Botany: The Science of Plant Life

by Norman Taylor

4. More Recent Ancestors of Our Modern Flora

The vegetation at the ending of Carboniferous times was much affected by the great changes in the earth’s surface which happened then. The thrusting up of great mountain chains, the slow encroachment of continental glaciers, and the other phenomena characterizing that period could not but be reflected in the plant population. For one thing the giant club mosses and horsetails were much reduced in extent and finally disappeared, leaving only the immediate ancestors of our present-day forms. Cordaitales gave place to trees not unlike some of the modern yew trees. True ferns as well as the cycadlike ferns with seeds appear to have lived side by side with true cycads, which subsequently supplanted their obvious cycadlike fern ancestors. There was an obvious dwindling of ancient Carboniferous forms, some of which, however, persisted in considerable numbers. Many other plants existed then, some of which died out there, and some of which still survive in descendants, particularly among our conifers and ferns. But there happened toward the upper end of this period an event in the history of the plant kingdom so dramatic, of such far-reaching results, that its appearance might be likened to the overthrow of the Czar in Russian history or to the downfall of the Kaiser in Germany. For with it dawned a new era for the plant world, the effects of which we see all about us to-day.

Somewhere in the rock strata of this period we find the first angiosperm, or plant that matures its seed in a closed ovary, and with the origin of that habit there began such a development of plants of this type that its impetus has not yet been lost. It is impossible to tell at this distance from the origin of that first angiosperm from what it developed, nor how many ages it may have existed before the accident of its preservation as a fossil revealed its presence. It is certainly not without significance that it bore conelike fruits, such as all its associates and predecessors among flowering plants had done, but its possession of large, showy petals is the first evidence of a flower characteristic that was destined to make our present vegetation the lovely thing it is. This exceedingly interesting plant was a Magnolia ([Figure 110]), or so like our present plants of that genus as to be their obvious ancestor. Somewhere here, too, must have arisen the insect fertilization of flowers which we have seen to be such an important part of flower economy at the present time. Most of this ancient magnolia’s associates must have relied on wind pollination for seed production, as many modern plants still do, but the origin of insect fertilization appears to have come with the appearance of the first really petaliferous flower.

With this Magnolia were found other flowering plants which soon increased tremendously in numbers of individuals and differences of structure, such as our sassafras, the tulip tree, the poplar tree, and some others. All of these are trees or shrubs and we do not yet know whether herbs grew in the strange surroundings of that ancient forest or not. Their soft tissue may have prevented their preservation as fossils, but, at any rate, no herb has left its rock-written record from as early in this period as the trees and shrubs. All of these ancient trees have been recorded only in the northern hemisphere and it may be true that this part of the earth was the cradle of all those hosts of the flowering plants that now number over 150,000 species.

There must have been a mighty struggle for occupancy of the really desirable plant sites soon after the rise of these immediate ancestors of our modern plants. For there is every evidence of the progressive dwindling of those still more ancient holdovers from the Carboniferous, and the steady encroachment of the newly arisen and obviously vigorous young race. As we get higher up in the strata, or, in other words, nearer to the present, there are literally thousands of these immediate ancestors of our modern flora, and it is not very long before herbs, particularly grasses and sedges, begin to be common, together with other monocotyledonous plants such as palms. One not unlike our coconut palm has been found in some of these strata in France.

While this period records the origin of hundreds, and there are probably thousands of unrecorded species which are very near our modern descendants of them, it was also a period when the earth’s crust was in an almost constant state of restlessness. Ice periods, huge inland seas, great volcanic upheavals, and the thrusting up of mountain chains such as the Alps, Himalayas, and some others, were only a few of the disturbances to the orderly procession of this wholly new type of vegetation that doomed the older kinds and subsequently conquered the world. The spread of this new element in the plant kingdom was greatly helped and sometimes greatly hindered by land connections between continents, now separated by the oceans. The giant redwoods, now isolated in a few localities on our Pacific Coast, were found then nearly throughout the world. Because of these changes of land areas and some others of even greater influence on plant growth, such as climate, there was a constant shuffling of floral and, of course, animal elements, so that by the end of this period the new type of flora had spread throughout the world, but with here and there very local occurrence of certain genera and families, some of which have persisted to the present day. As we shall see in the last chapter of this book, certain whole families are confined to restricted areas; the cactus and pineapple family, and the genus Helianthus, or sunflowers, for instance, are, with one or two trifling exceptions, wholly American. And we have already seen how many food and other useful plants were first found here by the Spaniards—chocolate, tobacco, corn, the potato, and others.

It would fill the rest of this book to enumerate the plants that flourished toward the end of this period, and, in fact, it might almost be said that the flora of those days was not very different from our own, only it was distributed in different ways and mixed in very different proportions. With the disappearance or partial dwindling of more ancient groups, the rise of the plants that immediately preceded our own ushered in a new era in the history of the plant kingdom.

Fossil and Living Algæ Compared. C. A living algal pool colony near the Great Fountain Geyser, Yellowstone Park. (After Walcott.) B. Fossil calcareous algæ. Cryptozoön proliferum Hall, from the Cryptozoön ledge in Lester Park, near Saratoga Springs, N. Y. These algæ, which are among the oldest plants of the earth, grew in cabbage-shaped heads on the bottom of the ancient Cambrian sea and deposited lime in their tissue. The ledge has been planed down by the action of a great glacier which cut the plants across, showing their concentric interior structure. (Photographed by H. P. Cushing. Pictures and explanations of them from “The Origin and Evolution of Life,” by Professor Henry Fairfield Osborn, who kindly permitted their reproduction here.) (Courtesy of Brooklyn Botanic Garden.)

Probable Landscape in the Carboniferous Age. About the Time Coal Was in the Making. (After Patonie) 1. Tree fern 2. Giant ancestors of our horsetails. 3 and 4. Ancestors of our club mosses. 5. Cordaites, a primitive type, or perhaps even the ancestor of our modern evergreens. At this time no herbs and no plants with petals, were known, nor for ages after this period. (Courtesy of Brooklyn Botanic Garden.)

At the end of this period an event of commanding interest occurred, because it happened only some 40,000 years ago. With it came the encroachment from the north and south poles of the last of the great continental glaciers. There had been many before, stretching over a past period of time, but as the last of these great ice invasions it is the most interesting to us. It crowded all these temperate and even subtropical plants that then grew up in the far north toward the equator, and scraped clear of vegetation every part of the earth which it covered. In the volume on geology you will find an account of the extent and thickness of this great ice sheet, which ultimately receded to its present home. As it went back the plants crowded forward to occupy the freshly released land, the far northern or glacial first, followed by waves of other kinds. Some of the glacial or northern plants were left on the tops of the highest mountains, where to-day they persist in complete isolation, nearly all their friends and associates of that greatest of all winters having left them for points farther north. Many students of plant geography think that wave of plant life creeping northward to occupy the region uncovered by the retreating ice is still going on, and recent studies appear to show in at least one isolated mountain in the Adirondacks that the survivals of the ice age which have been isolated on its rocky peak ever since are in considerable danger of being crowded out by invaders from the lowlands.

Not all the geological changes which have remodeled the earth’s surface have been mentioned in this brief history of those plants that preceded our own, nor have anything like all the plants occurring in the different strata been even hinted at. But the thing which has been stressed and for us to fix in our minds is that all our present vegetation literally has its roots deep down in the earth. Some, as Lycopodium Selago, go back no one knows how many millions of years; others, like the flowering herbs, are much more recent. We come to understand how recent we are and what a comparatively brief flash in the pan all our modern development both of plants and in man has been since the last glacial period only by looking for a moment at what has happened in the past. In the account of the Carboniferous plants we found that there remained after that period only about 19 per cent of the earth’s age in which all the changes since then could have come about. If, as may well be possible, this period has been about 19,000,000 years, then the mere 40,000 years since the last Ice Age seems a brief period indeed. As some one has written, to contrast all man’s historic period, back to the days of the most ancient Chinese manuscript, with that long journey from the dim past which the plant world has slowly accomplished, is to realize that we are “as the flashing of a meteor through the sea of night.”

Fossil plants then, and this delving into the dead past of the plant world, reveals to us as nothing else can how much the modern plant kingdom is literally built upon a mighty race of ancestors. Some perished as did the Cordaitales, but left descendants who themselves gave rise to other groups that survive to-day. To look over a list of the fossil plant genera of the different strata is to visualize a drama the like of which no one living will ever see replayed, the results of which are recorded all over the world with its changing panorama of vegetation.