important differences lie. It has been remarked
already that increase in complexity of the plant form usually goes with
an increase in complexity of the cells and variety of the tissues. The
general ground tissue in nearly all plants is very similar; it is
principally in the vascular system that the advance and variety lie.
Plant anatomists lay particular stress on the vascular system, which, in
comparison with animal anatomy, holds an even more important position
than does the skeleton. To understand the essential characters of stems,
both living and fossil, and to appreciate their points of likeness or
difference, it is necessary to have some knowledge of the general facts
of anatomy; hence the main points on which stress is laid will be given
now in brief outline.
Leaving aside consideration of the more rudimentary and less defined
structure of the algæ and mosses, all plants may be said to possess a
“vascular system”. This is typically composed of elongated wood (or
xylem) with accessory cells (see p. 57, table), and bast (phloem), also
with accessory cells. These specialized conducting elements lie in the
ground tissue, and in nearly all cases are cut off from direct contact
with it by a definite sheath, called the endodermis (see p. 55, fig. 26).
Very often there are also groups or rings of hard thick-walled cells
associated with the vascular tissues, which protect them and play an
important part in the consolidation of the whole stem.
[Illustration: Fig. 36.—Diagram of Simplest Arrangement of Complete
Stele in a Stem
W, Central solid wood; P, ring of bast; E, enclosing sheath of
endodermis; C, ground tissue or cortex.]
The simplest, and probably evolutionally the most primitive form which is
taken by the vascular tissues, is that of a single central strand, with
the wood in the middle, the bast round it, and a circular endodermis
enclosing all, as in fig. 36, which shows a diagram of this arrangement.
Such a mass of wood and bast surrounded by an endodermis, is technically
known as a _stele_, a very convenient term which is much used by
anatomists. In its simplest form (as in fig. 36) it is called a
_protostele_, and is to be found in both living and fossil plants. A
number of plants which get more complex steles later on, have protosteles
in the early stages of their development, as in _Pteris aurita_ for
example, a species allied to the bracken fern, which has a hollow ring
stele when mature.
[Illustration: Fig. 37.—Diagram of a Stele with a few Cells of Pith _p_
in the Middle of the Wood. Lettering as in fig. 36]
[Illustration: Fig. 38.—Diagram showing Extensive Pith _p_ in the Wood.
Lettering as in fig. 36]
The next type of stele is quite similar to the protostele, but with the
addition of a few large unspecialized cells in the middle of the wood
(_p_, fig. 37); these are the commencement of the hollowing process which
goes on in the wood, resulting later in the formation of a considerable
pith, as is seen in fig. 38, where the wood is now a hollow cylinder, as
the phloem has been from the first. When this is the case, a second
sheath or endodermis generally develops on the inner side of the wood,
outside the pith, and cuts the vascular tissues off from the inner
parenchyma. A further step is the development of an inner cylinder of
bast so that the vascular ring is completely double, with endodermis on
both sides of the cylinder, as is seen in fig. 39.
[Illustration: Fig. 39.—A Cylindrical Stele, with _e_, inner
endodermis, and _ph_, inner phloem; W, wood; P, outer phloem; E, outer
endodermis. L, part of the stele going out to supply a large leaf, thus
breaking what would otherwise appear as a closed ring stele]
In all these cases there is but one strand or cylinder, of vascular
tissue in the stem, but one stele, and this type of anatomy is known as
the _monostelic_ or single-steled type.
[Illustration: Fig. 40.—A Ring Stele apparently broken up into a Number
of Protosteles by many Leaf Gaps]
When from the double cylinder just described a strand of tissue goes off
to supply a large leaf, a considerable part of the stele goes out and
breaks the ring. This is shown in fig. 39, where L is the part of the
stele going to a leaf, and the rest the broken central cylinder. When the
stem is short, and leaves grow thickly so that bundles are constantly
going out from the main cylinder, this gets permanently broken, and its
appearance when cut across at any given point is that of a group of
several steles arranged in a ring, each separate stele being like the
simple protostele in its structure. See fig. 40. This type of stem has
long been known as _polystelic_ (_i.e._ many-steled), and it is still a
convenient term to describe it by. There has been much theoretical
discussion about the true meaning of such a “polystelic” stem, which
cannot be entered into here; it may be noted, however, that the various
strands of the broken ring join up and form a meshwork when we consider
the stem as a whole, it is only in a single section that they appear as
quite independent protosteles. Nevertheless, as we generally consider the
anatomy of stems in terms of single sections, and as the descriptive word
“polystelic” is a very convenient and widely understood term, it will be
used throughout the book when speaking of this type of stem anatomy.
Such a type as this, shown in fig. 40, is already complex, but it often
happens that the steles branch and divide still further, until there is a
highly complicated and sometimes bewildering system of vascular strands
running through the ground tissue in many directions, but cut off from it
by their protective endodermal sheaths. Such complex systems are to be
found both in living and fossil plants, more especially in many of the
larger ferns (see fig. 88).
Higher plants in general, however, and in particular flowering plants, do
not have a polystelic vascular arrangement, but a specialized type of
monostele.
[Illustration: Fig. 41.—Monostele in which the Central Pith is
Star-shaped, and the Wood breaking up into Separate Groups
_p_, Pith; W, wood; P, phloem; E, endodermis; C, cortex.]
Referring again to fig. 37 as a starting-point, imagine the pith in the
centre to spread in a star-shaped form till the points of the star
touched the edges of the ring, and thus to break the wood ring into
groups. A stage in this process (which is not yet completed) is shown in
fig. 41, while in fig. 42 the wood and bast groups are entirely distinct.
In the flowering plants the cells of the endodermis are frequently poorly
characterized, and the pith cells resemble those of the cortical ground
tissue, so that the separate groups of wood and bast (usually known as
“vascular bundles”, in distinction from the “steles” of fig. 40) appear
to lie independently in the ground tissue. These strands, however, must
not be confused with steles, they are only fragments of the single
apparently broken up stele which runs in the stem.
[Illustration: Fig. 42.—Monostele in which the Pith has invaded all the
Tissues as far as the Endodermis, and broken the Wood and Phloem up
into Separate Bundles. These are usually called “vascular bundles” in
the flowering plants]
[Illustration: Fig. 43.—Showing actively growing Zone _c_ (Cambium) in
the Vascular Bundles, and joining across the ground tissue between them]
The vascular bundle, of all except the Monocotyledons, has a potentiality
for continued growth and expansion which places it far above the stele in
value for a plant of long life and considerable growth. The cells lying
between the wood and the bast, the soft parenchyma cells always
accompanying such tissues, retain their vitality and continue to divide
with great regularity, and to give rise to a continuous succession of new
cells of wood on the one side and bast on the other; see fig. 33, _c_,
_b_. In this way the primary, distinct vascular bundles are joined by a
ring of wood, see fig. 43, to which are added further rings every season,
till the mass of wood becomes a strong solid shaft. This ever-recurring
activity of the cambium gives rise to what are known as “annual rings” in
stems, see fig. 44, in which the wood shows both primary distinct groups
in the centre, and the rings of growth of later years.
Cambium with this power of long-continued activity is found in nearly all
the higher plants of to-day (except the Monocotyledons), but in the fern
and lycopod groups it is in abeyance. Certain cases from nearly every
family of the Pteridophytes are known, where some slight development of
cambium with its secondary thickening takes place, but in the groups
below the Gymnosperms cambium has almost no part to play. On the other
hand, so far back as the Carboniferous period, the masses of wood in the
Pteridophyte trees were formed by cambium in just the same way as they
are now in the higher forms. Its presence was almost universal at that
time in the lower groups where to-day there are hardly any traces of it
to be found.
[Illustration: Fig. 44.—Stem with Solid Cylinder of Wood developed from
the Cambium, showing three “annual rings”. In the centre may still be
seen the separate groups of the wood of the primary “vascular bundles”]
It will be seen from this short outline of the vascular system of plants,
that there is much variety possible from modifications of the fundamental
protostele. It is also to be noted that the plants of the Coal Measures
had already evolved all the main varieties of steles which are known to
us even now,[6] and that the development of secondary thickening was very
widespread. In several cases the complexity of type exceeds that of
modern plants (see Chap. VII), and there are to be found vascular
arrangements no longer extant.
When we turn to the _Reproductive Organs_, we find that the points of
likeness between the living and the fossil forms are not so numerous or
so direct as they are in the case of the vegetative system.
[Illustration: Fig. 45.—Fern Sporangia
A, fossil; B, living.]
As has been indicated, the families of plants typical of the Coal
Measures were not those which are the most prominent to-day, but belonged
to the lower series of Pteridophytes. In their simpler forms the
fructifications then and now resemble each other very closely, but in the
more elaborate developments the points of variety are more striking, so
that they will be dealt with in the following chapter. Cases of likeness
are seen in the sporangia of ferns, some of which appear to have been
practically identical with those now living. This is illustrated in fig.
45, which shows the outline of the cells of the sporangia of living and
fossil side by side.
[Illustration: Fig. 46.—A, Living Lycopod cone; B, _Lepidodendron_
(fossil) cone. _a_, Axis; _s_, scale; S, sporangium with spores. One
side of a longitudinal section]
In the general structure also of the cones of the simpler types of
_Lepidodendron_ (fossil, see frontispiece) there is a close agreement
with the living Lycopods, though as regards size and output of spores
there was a considerable difference in favour of the fossils. The plan of
each is that round the axis of the cone simple scales are arranged, on
each of which, on its upper side, is seated a large sporangium bearing
numerous spores all of one kind (see fig. 46).
Equally similar are the cones of the living Equisetum and some of the
simple members of the fossil family Calamiteæ, but the more interesting
cases are those where differences of an important morphological nature
are to be seen.
As regards the second[7] generation there is some very important
evidence, from extremely young stages, which has recently been given to
the world. In a fern sporangium _germinating spores_ were fossilized so
as to show the first divisions of the spore cell. These seem to be
identical with the first divisions of some recent ferns (see fig. 47).
This is not only of interest as showing the close similarity in detail
between plants of such widely different ages, but is a remarkable case of
delicate preservation of soft and most perishable structures in the “coal
balls”.
[Illustration: Fig. 47.—Germinating Fern Spores
A and B, from carboniferous fossils; C, living fern. (A and B after
Scott.)]
While these few cases illustrate points of likeness between the
fructifications of the Coal Measures and of to-day, the large size and
successful character of the primitive Coal Measure plants was accompanied
by many developments on the part of their reproductive organs which are
no longer seen in living forms, and the greater number of palæozoic
fructifications must be considered in the next chapter.