Botany: The Science of Plant Life

by Norman Taylor

There are living to-day somewhere about 150,000 species of flowering plants; half a hundred conifers or gymnosperms; about 3,500 ferns; 500 club mosses; over 70,000 bacteria, fungi, and lichens, and probably over 20,000 species of algæ. The estimates of the last three divisions are more or less uncertain, as many species are still being discovered.

From what has just been read regarding the plants of earlier periods it is at once clear how completely the flowering plants have conquered all their ancient forbears, and what a pitifully small remnant of once proud and ancient forests are now represented by our club mosses and horsetails. That process of crowding, of the dominance of one kind to the exclusion or even extinction of others is still going on, and, as we shall see in the last chapter, often on a great scale.

If the period just after plants were first known on the earth may be called the Reign of Algæ, and subsequent periods were typified by still other kinds of plants, then our present period is par excellence the reign of flowering plants. In numbers of individuals they are still far outclassed by such cryptogams as the fungi, bacteria, algæ, etc., but their dominating influence in the plant world is unquestioned.

While all our present vegetation must have been derived from preexisting types, all of it is not necessarily directly descended from species which from their fossil records we know to have existed in geological periods older than our own. While the fossil record of the times immediately preceding the last ice age is a much more complete one than for many other periods, it fails to account directly for the great bulk of our varied flora of to-day. While ferns in great variety, gymnosperms, and hundreds of flowering plants are known quite definitely, they total only a minute fraction of these groups to-day. Even granting the always imperfect nature of the fossil record, and we have seen what an accident the formation of a plant fossil may be, and it is common knowledge how few comparatively have ever been recovered—even granting all this, there still remains a large part of our present flora of which the origin probably dates from comparatively recent times. So overwhelmingly true is this that of the Compositæ, or daisy, family, now numbering over 11,000 species, scarcely a handful of fossil species have been found. And in all collections of fossils the woody plants far outnumber the herbs, perhaps because of the greater probability of their being thus preserved rather than to any actual scarcity of herbs in the upper strata. And yet herbs to-day outnumber woody species over two to one. While it is true, then, that our present flora must have been derived from preexisting races, it is also true that much of it is apparently derived from plants that do not date very far back into the past. A few main types of flowering plants unquestionably are to be linked with fossil genera, but these types have now branched out into a wealth of detail that may not have existed and is certainly not recorded in the fossil record.

Some of these types stand out with remarkable clearness, notably magnolia, willows, poplars, walnuts, birches, oaks, figs, sassafras and its relatives, the rosales, the pea family, the spurges, maples, grapes, linden, myrtle, ginseng, and some others. All these, and in not very different aspect from their modern representatives, have been found in the fossils of the different and usually more recent strata before the last Ice Age. But the total fossil record of even these well-known genera is only a fraction of their modern development, and we are constantly confronted with the apparent dilemma of accounting for a present wealth of forms based upon an obvious poverty of ancestry. While the whole race of flowering plants is certainly a new one, as such things are reckoned fossilwise, there has been a fecundity in the origin of new species among these lusty upstarts that is simply amazing. How that, in part at least, has been accomplished will be considered in the final section of this chapter. Not only among these present-day plants, but all through the story of the development of the plant kingdom, we have been reading and writing of the changes of form and structure, some of which have been of far-reaching consequences. It is clear enough that if new types of vegetation and different races of plants have come into being and so modified the complexion of the plant kingdom, those changes must have first arisen in individuals which had within them some capacity for change, and furthermore the ability to use the change to their advantage. While, as we have seen, the losses have been tremendous, no one, with even this brief history of their development in mind, can doubt that there has been progress toward our present perfection of plant life.

6. How Plants Change Their Characters and Become New Species

It was with something very different in mind than the changing of plant characters that Cardinal Newman once said: “To live is to change, and to be perfect is to change often.” And yet nothing better expresses the facts of plants’ ability to change and the results of it than this reply of a great churchman to critics who could not or would not understand the truth of his now famous reply.

It is perhaps best to begin any discussion of the changes in plants by remembering a few simple facts regarding changes in ourselves. “Like father like son” is something more than an old saw which we repeat for centuries without stopping to think whether it is true or only half true. As with so many speeches of the sort, this is just precisely a half truth, for while sons are more apt to be like their fathers than other men, we all have within us the capacity, whether expressed or not, to change very considerably. In other words, all living things may be said to be a reflection or, perhaps better, the result of two divergent tendencies, one of which tends to make like produce like, and the other to produce something different.

Upon the ability of like to produce like rests the continuity of those plant groups, well exemplified by Lycopodium Selago and the ginkgo, which, through all the changing panorama of the history of the plant world, have steadily produced individuals so close to the ancestral type as to be essentially indistinguishable from it. It is upon the possession of this ability that all the different races of plants depend for the unchanged perpetuation of their kind. And, as we shall presently see, it is also upon this very ability that the new forms that do arise, rely for holding fast to their differences.

While it is true, then, that like tends to produce like, it is also and perhaps even more true that they do not precisely do so. In fact, they never do absolutely, and it is the degree of divergence from the type that different plants or animals exhibit, which is the measure of their ability to vary, or “produce something different.” Upon this capacity to vary, from whatever cause, rest all the changes which have occurred in the plant world, and, as we have seen in previous chapters, that has been by no means an insignificant affair. We know, in fact, that while one plant of Lycopodium Selago, than which scarcely any other now living has had greater opportunity to become fixed in its characters, is much like another, no two of them are actually identical. Nor are any two plants of the same species ever precisely alike, any more than two children, even of the same parents, are. The tendency for like to produce like is matched then, or sometimes exceeded, by an almost equally strong tendency to vary.