FOSSIL PLANTS AS RECORDS OF ANCIENT COUNTRIES
The land which to-day appears so firm and unchanging has been under the
sea many times, and in many different ways has been united to other land
masses to form continents. At each period, doubtless, the solid earth
appeared as stable as it is now, while the country was as well
characterized, and had its typical scenery, plants, and animals. We know
what an important feature of the character of any present country is its
flora; and we have no reason to suspect that it was ever less so than it
is to-day. Indeed, in the ages before men interfered with forest growth,
and built their cities, with their destructive influences, the plants
were relatively more important in the world landscape than they are
to-day.
As we go back in the periods of geological history we find the plants had
an ever-increasing area of distribution. To-day most individual species
and many genera are limited to islands or parts of continents, but before
the Glacial epoch many were distributed over both America and Europe. In
the Mesozoic _Ginkgo_ was spread all over the world, and in the present
epoch it was confined to China and Japan till it was distributed again by
cultivation; while in the Palæozoic period _Lepidodendron_ seemed to
stretch wellnigh from pole to pole.
The importance of the relation of plant structure to the climate and
local physical conditions under which it was growing cannot be too much
insisted upon. Modern biology and ecology are continually enlarging and
rendering more precise our views of this interrelation, so that we can
safely search the details of anatomical structure of the fossil plants
for sidelights on the character of the countries they inhabited and their
climates.
It has been remarked already that most of the fossils which we have well
preserved, whether of plants or animals, were fossilized in rocks which
collected under sea water; yet it was also noted that of marine plants we
have almost no reliable fossils at all. How comes this seeming
contradiction?
The lack of marine plant fossils probably depends on their easily
decomposable nature, while the presence of the numerous land plants
resulted from their drifting out to sea in streams and rivers, or
dropping into the still salt marshes where they grew. Hence, in the rocks
deposited in a sea, we have the plants preserved which grew on adjacent
lands. In fresh water, also, the plants of the neighbourhood were often
fossilized; but actually on the land itself but little was preserved. The
winds and rains and decay that are always at work on a land area tend to
break down and wash away its surface, not to build it up.
There are many different details which are used in determining the
evidence of a fossil plant. Where leaf impressions are preserved which
exhibit a close similarity to living species (as often happens in the
Tertiary period), it is directly assumed that they lived under conditions
like those under which the present plants of that kind are living; while,
if the anatomy is well preserved (as in the Palæozoic and several
Mesozoic types), we can compare its details with that of similar plants
growing under known conditions, and judge of the climate that had
nurtured the fossil plant while it grew.
Previous to the present period there was what is so well known as the
Glacial epoch. In the earthy deposits of this age in which fossils are
found plants are not uncommon. They are of the same kind as those now
growing in the cold regions of the Arctic circle, and on the heights of
hills whose temperature is much lower than that of the surrounding
lowlands. Glacial epochs occurred in other parts of the world at
different times; for example, in South Africa, in the Permo-Carboniferous
period, during which time the fossils indicate that the warmth-loving
plants were driven much farther north than is now the case.
It is largely from the nature of the plant fossils that we know the
climate of England at the time preceding the Glacial epoch. Impressions
of leaves and stems, and even of fruits, are abundant from the various
periods of the Tertiary. Many of them were Angiosperms (see Chap. VIII),
and were of the families and even genera which are now living, of which
not a few belong to the warm regions of the earth, and are subtropical.
It is generally assumed that the fossils related to, or identical with,
these plants must therefore have found in Tertiary Northern Europe a much
warmer climate than now exists. Not only in Northern Europe, but right up
into the Arctic circle, such plants occur in Tertiary rocks, and even if
we had not their living representatives with which to compare them, the
large size and thin texture of their leaves, their smoothness, and a
number of other characteristics would make it certain that the climate
was very much milder than it is at present, though the value of some of
the evidence has been overestimated.
From the Tertiary we are dependent chiefly on impressions of fossils;
anatomical structure would doubtless yield more details, but even as it
is we have quite enough evidence to throw much light on the physiography
of the Tertiary period. The causes for such marked changes of climate
must be left for the consideration of geologists and astronomers. Plants
are passive, driven before great climatic changes, though they have a
considerable influence on rainfall, as has been proved repeatedly in
India in recent times.
From the more distant periods it is the plants of the Carboniferous,
whose structure we know so well, that teach us most. Although there is
still very much to be done before knowledge is as complete as we should
wish, there are sufficient facts now discovered to correct several
popular illusions concerning the Palæozoic period. The “deep,
all-enveloping mists, through which the sun’s rays could scarcely
penetrate”, which have taken the popular imagination, appear to have no
foundation in fact. There is nothing in the actual structure of the
plants to indicate that the light intensity of the climate in which they
grew was any less than it is in a smoke-free atmosphere to-day.
Look at the “shade leaves” of any ordinary tree, such as a Lime or Maple,
and compare them with those growing in the sunlight, even on the same
tree. They are larger and softer and thinner. To absorb the same amount
of energy as the more brilliantly lighted leaves, they must expose a
larger surface to the light. Hence if the Coal Measure plants grew in
very great shade, to supply their large growth with the necessary sun
energy we should expect to find enormous spreading leaves. But what is
the fact? No such large leaves are known. _Calamites_ and
_Lepidodendron_, the commonest and most successful plants of the period,
had narrow simple leaves with but a small area of surface. They were, in
fact, leaves of the type we now find growing in exposed places. The ferns
had large divided leaves, but they were finely lobed and did not expose a
large continuous area as a true “shade leaf” does; while the height of
their stems indicates that they were growing in partial shade—at least,
the shade cast by the small-leaved Calamites and Lepidodendrons which
overtopped them.
Indeed there is no indication from geological evidence that so late as
Palæozoic times there was any great abnormality of atmosphere, and from
the internal evidence of the plants then growing there is everything to
indicate a dry or physiologically dry[14] sunny condition.
Of the plant fossils from the Coal Measures we have at least two types.
One, those commonly found in nodules _in_ the coal itself; and the other,
nodules in the rocks above the coal which had drifted from high lands
into the sea.
The former are the plants which actually formed the coal itself, and from
their internal organization we see that these plants were growing with
partly submerged roots in brackish swamps. Their roots are those of water
plants (see p. 150, young root of Calamite), but their leaves are those
of the “protected” type with narrow surface and various devices for
preventing a loss of water by rapid transpiration. If the water they grew
in had been fresh they would not have had such leaves, for there would
have been no need for them to economize their water, but, as we see in
bogs and brackish or salt water to-day (which is physiologically usable
in only small quantities by the plant), plants even partly submerged
protect their exposed leaves from transpiring largely.
There are details too numerous to mention in connection with these
coal-forming plants which go to prove that there were large regions of
swampy ground near the sea where they were growing in a bright atmosphere
and uniform climate. Extensive areas of coal, and geological evidence of
still more extensive deposits, show that in Europe in the Coal Measure
period there were vast flats, so near the sea level that they were
constantly being submerged and appearing again as débris drifted and
collected over them. Such a land area must have differed greatly from the
Europe now existing, in all its features. But the whole continent did not
consist of these flats; there were hills and higher ground, largely to
the north-east, on which a dry land flora grew, a flora where several of
the Pteridosperms and _Cordaites_ with its allies were the principal
plants. These plants have leaves so organized as to suggest that they
grew in a region where the climate was bright and dry.
A fossil flora which has aroused much interest, particularly among
geologists, is that known as the Glossopteris flora. This Palæozoic flora
has in general characters similar to those of the European
Permo-Carboniferous, but it has special features of its own, in
particular the genus _Glossopteris_ and also the genera _Phyllotheca_ and
_Schizoneura_.
These genera, with a few others, are characteristic of the
Permo-Carboniferous period in the regions in the Southern Hemisphere now
known by the names of Australasia, South Africa, and South America, and
in India. These regions, at that date, formed what is called by
geologists “Gondwanaland”. In the rocks below those containing the plants
there is evidence of glacial conditions, and it is not impossible that
this great difference in climate accounts for the differences which exist
between the flora of the Gondwanaland region and the Northern Hemisphere.
Unfortunately we have not microscopically preserved specimens of the
Glossopteris flora, which could be compared with those of our own
Palæozoic.[15]
To describe in detail the series of changes through which the seas and
continents have passed belongs to the realm of pure geology. Here it is
only necessary to point out how the evidence from the fossil plants may
afford much information concerning these continents, and as our knowledge
of fossil anatomy and of recent ecology increases, their evidence will
become still more weighty. Even now, had we no other sources of
information, we could tell from the plants alone where in the past
continents were snow and ice, heat and drought, swamps and hilly land.
However different in their systematic position or scale of evolutional
development, plants have always had similar minute structure and similar
physiological response to the conditions of climate and land surface, so
that in their petrified cells are preserved the histories of countries
and conditions long past.