CHAPTER XVII

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.

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.)