EVOLUTION OF PLANTS
Have we any knowledge regarding the beginning of life on our planet?
Our answer to this question must be decidedly in the negative. We can,
however, be very positive in regard to two important matters concerning
life in early geological time, namely, that plants must have existed
before animals, and that the very oldest known (Archeozoic) rocks of
the earth contain vestiges of organisms. We may be sure that plants
preceded animals because animal life ultimately depends upon plants
for its food supply or, in other words, all animals could never have
been carnivorous. Now, if we can prove that organisms existed during
Archeozoic time, it is evident that plants at least must have lived in
that oldest known era of earth history. That living things did then
exist is proved by the common occurrence of graphite, a crystallized
form of carbon, in the oldest known of the Archeozoic rocks. The facts
that flakes of graphite are abundantly scattered through many layers of
strata of Archeozoic Age, and that adjacent layers of strata contain
such varying amounts of graphite, render it practically certain that
such graphite represents the carbon of organisms. Graphite existing
under such conditions could not be of igneous origin. Carbonaceous
or bituminous strata, so called because they contain more or less
decomposed organic matter, would, when crystallized under conditions
of metamorphism, yield graphite-bearing rocks exactly like those of
Archeozoic Age, and there is every reason to believe that this was
their origin. But, since only graphite (carbon) of the Archeozoic
organisms remains, the rest having disappeared through chemical change
or decomposition, it is impossible to say whether much or all of it
represents original plants or animals. In any case we can be very sure
about the existence of plants (probably very simple or primitive types)
in Archeozoic time, but the presence of any form of animal life has not
been proved.
In the next, or Proterozoic era, some plants and animals of definite
types are known to have existed and, from here on in the present
chapter, it is our purpose to consider the salient points in the
geological history of plants, taking up the main types in the regular
order of their appearance from the remote Proterozoic days to the
present. The very oldest known definitely determinable fossils of any
kind are the more or less rounded masses of crudely concentric layers
of carbonate of lime from one to fifteen inches in diameter found in
middle Proterozoic limestone of western Ontario, Canada. Similar forms
are abundant in late Proterozoic strata of Montana. They occur in large
numbers as layers or reefs, in many cases repeating themselves through
hundreds or even thousands of feet of strata. Careful studies have
shown that these forms are the limey secretions of some of the very
simplest types of plants, that is thallophytes (e.g., seaweeds), which
lived in water.
Before proceeding to describe the plants of Paleozoic and later time,
the reader should be impressed with the important fact that plants of
higher and higher types came into existence throughout geological time
in almost exactly the botanical order of their classification, that
is to say, from the very simplest types (thallophytes) of Proterozoic
time there were gradually evolved, through the long geological ages,
higher and higher plant forms reaching a climax in the complex
and highly organized plants of the present time. This is the most
significant general fact in regard to the geological history of plants.
For the convenience of the reader the largest subdivisions in the
classification of plants are here given.
OUTLINE CLASSIFICATION OF PLANTS
I. Cryptogams {1. Thallophytes (e.g., seaweeds, mushrooms)
(seedless and {2. Bryophytes (e.g., mosses)
flowerless) {3. Pteridophytes (e.g., "club mosses,"
"horsetails," ferns)
II. Pteridosperms {
(seed-bearing, { (e.g., seed ferns--wholly extinct)
flowerless) {
III. Phanerogams {1. Gymnosperms (e.g., cycads, conifers)
(seed-bearing, {2. Angiosperms (e.g., grasses, lilies,
flowering) { oaks, roses)
Throughout the first two periods--Cambrian and Ordovician--of the
Paleozoic era, plant life appears to have made little or no progress
toward higher forms. The very simple Thallophytes (e.g., seaweeds)
continued to secrete concentric layers of carbonate of lime in almost
exactly the same way as during the middle and late Proterozoic era.
Remarkable reefs of such forms occur in the late Cambrian limestone
near Saratoga Springs, New York, where one locality has been set aside
as a state park. During the Ordovician there were seaweeds of the more
familiar branching types without carbonate of lime supports, and these
have left very perfect impressions in some of the Ordovician strata.
During the Silurian period seaweeds continued, as, in fact, they did
throughout succeeding geologic time to the present. The Silurian strata
seem to contain some vestiges of the first-known land plants, though
the records are meager and some of the specimens are of a doubtful
character. Most interesting of all is a fern or fernlike plant found
in France. When we consider the profusion of land plants (all of
relatively simple types) of the next or Devonian period, it seems
certain that their progenitors must have existed in the Silurian, and
their remains may very likely be discovered.
Beginning with the Devonian period of the Paleozoic era the records
show that important advances had taken place in the evolution of the
plant kingdom. Among the very simple Thallophyte plants some seaweeds
of unusually large size occur in fossil form, but the important
fact is that all the principal subdivisions of the typical higher
non-flowering plants (Pteridophytes) as well as Pteridosperms, and even
some primitive representatives of the lower order flowering plants
(gymnosperms) were well represented in the Devonian. Our knowledge
of land plants earlier than the Devonian amounts to almost nothing
and they certainly could not have been at all prominent, but the
fossil records make it very clear that many Devonian land areas were
clad with rich and diversified plant life. There were even forests,
probably the first on earth, but they were far different, both in
general and in particular, from those of to-day because the trees
were all of exceptionally low organization types. During the next two
periods--Mississippian and Pennsylvanian--there was no really important
progress in the evolution of plants, and since these remarkable
types of land plants have left such wonderfully preserved records in
strata of the Pennsylvanian or great Coal Age, we shall proceed to
descriptions of the main types of that time, especially those which
contributed to the formation of beds of coal.
As shown by the abundant records, the land plant life of Pennsylvanian
time must have been not only prolific but exceedingly varied. Thousands
of species have been unearthed from the coal-bearing formations
alone, and these must represent only a fraction of all species of
plants which lived during the period. Most prominent of all were
the giant Lycopods constituting the lowest main subdivision of the
Pteridophytes (see above classification). These great, non-flowering
plants were at once the biggest, most common and conspicuous trees of
the extensive swamp forests, and they were the greatest contributors
to the formation of coal (Plate 15). Many species have been described.
They commonly attained heights of 50 to 100 feet and diameters of 2 to
6 feet. In one important type the fairly numerous branches bristled
with stiff, needle-shaped leaves. When the leaves dropped off the
older or trunk portions, scars were left spirally arranged around the
trunks of the trees. In another important type the leaf scars were
vertically arranged on the lower portions of the tree trunks. The upper
portions of the trunks (rarely branched) were thickly set with long,
slender leaves, which in some species were two or three feet long. An
interesting fact is that the inner parts of the trunks of the great
Lycopods were filled with soft, pithy material. This explains why the
fossil trees are nearly always flattened out, as a result of burial
within the earth. The nonbranching type of Lycopod has been totally
extinct for millions of years, while the branching type is to-day
represented only by small, mostly delicate, trailing plants familiarly
known as "club mosses" and "ground pines." The most conspicuous
trees of the great Pennsylvanian lowlands and swamps have, indeed,
left meager modern representatives, and here we have an excellent
illustration of a once prominent group of plants which has dwindled
away almost to extinction.
Another common type of Pennsylvanian vegetation was the so-called
"horsetail" plant or giant rush. The much smaller scouring rush,
represented by several species to-day, is the direct descendant of
this type which, during later Paleozoic time, grew to be 50 to 90 feet
high and 1 to 2 feet in diameter. The long, slender trunks filled with
pith were segmented with variously shaped leaves arranged in whorls
around the joints. A fine, vertical-fluted structure without leaf scars
characterized the surfaces of the trunk.
Recent study has shown that many of the Pennsylvanian plants, long
classed as true ferns, were really "seed ferns," as described below.
Many of the true ferns grew to be real trees up to fifty or sixty feet
high, but all Paleozoic types were primitive in structure as compared
with modern ferns.
Very remarkable among the later Paleozoic plants were the
Pteridosperms, represented by the so-called "seed ferns." These now
wholly extinct plants seem to have formed the connecting link between
the seedless, flowerless plants (Cryptogams) and the seed-bearing,
flowering plants (Phanerogams), because they bore seeds but not
flowers. Many of them were small and herbaceous, but others were tall
trees, in general appearance resembling the tree ferns. "Seed ferns,"
which play such an important part in the evolution of plants, are not
known to have existed after Paleozoic time.
During the latter half of the Paleozoic era some very primitive types
of flowering plants (Gymnosperms) existed. Most abundant of these were
the so-called Cordaites, which were the tallest trees of the time,
some having reached heights of over 100 feet. The upper portions only
bore numerous branches supplied with many simple, parallel-veined,
strap-shaped leaves up to six feet long and six inches wide. Excepting
the pithy cores the trunks of these trees were of real wood covered
with thick bark. Trees of this kind became extinct in the early
Mesozoic era.
Very late in the Paleozoic (Permian period) two other types of the
simple flowering plants (Gymnosperms) made their appearance. These were
the cycads and conifers, which were the most conspicuous trees during
the first two periods of the Mesozoic era. The cycads reached their
culmination in the Jurassic period, but they still exist in modified
form in some parts of the world. The short, stout trunk was crowned
with long, stiff, palm-like leaves. In fact, the cycads are distantly
related to the palms, which belong to a higher group of plants. Some
specimens of cycads, especially from the Mesozoic strata of South
Dakota, are so wonderfully fossilized that even the detailed structures
of trunks, leaves, flowers, and seeds are so perfectly preserved that
almost as much is known about these plants of millions of years ago as
though they were living forms.
The conifers, with which are classed present-day pines, spruces, and
many other evergreen trees, gradually took on a more modern aspect, so
that late in the Mesozoic era they were much like those now living.
Among the most interesting trees were the sequoias, to which the
living "big trees" and red-woods of California belong. These began in
relatively late Mesozoic time, reached their climax in numbers, variety
of species, and widespread distribution in the early Cenozoic era;
and are now almost extinct, being represented by only two species in
local portions of California. Cordaites, trees which were so large and
abundant in later Paleozoic time, were reduced to extinction in the
early Mesozoic era.
During Mesozoic time the Thallophytes, represented by seaweeds, were
common. Among the Pteridophytes the ferns and "horsetail" plants
were fairly common, but the very large forms gradually gave way to
much smaller ones during Mesozoic time. The giant Lycopods of later
Paleozoic time dwindled almost to extinction even in early Mesozoic
time, so that from that time to the present they have been very small
and relatively insignificant.
Tens of millions of years of earth history had passed before the
true flowering plants--the Angiosperms--appeared upon the earth. The
Cretaceous period marks their advent. So far as known, these plants
originated along the eastern side of North America, and very soon after
their establishment they spread over the earth with amazing rapidity
and dominated the vegetation as they do to-day, more than half of the
existing species of plants being Angiosperms. Among the common types
which have been unearthed from Cretaceous strata are palms, grasses,
maples, oaks, elms, figs, magnolias, willows, beeches, chestnuts, and
poplars.
[Illustration: Plate 13.--A Slab of Very Early Paleozoic (Cambrian)
Rock, Covered with Some of the Oldest Known Definitely Determinable
Animal Remains. These creatures lived in a sea which overspread the
site of the Rocky Mountains of southern Canada fully 25,000,000 years
ago. Most of the fossils are trilobites (including some very small
ones) and other related crustacean forms (lighter portions). (_After C.
D. Wolcott, Smithsonian Institution, Washington, D. C._)]
[Illustration: Plate 14.--(_a_) Photographs of Small Slabs of
Ordovician Strata Full of Fossils. These slabs are actual bits of
sea bottom at least 20,000,000 years old. The left picture shows
"stone-lily" stems, so-called "sea mosses," brachiopods. Right picture
shows various species of brachiopods. (_Photo by the author._)]
[Illustration: Plate 14.--(_b_) An Outcrop of Middle Ordovician
Stratified Limestone in Northern New York. This ledge is full of
fossils similar to those above. The material was deposited on the floor
of the Ordovician sea which overspread much of the continent. (_Photo
by the author._)]
The introduction of the higher flowering plants (Angiosperms) "was,
perhaps, the most important and far-reaching event in the whole
history of vegetation, not only because they almost immediately became
dominant, but also because of their influence upon the animal life of
the succeeding periods. Hardly had flowers appeared, before a great
horde of insects, which fed upon their honey or pollen, seem to have
sprung into existence. The nutritious grasses and the various nuts,
seeds, and fruits afforded a better food for noncarnivores than ever
before in the history of the world. It was to be expected, therefore,
that some new type of animal life would be developed to take advantage
of this superior food supply. As we shall see in the discussion of
the Tertiary (next chapter), the mammals, which kept a subordinate
position throughout the Mesozoic, rapidly took on bulk and variety and
acquired possession of the earth as soon as they became adapted to this
new food, quickly supplanting the great reptiles of the Mesozoic."
(Cleland.)
During the present or Cenozoic era vegetation gradually took on a more
and more modern aspect until the existing species were developed. The
grasses especially developed and spread rapidly, but the cereals did
not evolve until late in the era. Certain single-celled plants, called
diatoms, may be especially mentioned, for they must have literally
swarmed in some of the Tertiary seas which spread over parts of the
present lands. "The microscopic plants which form siliceous shells,
called diatoms, make extensive deposits in some places. One stratum
near Richmond, Virginia, is thirty feet thick and is many miles in
extent; another, near Monterey, California, is fifty feet thick, and
the material is as white and fine as chalk, which it resembles in
appearance; another, near Bilin, in Bohemia, is fourteen feet thick....
Ehrenberg has calculated that a cubic inch of the fine, earthy
rock contains about forty-one thousand millions of organisms. Such
accumulations of diatoms are made both in fresh waters and salt, and in
those of the ocean at all depths." (J. D. Dana.)