Botany: The Science of Plant Life

by Norman Taylor

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.

Heredity on the one hand and variation on the other are the two forces upon which the origin of new species or kinds of plants is based. Both of these work in rather definite ways, some of which are fairly well understood, but many of which are still among the things that scientists are striving to clear up. As the capacity to inherit characters from one generation to another reflects itself in the generally stable conditions which the plant world exhibits, while the capacity to vary is the only source of new forms, it is quite naturally the variations of plants from one generation to another which have been most studied. And the study of variation in plants is not the simple thing we might assume it to be, having in mind only the well-known fact that no two organisms are exactly alike. Wherein do they differ? Are their perhaps temporary or even quite casual differences passed on to their progeny? These and many other questions about variation make it at once the most complicated and often one of the most fruitful subjects of plant research. It is clear enough that with the bewildering variety of different plants in nature it is next to impossible not only to record accurately the amount of variation or its probability of being handed on, but least of all to arrive at any clue as to the origin of that variation. Because of this, and still more because practically no one has ever seen the actual origin of a new species in nature, for we only see the finished product, practically all our knowledge of the laws of variation has been derived from studying cultivated plants. The ease of controlling them and of recording thousands of observations of their characteristics has made the work of the plant breeder, and others who study variation in the vegetable world, much more of an exact science to-day than the mass of often interesting but usually unrelated data that crowd the pages of older botanical literature.

One of the main facts about variation is that it is itself a very variable thing, and the nature of those “fluctuating variations” which are so common in nature well illustrate the point. Within what we know as a species there are many individuals that vary one way or the other from a fairly central, we might almost call them normal, mass of individuals which are typical of that species. Nearly all these forms on the fringe of the species due to the environmental changes and not to changes of hereditary constitution will, if left to themselves, tend through their progeny to become more like the central mass as time goes on, while their position, or some other equally nontypical edge of the fringe, will be taken up by other variants from the average conditions. The amount of this fluctuating variability among plants is beyond calculation, and its action has often been likened to the swinging of a pendulum, which of course spends twice the time passing through the center of its arc, that it does on the limits of it. This very nearly expresses the proportion of fluctuating variants to the mass of typical individuals in many species of plants. In many others, often peculiarly unstable species, the number of individuals at the fringe is very large indeed. Sometimes there may be one or more that do hold their characteristics, in which case we know that they are not true environmental variations but have actually a different constitution. These will be considered presently under another and different sort of variability. But, speaking generally, these fluctuating or environmental variants are merely forms of the species, and, other things being equal, they will not actually originate new species.

It should be emphasized here, and before we go further in our discussion of variation, that species and varieties are after all largely creations of the mind of man rather than the reflection of actuality in nature. When we speak of a “species” it is merely a term which through usage by botanists becomes the symbol of a group of plants more like one another than like anything else. It is obvious that it is therefore necessarily an inaccurate designation of the actual conditions found among plants, which might almost be considered as all belonging to one great group of which, for our convenience in referring to them, we mark off units (families, genera, species, etc.), much as units are marked off on a rule. Species, then, and varieties of plants, notwithstanding the utmost refinement of method used in designating and describing them, and this is historically the most ancient and the most widely developed phase of botanical science, cannot reflect the true conditions, and for a number of reasons. The chief one is that species differ in usually several characters one from another and in large genera there is often a bewildering recombination of characters of the genus in the species belonging to it. Species and varieties are concepts of convenience, nay of absolute necessity, in talking or writing about plants, but hardly expressions of exact truth.

With this in mind we can appreciate the position of those plant breeders who insist that the basis of differences in all plants are the simplest, so-called, factor expressions, which can be isolated and studied with some approach to exactitude in experimental cultures. A factor may be defined as the hereditary determiner or base, which, either singly or in conjunction with other factors, is expressed as a character, such as tallness in peas, or brown eyes in human beings. Such studies have built up a body of information about variation in plants that show it to be of several kinds and with different chances of being passed on from one generation to another.

It was noted in the paragraph before the last that fluctuating variants were sometimes so far off the usual that they might almost be considered distinct forms or varieties. Many such changes appear to be the result of different conditions of the local environment, due to changed conditions rather than to any internal difference in the constitution of the plant itself. A familiar illustration of environmental variation may be seen in lima beans. In any considerable number of plants one often finds smaller and larger pods, either sparsely or well filled with beans. If the beans from the small-podded, few-seeded variants are planted they will produce, apparently quite indiscriminately, large and small podded progeny, just as there will result a mixed progeny if only beans from the well-filled and large-podded kind are sown. In other words the plant fluctuates about a general average which typifies the usual or mass characteristics of the species. One should not, however, regard all variations of the character in lima beans, or any other plant as environmental or fluctuating ones, for some of them may be due to differences in hereditary constitution. And these could only be determined by breeding tests.

Environmental variations are as frequent as the ever-changing conditions of plant growth may determine, and it is common knowledge that such diversity of the environment and the variants resulting from it are extremely frequent.

A much more fruitful source of new forms of plant life results from natural cross-fertilization, which, as we saw in an earlier chapter, is the nearly universal condition in the plant world. If species and varieties can be distinguished only by factor differences, as the plant breeders no doubt correctly insist, it becomes obvious enough that we have in cross-fertilization to consider not alone the factor differences of the pistillate or female, but also of the staminate or male contribution to the union, and how these are reflected in the progeny. Our knowledge of this has practically all been based on work done on cultivated plants under control conditions and it shows some interesting developments which occur from crossing.

If garden peas with, let us say, reddish-purple flowers are crossed with white-flowered ones, the progeny will not be a mixture of these colors but all reddish-purple. If all danger of subsequent cross-fertilization is excluded this first generation of reddish-purple progeny will themselves produce reddish-purple and white progeny in the ratio of three to one. But the extraordinary part of it is that in the third generation all the white and about one-third of the reddish-purple plants will breed true to color. The balance of the reddish-purple plants, which comprise about two-thirds of the second generation, will, if their seeds are germinated, produce colored as against white-flowered progeny in the three-to-one ratio. In other words, these artificial crosses, made by the plant breeder, and this splitting up of hybrids which has been many times verified, are seen to be very fertile causes of the origin of new forms of plant life, if only the factor and character differences in the ancestry be sufficiently complex. With no two plants precisely alike, with cross-fertilization so nearly universal, and with all characters, not a single character or factor expression, as in control conditions likely to be affected by the cross, it may be seen how fruitful a source of new forms this natural crossing may be. It is, in fact, not surprising that plants vary, but that the force of heredity will hold them into such recognizable categories that the red maple, or white ash, or blue cohosh are, with thousands of other species, after all fairly definite designations without which talking and writing about plants would be all but impossible. Some of our most beautiful garden plants have arisen either as the result of natural crossing, or crosses deliberately made by the plant breeder. The scores of forms of the common garden lilac have mostly come about by such crosses, although many other garden plants have arisen by still another kind of variability.

The effects of crossing which have been so briefly noted were not understood, as indeed the cause of them is still unknown, before 1865, when Gregor Mendel, an Augustinian monk, published the results of his work on peas, which furnished the basis for all subsequent work on this kind of variability. His work was neglected until 1900, when what is now known as Mendel’s law, involving the Mendelian ratio already noted, was rediscovered by three independent workers. It is now practically universally accepted as the way in which natural or induced hybrids transmit their characters.

There remains still another type of variability which has been noticed from very early days, and received the name sport, because quite suddenly, from a crop of otherwise similar specimens, one or a few plants showed marked and permanently transmitted differences from the average condition. Such sudden offshoots, which occur rather frequently in many plants, are known as mutants, the process as mutation. Hugo de Vries, a Dutch botanist of world-wide fame, was the chief modern figure who drew attention to mutants, and explained how they differ from fluctuating variants in that while these tend to revert to the average or mass conditions the mutant, once it appeared, held true to type. A well-known example of mutation is the cabbage, brussels sprouts, cauliflower, and kohl-rabi, all of which are sports or mutants from a weedy seaside plant of the mustard family, native in Europe. Since their appearance, hundreds of years ago, they have held their essential characters. If they had been environmental variants they would in all probability have reverted to their weedy ancestor. Hundreds of sports or mutants have been recognized and isolated, so that many of our most valuable garden plants have arisen through this ability of plants to vary in often sudden and rather startling degree. The gardener and horticulturist, from long observation and a keen sight for valuable novelties, have always known that sports are fruitful sources of new forms of plants, but De Vries first scientifically studied them and worked out the principles by which they apparently react. The cause of them is still unknown.

While the cause of mutants has not yet been revealed, we have already seen that the two remaining kinds of variability are due to changes in the environment, or to crossing. Charles Darwin when he published his “Origin of Species,” than which no other book has so completely revolutionized modern thought, did not state the cause of those variations of which he was our greatest observer. He did state the now universally accepted law of the “Survival of the Fittest” which explains how, once these variations make their appearance, the inexorable conflict of nature would automatically weed out the unfit. We have seen all through the course of this history of the plant kingdom how whole types of vegetation have been overthrown to give way to other types better fitted to survive. That process is going on with just as inexorable results to-day as it has down through the ages. While Darwin never claimed that such a purely selective process could initiate new species, many of his partisans who waged battle for him during the first years of the tremendous opposition his views encountered, did so claim, and probably wrongly. The actual cause of the origin of new species, except those demonstrated to result from new combinations of already known characters, through crossing, cannot be explained through the natural selection of plants or animals which exhibit favorable variations. We see their effects; it is obvious enough, that those of value tend for the survival of plants having such variations, and it was natural enough that older students of the problem should mistake these effects for the cause of them. The process of selective elimination constantly going on does tend to fix certain favorable variations and untold millions of plants have had their day in the past due to their possession of such, and the killing off of their less fortunately provided associates. We speak of this great march from the simplest organism up to our most complex plants and animals as their evolution, but we must never forget that it has gone step by step, by one or the other methods by which we have seen that plants vary, or perchance by some undetected method, and that while the results of it are for all to see, the causes of that infinitely slow and quite often wayward variation are not understood. Upon such a conception our modern plant life is seen to be a development of plants that have gone before, that all existing life is derived from preexisting, and not from providential interposition or special creations. All through the long marches of plant evolution there appears to be a definite and final goal toward which it tends, but we do not know the direction, least of all the object, of that goal. In fact, there may be many goals, just as there are the diversity of ambitions among human beings. In tracing the present ascendancy of our flowering plants from their links with the past perhaps we shall find no better statement of their present condition or destiny than to repeat Cardinal Newman’s reply: “To live is to change, and to be perfect is to change often.”

CHAPTER VIII
DISTRIBUTION OF PLANTS

WE have seen in the previous chapters how many and how varied are the activities of the plant world and in this final one we shall get a glimpse of what these activities have produced. All the delicate mechanism of food getting and the manufacture of starch; the fertilization of flowers by insects, the wind or water; the response to changing light and to climate—these and scores of other activities of plants have resulted in the present vegetation of the world being what it is. Here we see the final reflection or register of not one but all the kaleidoscopic evidences of plant response and activities and history working in harmony, or, as we shall see presently, sometimes in violent conflict, and leaving as the result the wonderfully varied vegetation that now covers the earth. If we could read aright the story of which the vegetation of any particular country is the silent narrator, it would tell us not only what happened in the past but what is likely to happen in the future.

Plants, by what amounts to a kind of fatality, are rooted to the spot where they grow so that, unlike animals, their rapid distribution appears to be almost impossible, and yet the tremendous distances that some species have traveled seem like a pretty successful protest against the fact of the anchorage of individuals to the point of their origin. It is more than a successful protest, for it amounts in many species to an active campaign for dominance, to the exclusion or extinction of less aggressive neighbors, so that in any field or meadow or forest there are silent struggles constantly going on. Some of these are so inexorable in their results that they change not only the frequency of occurrence of the individuals involved, but sometimes the whole type of vegetation.

The competition to occupy just as much of the favorable plant sites as possible has been much aided by many species possessing means for the dispersal of their seeds or fruits that are ingenious in the extreme. Some of these are written plainly enough in the structure of the seed and its wonderful adaptability for the peculiar conditions to which it will be subjected. Before considering some of these structures we may profitably see how some plants look after the dispersal of their seeds within their own limited sphere of action.

In hundreds of plants the ripened pods, instead of being erect as their flowers have been, are pointed downward about the time the seeds are ready to be released, and their harvest is sown, sometimes by deliberate movements, in the immediate vicinity. No great areas are captured by such plants, except by the slow process of successive generations extending their range a few inches or at most a few feet a year. The great bulk of all seeds never do grow into new plants, but in those that only shed their seeds close to the parent plant the opportunity to reach new sites is by that much restricted. The chance of the species getting very far afield except by slow invasion of the neighboring region is limited. A few of such plants show remarkable ingenuity in reaching the utmost distance possible, perhaps the most effective cases being those that shoot their seeds by explosive bursting of their pods. Nearly all the violets do this, often shooting seeds several feet from the parent plant. Many plants of the pea family have pods that are twisted, which upon splitting release the previously pinched seeds so suddenly that they are shot considerable distances. In the common witch-hazel ([Figure 111]), the seed is shot through the air often as much as thirty feet.

But with even the greatest ingenuity and the most explosive bursting of pods, most plants could never capture much new ground, and their very existence as a species is often contingent on their ability to spread, if these various methods by which plants shed their seeds were not aided by outside help. Some of these have had conspicuous and almost startling results.