PAST HISTORIES OF PLANT FAMILIES
II. Higher Gymnosperms
The more recent history of the higher Gymnosperms, in the Upper
Cretaceous and Tertiary periods, much resembles that of the flowering
plants as sketched in the previous chapter. Many of the genera appear to
have been those still living, and some of the species even may have come
very close to or have been identical with those of to-day. The forms now
characteristic of the different continents were growing together, and
appear to have been widely distributed over the globe. For example,
_Sequoia_ and _Taxodium_, two types now characteristic of America, and
_Glyptostrobus_, at present found in Asia, were still growing with the
other European types in Europe so late as middle Tertiary times.
As in the case of the Angiosperms, the fossils we have of Cretaceous and
Tertiary Gymnosperms are nearly all impressions and casts, though some
more or less isolated stems have their structure preserved. Hence our
knowledge of these later Gymnosperms is far from complete. From the older
rocks, however, we have both impressions and microscopically preserved
material, and are more fully acquainted with them than with those which
lived nearer our own time. Hard, resistant leaves, which are so
characteristic of most of the living genera of Gymnosperms, seem to have
been also developed in the past members of the group, and these tend to
leave clear impressions in the rocks, so that we have reliable data for
reconstructing the external appearance of the fossil forms from the
Palæozoic period.
The resinous character of Gymnosperm wood probably greatly assisted its
preservation, and fragments of it are very common in rocks of all ages,
generally preserved in silica so as to show microscopic structure. The
isolated wood of Gymnosperms, however, is not very instructive, for from
the wood alone (and usually it is just fragments of the secondary wood
which are preserved) but little of either physiological or evolutional
value can be learned. When twigs with primary tissues and bark and leaves
attached are preserved, then the specimens are of importance, for their
true character can be recognized. Fortunately among the coal balls there
are many such fragments, some of which are accompanied by fruits and male
cones, so that we know much of the Palæozoic Gymnosperms, and find that
in some respects they differ widely from those now living.
There is, therefore, much more to be said about the fossil Gymnosperms
than about the Angiosperms, both because of the better quality of their
preservation and because their history dates back to a very much earlier
period than does the Angiospermic record. Indeed, we do not know when the
Gymnosperms began; the well-developed and ancient group of _Cordaiteæ_
was flourishing before the Carboniferous period, and must therefore date
back to the rocks of which we have no reliable information from this
point of view, and the origin of the Gymnosperms must lie in the
pre-Carboniferous period.
The group of Gymnosperms includes a number of genera of different types,
most of which may be arranged under seven principal families. In a sketch
of this nature it is, of course, quite impossible to deal with all the
less-important families and genera. Those that will be considered here
are the following:—
Coniferales (see p. 90).
_Araucareæ_, _e.g._ Monkey-puzzle
Genera both living and fossil.
Fossil forms undoubted so far back as the Jurassic, and
presumably further.
_Abietineæ_, _e.g._ Pine and Larch
Genera both living and fossil.
Fossils recognized as far back as the Lower Cretaceous.
_Cupresseæ_, _e.g._ Juniper, Cypress
Genera both living and fossil.
Fossils recognized as far back as the Jurassic.
_Taxeæ_, _e.g._ Yew
Genera living and fossil.
Fossils recognized as far back as the Cretaceous.
Cordaitales (see p. 92).
_Cordaiteæ_, _e.g._ Cordaites
Fossil only.
Characteristic of Devonian, Carboniferous, and Permian periods.
_Poroxyleæ_, _e.g._ Poroxylon
Fossil only.
Characteristic of the Carboniferous and Permian.
Ginkgoales (see p. 98).
_Ginkgoaceæ_, _e.g._ Ginkgo
Fossil and living, dating back, apparently with little change,
to Palæozoic times.
We must pay the most attention to the two last groups, as they are so
important as fossils, and the _Cordaiteæ_ were a very numerous family in
Coal Measure times. They had their period of principal development so
long ago that it is probable that no direct descendants remain to the
present time, though some botanists consider that the _Taxeæ_ are allied
to them.
Of the groups still living it is difficult, almost impossible, to say
which is the highest, the most evolved type. In the consideration of the
Gymnosperm family it is brought home with great emphasis how incomplete
and partial our knowledge is as yet. Many hold that the _Araucareæ_ are
the most primitive of the higher Gymnosperms. In support of this view the
following facts are noted. They have a simple type of fructification,
with a single seed on a simple scale, and many scales arranged round an
axis to form a cone. In the microscopic structure of their wood they have
double rows of bordered pits, a kind of wood cell which comes closer to
the old fossil types than does the wood of any of the other living
genera. Further than this, wood which is almost indistinguishable from
the wood of recent Araucarias is found very far back in the rocks, while
their leaves are broad and simple, and attached directly to the stem in a
way similar to the leaves of the fossil _Cordaiteæ_, and very different
from the needle leaves on the secondary stems of the Pine family; so that
there appears good ground for considering the group an ancient and
probably a primitive one.[10]
On the other hand, there are not wanting scientists who consider the
_Abietineæ_ the living representatives of the most primitive and ancient
stock, though on the whole the evidence seems to indicate more clearly
that the Pine-tree group is specialized and highly modified. Their double
series of foliage leaves, their complex cones (whose structures are not
yet fully understood), and their wood all support the latter view.
Some, again, consider the _Taxeæ_ as a very primitive group, and would
place them near the Cordaiteæ, with which they may be related. Their
fleshy seeds, growing not in cones but on short special axes, support
this view, and it is certainly true that in many ways the large seeds,
with their succulent coats and big endosperm, are much like those of the
lower Gymnosperms and of several fossil types. Those, however, who hold
to the view that the Abietineæ are primitive, see in the _Taxeæ_ the
latest and most modified type of Gymnosperm.
It will be seen from this that there is no lack of variety regarding the
interpretation of Gymnosperm structures.
The Gymnosperms do not stand in such an isolated position as do the
Angiosperms. Whatever the variety of views held about the details of the
relative placing of the families within the group, all agree in
recognizing the evidence which enables us to trace with confidence the
connection between the lower Gymnosperms and the families of ferns. There
are many indications of the intimate connection between higher and lower
Gymnosperms. Between the series exist what might be described as
different degrees of cousinship, and in the lower groups lie unmistakable
clues to their connection with more ancient groups in the past which
bridge over the gaps between them and the ferns.
For the present, however, let us confine ourselves to the history of the
more important Gymnosperms, the discussion of their origin and the groups
from which they may have arisen must be postponed until the necessary
details about those groups have been mentioned.
To a consideration of the living families of _Araucareæ_, _Abietineæ_,
_Cupresseæ_, and _Taxeæ_ we can allow but a short space; their general
characters and appearance are likely to be known to the reader, and their
details can be studied from living specimens if they are not. For
purposes of comparison with the fossils, however, it will be necessary to
mention a few of the principal features which are of special importance
in discussing phylogeny.
The Araucariaceæ are woody trees which attain a considerable size, with
broad-based, large leaves attached directly to the stem. In the leaves
are a series of numerous parallel vascular bundles. The wood cells in
microscopic section show two rows or more of round bordered pits. The
cones are very large, but the male and female are different in size and
organization. The female cone is composed of series of simple scales
arranged spirally round the axis, and each scale bears a single seed and
a small ligule.
The pollen grains from the male cone are caught on the ligule and the
pollen tubes enter the micropyle of the ovule, bringing in passive male
cells which may develop in large numbers in each grain. The seeds when
ripe are stony, and some are provided with a wing from part of the tissue
of the scale. In the ripe cones the scales separate from the cone axis.
The Abietineæ are woody trees, some reaching a great height, all with a
strong main stem. The leaves are of two kinds: primary ones borne
directly attached to the stem (as in first-year shoots of the Larch), and
secondary ones borne in tufts of two (in Pine) or a large number (in
older branches of Larch) on special short branches, the primary leaves
only developing as brown scales closely attached to the stems. Leaves
generally very fine and needlelike, and with a central vascular bundle.
The wood in microscopic section shows a single row of round bordered pits
on the narrow tracheæ.
The female cones are large, male and female differing greatly in size and
organization. The female cone, composed of a spiral series of pairs of
scales, which often fuse together as the cone ripens. Each upper scale of
the pair bears two seeds. The pollen grains from the male cone enter the
micropyle of the seed and are caught in the tissue (apex of nucellus)
there; the pollen tubes discharge passive male cells, only two of which
develop in each grain. The seeds when ripe are stony and provided with a
wing from the tissue of the scale on which they were borne.
The Cupresseæ are woody trees reaching no great height, and of a bushy,
branching growth. The leaves are attached directly to the main stem, and
arrange themselves in alternating pairs of very small leaves, closely
pressed to the stem. The wood in microscopic section shows a single row
of round bordered pits on the tracheæ.
The cones are small, and the scales forming them arranged in cycles. The
female scales bear a varying number of seeds. The pollen grain has two
passive male cells. The seeds when ripe are stony, with wings, though in
some cases (species of Juniper) the cone scales close up and become
fleshy, so that the whole fruit resembles a berry.
The Taxeæ are woody, though not great trees, bushily branched. The leaves
are attached spirally all round the stem, but place themselves so as to
appear to lie in pairs arranged in one horizontal direction. The wood in
microscopic section shows a single row of round bordered pits on the
tracheæ.
There are small male cones, but the seeds are not borne on cones, growing
instead on special short axes, where there may be several young ovules,
but on which usually two seeds ripen. The seeds are big, and have an
inner stone and outer fleshy covering. Some have special outer fleshy
structures known as “arils”, _e.g._ the red outer cup round the yew
“berry” (which is not a berry at all, but a single unenclosed seed with a
fleshy coat).
When we turn to the Cordaiteæ we come to a group of plants which bears
distinct relationship to the preceding, but which has a number of
individual characters. It is a group of which we should know nothing were
it not for the fossils preserved in the Palæozoic rocks; yet,
notwithstanding the fact that it flourished so long ago, it is a family
of which we know much. At the time of the Coal Measures and the
succeeding Permo-carboniferous period, it was of great importance, and,
indeed, in some of the French deposits it would seem as though whole
layers of coal were composed entirely of its leaves.
Among the fossil remains of this family there are impressions, casts, and
true petrifactions, so that we know both its external appearance and the
internal anatomy of nearly every part of several species of the genus.
For a long time the various fossil remains of the plant were not
recognized as belonging to each other and together forming the records of
one and the same plant—the broad, long leaves with their parallel veins
were looked on as Monocotyledons (see fig. 61); the pith casts (see fig.
63) were thought to be peculiar constricted stems, and were called
_Sternbergia_; while the wood, which was known from its microscopic
structure, was called _Araucarioxylon_—but the careful work of many
masters of fossil botany, whose laborious studies we cannot describe in
detail here, brought all these fragments together and proved them to
belong to _Cordaites_.
[Illustration: Fig. 61.—Leaf of _Cordaites_, _l_, attached by its broad
base to a Stem, _s_]
We now know that _Cordaites_ were large trees, with strong upright shafts
of wood, to whose branches large simple leaves were attached. The leaves
were much bigger than those of any living Gymnosperm, even than those of
the Kauri Pine (a member of the Araucariaceæ), and seem in some species
to have exceeded 3 ft. in length. The trees branched only at the top of
the main shaft, and with their huge sword-like leaves must have differed
greatly in appearance from any plant now living. The leaves had many
parallel veins, as can be seen in fig. 61, and were attached by a broad
base directly to the main stem; thus coming closer to the Araucarias than
the other groups of Gymnosperms in their leaf characters.
[Illustration: Fig. 62A.—Microscopic Section of Part of a Leaf of
_Cordaites_
V, Vascular bundle; W, wood of bundle; _sh_, its sheath; S^1, large
sclerenchyma mass alternating with bundles; S^2 and S^3, sclerenchyma
caps of bundle; P, soft tissue of leaf.]
The internal anatomy is often well preserved, and there is a number of
species of leaves whose anatomy is known. As will be expected from the
parallel veins, in each section there are many vascular bundles running
equidistantly through the tissue. Fig. 62A shows the microscopic details
from a well-preserved leaf. In all the species patches of sclerenchyma
were developed, and everything indicates that they were tough and well
protected against loss of water, even to a greater extent than are most
of the leaves of living Gymnosperms.
In the stems the pith was much larger than that in living Gymnosperms
(where the wood is generally very solid), and it was hollow in older
stems, except for discs of tissue across the cavity. The internal cast
from these stems has been described before, and is seen in fig. 63.
[Illustration: Fig. 62B.—Much-magnified Wood Elements from _Cordaites_
Stem seen in longitudinal section, the type known as _Araucarioxylon_.
Note the hexagonal outlines of the bordered pits, which lie in several
rows]
The wood was formed in closely packed radiating rows by a normal cambium
(see p. 66), and the tracheæ so formed had characteristic rows of
bordered pits (see fig. 62B). The wood comes nearer to that of the living
Araucarias than any other, and indeed the numerous pieces of fossil wood
of this type which are known from all the geological periods are called
_Araucarioxylon_.[11] A double strand goes out from the main mass of
wood, which afterwards divides and subdivides to provide the numerous
bundles of the leaf.
[Illustration: Fig. 63.—Cast of Hollow Pith of _Cordaites_, the
constrictions corresponding to discs of solid tissue across the cavity]
In the case of these fossils we are fortunate enough to have the
fructifications, both male and female, in a good state of preservation.
As in other Gymnosperms, the male and female cones are separate, but they
differed less from each other in their arrangement than do those of any
of the living types hitherto mentioned. They can hardly be described as
true cones, though they had something of that nature; the seeds seem to
be borne on special short stems, round which are also sterile scales. In
the seed and the way it is borne perhaps the Cordaiteæ may be compared
more nearly with the Taxeæ than with the other groups. A seed, not yet
ripe, is shown in slightly diagrammatic form in fig. 64, where the
essential details are illustrated. The seeds of this family sometimes
reached a considerable size, and had a fleshy layer which was thick in
comparison with the stone, and externally comparable with a
cherry—though, of course, of very different nature in reality, for
_Cordaites_, like _Taxus_, is a Gymnosperm, with simple naked seeds,
while a cherry is the fruit of an Angiosperm.
In a few words, these are the main characters of the large group of
_Cordaites_, which held the dominant position among Gymnosperms in the
Palæozoic era. They have relationships, or perhaps one should say
likenesses, to many groups. Their stem- and root-anatomy is similar to
the Coniferæ of the present day, the position of the ovules is like that
in the Taxaceæ, the male cones in some measure recall those of _Ginkgo_,
the anatomy of their leaves has points which are comparable with those of
the Cycads, to which group also the large pith in the stem and the
structure of some details in the seeds unite them. Their own specially
distinctive characters lie in their crown of huge leaves, and unbranched
shaft of stem, the similarity of their male and female inflorescences,
and some points in their pollen grains which have not been mentioned. The
type is a very complex one, possibly coming near the stock which, having
branched out in various directions, gave rise to several of the living
families.
[Illustration: Fig. 64.—Representation of _Cordaites_ Seed and its Axis
with Scales, slightly diagrammatic, modified from Renault.
A, Axis with _s_, scales; _c_, coat of the seed, from which the inner
parts have shrunk away; _n_, nucellus; _p.c_, pollen chamber containing
pollen grains which enter through _m_.]
Plants which come very near to the Cordaiteæ are the Poroxyleæ. Of this
group we have unfortunately no remains of fructifications in organic
connection, so that its actual position must remain a little doubtful
till they are discovered. There seems no doubt that they must have borne
seeds.
Still, it has been abundantly demonstrated in recent years that the
anatomy of the root, stem, and leaves indicates with considerable
exactness the position of any plant, so that, as these are known, we can
deduce from them, with a feeling of safety, the position that _Poroxylon_
takes in the natural system. In its anatomy the characters are those of
the Cordaiteæ, with certain details which show a more primitive nature
and seem to be characteristic of the groups below it in organization.
_Poroxylon_ is not common, and until recently had not been found in the
Lower Coal Measures of England. The plants appear to have been much
smaller than _Cordaites_, with delicate stems which bore relatively large
simple leaves. The anatomy of the root was that common in Gymnosperms,
but the stem had a very large pith, and the leaves were much like those
of _Cordaites_ in having parallel veins. An important character in the
anatomy of the stem was the presence of what is known as _centripetal
wood_. This must be shortly explained. In all the stems hitherto
considered, the first-formed wood cells (protoxylems, see p. 57)
developed at the central point of the wood, towards the pith (see fig.
19, _px_, p. 49). This is characteristic of all Angiosperms and the
higher Gymnosperms (except in a couple of recently investigated Pines),
but among the lower plants we find that part of the later wood develops
to the inner side of these protoxylem masses. The distinction is shown in
fig. 65.
[Illustration: Fig. 65.—A, Normal bundle of higher plant; _x_,
protoxylem on inner side next the pith _p_, and the older wood _w_
outside it, _centrifugal_ wood. B, Bundle with wood cells _c_ developed
on inner side of protoxylem, _centripetal_ wood; the arrow indicates
the direction of the centre of the stem.]
This point is one to which botanists have given much attention, and on
which they have laid much weight in considering the affinities of the
lower Gymnosperms and the intermediate groups between them and the ferns,
which are found among the fossils. In _Cordaites_ this point of
connection with the lower types is not seen, but in _Poroxylon_, which
has otherwise a stem anatomy very similar to _Cordaites_, we find groups
of _centripetal_ wood developed inside the protoxylem of primary bundles.
For this reason, principally, is _Poroxylon_ of interest at present, as
in its stem anatomy it seems to connect the _Cordaites_ type with that of
the group below it in general organization.
Ginkgoales.—Reference to p. 44 shows that _Ginkgo_, the Maidenhair tree,
belongs to the Ginkgoales, a group taking equal rank with the large and
complex series of the Coniferales. The Ginkgoales of the present day,
however, have but one living representative. _Ginkgo_ stands alone, the
single living species of its genus, representing a family so different
from any other living family that it forms a prime group by itself.
Had the tree not been held sacred in China and Japan, it is probable that
it would long since have been extinct, for it is now known only in
cultivation. It is indeed a relic from the past which has been
fortunately preserved alive for our examination. It belongs to the fossil
world, as a belated November rose belongs to the summer.
Because of its beauty and interest the plant is now widely distributed
under cultivation, and is available for study almost as freely as the
other types of living Gymnosperms already mentioned, so that but a short
summary of its more important features is needed here.
Old plants, such as can be seen growing freely in Japan (in Kew Gardens
there is also a fine specimen), are very tall handsome woody trees, with
noble shafts and many branches. The leaves grow on little side shoots and
are the most characteristic external feature of the tree; their living
form is illustrated in fig. 66, which shows the typical simple shape as
well as the lobed form of the leaf which are to be found, with all
intermediate stages, on the same tree. No other plant (save a few ferns,
which can generally be distinguished from it without difficulty) has
leaves at all like these, so that it is particularly easy to identify the
fossil remains, of which there are many.
[Illustration: Fig. 66.—A, Tuft of _Ginkgo_ Leaves, showing their
“maidenhair”-like shape. B, Single deeply-divided Leaf to be found on
the same tree, usually on young branches.]
The wood is compact and fine grained, the rings of secondary tissue being
developed from a normal cambium as in the case of the higher Gymnosperms,
and the individual tracheæ have round bordered pits. There are small male
cones, but the seeds are not borne in cones. They develop on special
stalks on which are no scales, but a small mass of tissue at the base of
the seed called the “collar”. Usually there are two young ovules, of
which often only one ripens to a fleshy seed, though both may mature.
[Illustration: Fig. 67.—Ripe Stage of _Ginkgo_ Seeds attached to their
Stalk. _c_, “Collar” of seed.]
The ripe seed reaches the size shown in the diagram, and is orange
coloured and very fleshy; within it is a stone encasing the endosperm,
which is large, _green_, and starchy, and contains the embryo with two
cotyledons. This embryo is small compared with the endosperm, cf. fig.
57, p. 76, which is somewhat similar to that of _Ginkgo_ in this stage.
Of the microscopic characters of the reproductive organs the most
remarkable is the male cell. This is not a passive nucleus, as in the
plants hitherto considered, but is an _actively swimming_ cell of some
size, provided with a spiral of cilia (hairlike structures) whose
movements propel it through the water. In the cavity of the unripe seed
these swim towards the female cell, and actively penetrate it. The
arrangements of the seed are diagrammatically shown in fig. 68, which
should be compared with that of _Cycas_, fig. 76, with which it has many
points in common.
[Illustration: Fig. 68.—Section through Seed of _Ginkgo_
_p.c_, Pollen chamber in the nucellus _n_, which is fused to the coat
_c_ to the level _l_; _sc_, stony layer in coat; S, the big spore,
filled with endosperm tissue (in this case green in colour); _e_, egg
cells, one of which will produce the embryo after fertilization.]
The nature of the male cell in _Cordaites_ is not yet known, but there is
reason to suspect it may have been actively swimming also. As this is
uncertain, however, we may consider _Ginkgo_ the most highly organized
plant which has such a primitive feature, a feature which is a bond of
union between it and the ferns, and which, when it was discovered about a
dozen years ago, caused a considerable sensation in the botanical world.
To turn now to the fossil records of this family. Leaf impressions of
_Ginkgo_ are found in rocks of nearly all ages back even to the Upper
Palæozoic. They show a considerable variety of form, and it is certain
that they do not all belong to the same _species_ as the living plant,
but probably they are closely allied. Fig. 69 shows a typical impression
from the Lower Mesozoic rocks. In this specimen, the cells of the
epidermis were fortunately sufficiently well preserved to be seen with
the microscope, and there is a distinct difference in the size and shape
of the cells of living and fossil species, see fig. 70; but this
difference is slight as compared with the great similarity of form and
appearance, as can be seen on comparing figs. 69 and 66, B, so that the
fossil is at the most a different species of the genus _Ginkgo_. Among
the fossil leaves there is greater variety than among the living ones,
and some which are very deeply lobed so as to form a divided palm-like
leaf go by different names, e.g. _Baiera_, but they are supposed to
belong to the same family. Fossil seeds and male cones are also known as
impressions, and are found far back in the Mesozoic rocks. From the
fossil impressions it is certain that _Ginkgo_ and plants closely allied
to it were very widespread in the past, as they are found all over Europe
as well as the other continents. Particularly in the Lower Mesozoic rocks
_Ginkgo_ seems to have been a world-wide type growing in great abundance.
[Illustration: Fig. 69.—Leaf Impression of _Ginkgo_ from Mesozoic Rocks
of Scotland]
[Illustration: Fig. 70.—Showing Epidermis with Stomates from the lower
side of the Leaf seen in fig. 69
_e_, Epidermis cells; _s_, stomates; _v_, long cells of epidermis lying
over the veins.]
In the Palæozoic the records are not so undoubted, but there is strong
evidence which leads us to suppose that if the genus now living were not
then extant, at least other closely related genera were, and there seems
to be good grounds for supposing that _Ginkgo_ and _Cordaites_ may have
both arisen from some ancient common stock.