PAST HISTORIES OF PLANT FAMILIES
I. Flowering Plants, Angiosperms
In comparison with the other groups of plants the flowering families are
of recent origin, yet in the sense in which the word is usually used they
are ancient indeed, and the earliest records of them must date at least
to periods hundreds of thousands of years ago.
Through all the Tertiary period (see p. 34) there were numerous flowering
plants, and there is evidence that many families of both Monocotyledons
and Dicotyledons existed in the Upper Cretaceous times. Further back than
this we have little reliable testimony, for the few specimens of
so-called flowering plants from the Lower Mesozoic are for the most part
of a doubtful nature.
The flowering plants seem to stand much isolated from the rest of the
plant world; there is no _direct_ evidence of connection between their
oldest representatives and any of the more primitive families. So far as
our actual knowledge goes, they might have sprung into being at the
middle of the Mesozoic period quite independently of the other plants
then living; though there are not wanting elaborate and almost convincing
theories of their connection with more than one group of their
predecessors (see p. 108).
It is a peculiarly unfortunate fact that although the rocks of the
Cretaceous and Tertiary are so much less ancient than those of the Coal
Measures, they have preserved for us far less well the plants which were
living when they were formed. Hitherto no one has found in Mesozoic
strata masses of exquisitely mineralized Angiosperm fragments[8] like
those found in the Coal Measures, which tell us so much about the more
ancient plants. Cases are known of more or less isolated fragments with
their microscopical tissues mineralized. For example, there are some
palms and ferns from South America which show their anatomical structure
very clearly preserved in silica, and which seem to resemble closely the
living species of their genera. The bulk of the plants preserved from
these periods are found in the form of casts or impressions (see p. 10),
which, as has been pointed out already, are much less satisfactory to
deal with, and give much less reliable results than specimens which have
also their internal structure petrified. The quantity of material,
however, is great, and impressions of single leaves innumerable, and of
specimens of leaves attached to stems, and even of flowers and fruits,
are to be found in the later beds of rock. These are generally clearly
recognizable as belonging to one or other of the living families of
flowering plants. Leaf impressions are by far the most frequent, and our
knowledge of the Tertiary flora is principally derived from a study of
them. Their outline and their veins are generally preserved, often also
their petioles and some indication of the thickness and character of the
fleshy part of the leaf. From the outline and veins alone an expert is
generally able to determine the species to which the plant belongs,
though it is not always quite safe to trust to these determinations or to
draw wide-reaching conclusions from them.
In fig. 59 is shown a photograph of the impression of a Tertiary leaf,
which illustrates the condition of an average good specimen from rocks of
the period. Its shape and the character of the veins are sufficient to
mark it out immediately as belonging to the Dicotyledonous group of the
flowering plants.
Seeds and fruits are also to be found; and in some very finely preserved
specimens from Japan stamens from a flower and delicate seeds are seen
clearly impressed on the light stone. In fig. 60 is illustrated a couple
of such seeds, which show not only their wings but also the small
antennæ-like stigmas. Specimens so perfectly preserved are practically as
good as herbarium material of recent plants, and in this way the
externals of the Tertiary plants are pretty well known to us.
[Illustration: Fig. 59.—Dicotyledonous Leaf Impression from Tertiary
Rocks]
[Illustration: Fig. 60.—Seeds from Japanese Tertiary Rocks; at _a_ are
seen the two stigmas still preserved]
A problem which has long been discussed, and which has aroused much
interest, is the relative antiquity of the Monocotyledonous and the
Dicotyledonous branches of the flowering plants. A peculiar fascination
seems to hang over this still unsolved riddle, and a battle of flowers
may be said to rage between the lily and the rose for priority. Recent
work has thrown no decisive light on the question, but it has undoubtedly
demolished the old view which supposed that the Monocotyledons (the lily
group) appeared at a far earlier date upon this earth than the
Dicotyledons. The old writers based their contention on incorrectly
determined fossils. For instance, seeds from the Palæozoic rocks were
described as Monocotyledons because of the three or six ribs which were
so characteristic of their shell; we know now that these seeds
(_Trigonocarpus_) belong to a family already mentioned in another
connection (p. 72), the Medulloseæ (see p. 122), the affinity of which
lies between the cycads and the ferns. Leaves of _Cordaites_, again,
which are broad and long with well-marked parallel veins, were described
as those of a Monocotyledonous plant like the Yucca of to-day; but we now
know them to belong to a family of true Gymnosperms possibly distantly
related to _Taxus_ (the Yew tree).
Recent work, which has carefully sifted the fossil evidence, can only say
that no true Monocotyledons have yet been found below the Lower
Cretaceous rocks, and that at that period we see also the sudden inrush
of Dicotyledons. Hence, so far as palæontology can show, the two parallel
groups of the flowering plants arose about the same time. It is of
interest to note, however, that the only petrifaction of a flower known
from any part of the world is an ovary which seems to be that of one of
the Liliaceæ. In the same nodules, however, there are several specimens
of Dicotyledonous woods, so that it does not throw any light on the
question of priority.
With the evidence derived from the comparative study of the anatomy of
recent flowering plants we cannot concern ourselves here, beyond noting
that the results weigh in favour of the Dicotyledons as being the more
primitive, though not necessarily developed much earlier in point of
time. Until very much more is discovered than is yet known of the origin
of the flowering plants as a whole, it is impossible to come to a more
definite conclusion about this much-discussed subject.
Let us now attempt to picture the vegetable communities since the
appearance of the flowering plants. The facts which form the bases of the
following conceptions have been gathered from many lands by numerous
workers in the field of fossil botany, from scattered plant remains such
as have been described.
When the flowering plants were heralded in they appeared in large
numbers, and already by the Cretaceous period there were very many
different species. Of these a number seem to belong to genera which are
still living, and many of them are extremely like living species. It
would be wearisome and of little value to give a list of all the recorded
species from this period, but a few of the commoner ones may be mentioned
to illustrate the nature of the plants then flourishing.
Several species of _Quercus_ (the Oak) appeared early, particularly
_Quercus Ilex_; leaves of the _Juglandaceæ_ (Walnut family) were very
common, and among the Tertiary fossils appear its fruits. Both _Populus_
(the Poplar) and _Salix_ (the Willow) date from the early rocks, while
_Ficus_ (the Fig) was very common, and _Casuarina_ (the Switch Plant)
seems to have been widely spread. Magnolias also were common, and it
appears that _Platanus_ (the Plane) and _Eucalyptus_ coexisted with them.
It will be immediately recognized that the above plants have all living
representatives, either wild or cultivated, growing in this country at
the present day, so that they are more or less familiar objects, and
there appears to have been no striking difference between the early
flowering plants and those of the present day. Between the ancient
Lycopods, for example, and those now living the differences are very
noteworthy; but the earliest of the known flowering plants seem to have
been essentially like those now flourishing. It must be remembered in
this connection that the existing flowering plants are immensely nearer
in point of time to their origin than are the existing Lycopods, and that
when such æons have passed as divide the present from the Palæozoic, the
flowering plants of the future may have dwindled to a subordinate
position corresponding to that held by the Lycopods now.
A noticeable character of the early flowering-plant flora, when taken as
a whole, is the relatively large proportion of plants in it which belong
to the family _Amentiferæ_ (oaks, willows, poplars, &c.). This is
supposed by some to indicate that the family is one of the most primitive
stocks of the Angiosperms. This view, however, hardly bears very close
scrutiny, because it derives its main support from the large numbers of
the Amentiferæ as compared with other groups. Now, the Amentiferæ were
(and are) largely woody resistant plants, whose very nature would render
them more liable to be preserved as impressions than delicate trees or
herbs, which would more readily decay and leave no trace. Similarly based
on uncertain evidence is the surmise that the group of flowers classed as
_Gamopetalæ_ (flowers with petals joined up in a tube, like convolvulus)
did not flourish in early times, but are the higher and later development
of the flower type. Now, _Viburnum_ (allied to the honeysuckle) belongs
to this group, and it is found right down in the Cretaceous, and
_Sambucus_ (Elder, of the same family) is known in the early Tertiary.
These two plants are woody shrubs or small trees, while many others of
the family are herbs, and it is noteworthy that it is just these woody,
resistant forms which are preserved as fossils; their presence
demonstrates the antiquity of the group as a whole, and the absence of
other members of it may be reasonably attributed to accidents of
preservation. In the Tertiary also we get a member of the heath family,
viz. _Andromeda_, and another tube-flower, _Bignonia_, as well as several
more _woody_ gamopetalous flowers.
Hence it is wise to be very cautious about drawing any important
conclusions from the relative numbers of the different species, or the
absence of any type of plant from the lists of those as yet known from
the Cretaceous. When quantities of structurally preserved material can be
examined containing the flowering plants in petrifactions, then it will
be possible to speak with some security of the nature of the Mesozoic
flora as a whole.
The positive evidence which is already accumulated, however, is of great
value, and from it certain deductions may be safely made. Specimens of
Cretaceous plants from various parts of the world seem to indicate that
there was a very striking uniformity in the flora of that period all over
the globe. In America and in Central Europe, for example, the same types
of plants were growing. We shall see that, as time advanced, the various
types became separated out, dying away in different places, until each
great continent and division of land had a special set of plants of its
own. At the commencement of the reign of flowering plants, however, they
seem to have lived together in the way we are told the beasts first lived
in the garden of Eden.
At the beginning of the Tertiary period there were still many tropical
forms, such as Palms, Cycads, _Nipa_, various _Artocarpaceæ_, _Lauraceæ_,
_Araliaceæ_, and others, growing side by side with such temperate forms
as _Quercus_, _Alnus_, _Betula_, _Populus_, _Viburnum_, and others of the
same kind. Before the middle of the Tertiary was reached the last Cycads
died in what is now known as Europe; and soon after the middle Tertiary
all the tropical types died out of this zone.
At the same time those plants whose leaves appear to have fallen at the
end of the warm season began to become common, which is taken as an
indication of a climatic influence at work. Some writers consider that in
the Cretaceous times there was no cold season, and therefore no regular
period of leaf fall, but as the climate became temperate the deciduous
trees increased in numbers; yet the Gymnospermic and Angiospermic woods
which are found with petrified structure show well-marked annual rings
and seem to contradict this view.
Toward the end of the Tertiary times there were practically no more
tropical forms in the European flora, though there still remained a
number of plants which are now found either only in America or only in
Asia.
The Glacial epoch at the close of the Tertiary appears to have driven all
the plants before it, and afterwards, when its glaciers retreated,
shrinking up to the North and up the sides of the high mountains, the
plant species that returned to take possession of the land in the
Quaternary or present period were those which are still inhabiting it,
and the floras of the tropics, Asia, and America were no longer mixed
with that of Europe.[9]