Rafflesia. One of the Strangest Products of the Rain Forest. It consists only of a giant flower, the largest in the world, which is attached directly to the roots or stems of relatives of the grape, upon which it is parasitic. (After Kerner and Oliver. Courtesy of Brooklyn Botanic Garden.)
autophytes, literally solitary or self-providing plants, and this thrifty mode of life is called autophytic. But a few kinds of plants, actually many millions of individuals, have more devious ways of getting their food and provide strong contrast to their sturdier associates.
These baser modes of life appear to have been rather insidiously developed, as though there had been some hesitation at even the smallest departure from the normal. Of course we must not forget that plants, while living things, are never reasoning ones, and that good and evil and all other qualities that are ascribed to plants are perfectly foreign to them. Throughout this book, and in many others, the habits of plants are spoken of as base, for instance, or good. What is actually the fact is that nature works in truly marvelous ways, and to our reasoning faculties these adjustments seem clothed with attributes they do not really possess. But the description of them in the terms of our everyday speech, the translation of their behavior into the current conceptions of mankind, does so fix them in our minds that they cease to be “just plants,” and we no longer put their habits in the category of those interesting things that nearly everyone forgets.
One of the first signs of departure from the usual methods of getting food is the association of certain minute organisms at the roots upon which plants, otherwise autophytic, depend for aid in securing nourishment. This characteristic is fairly common, notably in all the plants of the pea family, such as peas, beans, locust trees, vetch, clover, and hundreds of others. If the roots of any of these be examined, it will be seen that attached to the smaller divisions of them are small tubercles from the size of a pinhead to a pea, depending on the kind. These tubercles or galls are caused by and infested with bacteria, the smallest of all plants. The bacteria have the extraordinary power of changing nitrogen into nitrates, which is the only form in which nitrogen can be absorbed by roots. Not only do they accomplish this, but excess nitrogen is stored in the roots by the same agency. It is this fact that has resulted in the planting of vetch and kindred plants for soil enrichment, as each year there is a residue of nitrogen left in their roots and by so much they add plant food to the soil. For hundreds of years farmers have done this, but only quite recently have we known why they did so. The occurrence of bacteria or microbes at the roots of plants is much more common than was formerly supposed to be the case, and many other plants than those of the pea family depend, at least in part, upon them in getting food from the soil. While not wholly autophytic, such plants do make some return for what they gain, as some of them at least pay dividends in extra nitrogen, and all of them provide opportunity for the bacteria to live. The latter play an important part in populating the soil, which is not the comparatively sterile thing it appears to be. Actually it is infested with organisms that play a mighty, if rather inconspicuous, part in the work of preparing the soil for plant growth. These organisms are so minute and the chemical nature of their work is so complicated that merely to mention their existence must suffice here. This close association of certain roots and bacteria, which, as we have seen, is of mutual advantage, is known as symbiosis. It is really only a kind of exchange, not unlike the storybook community that helped out by taking in each other’s washing. Unlike that community the association between the two works to the actual advantage of both, but the process is undeniably a step away from those wholly autophytic plants which live free and independent of such aid.
A much more gruesome habit of certain plants is their reliance for food only upon the dead. In the Indian pipe, some kinds of shinleaf, and in many other plants their roots and root hairs are changed or often nearly lacking, and we find them growing only on the dead bodies of other plants. One peculiarly repulsive characteristic of such plants is that they secrete at their roots a substance that hastens the decay of the dead, and, as if this were not rapid enough, there are associated with them certain kinds of minute fungus organisms that also speed up decomposition. Plants with this charming mode of life are known as saprophytes, literally sapros, rotten, and phytes, plants. “Rotten plants” they may be in their mode of life, but the pearly white stems and flowers of the Indian pipe have a certain ghostly charm, an almost statuesque beauty among the normal greenery of the gloomy dark woods in which they always grow. It is not without significance that Indian pipe bears no leaves, has none or almost none of the life-giving green coloring matter which we have seen to be the almost priceless possession of plants which lead a different, and perhaps a better life. The great bulk of saprophytes bear no leaves, and some only partially wedded to the habit appear to be midway between bearing normal green leaves and bearing none, or much reduced ones that are quite unlike the busy factories we know normal green leaves to be. Plants with this method of getting their food, must of course grow in places where dead and decaying vegetation is plentiful, and often as such soil is turned up there may be noted a peculiar dank odor, suggestive not only of its origin, but of the fact that these “rotten plants” make their home in it. Some of our most beautiful orchids grow in this fashion, but even there, in spite of flowers that for beauty of form are without rivals, the plants have no green coloring matter in their leaves, which are often reduced or even wanting altogether.
It might almost seem as if demoralization, so far as food habits are concerned, had reached its lowest point in these plants that literally rob the dead, but there are still lower depths to which certain plants have been reduced. This consists of robbing the living, and such plants are called parasites, a word perfectly familiar in other connections. Parasitic plants have no roots, but attach themselves to the roots of other plants, somewhat generously called hosts, from which they derive their food. The best known case is the common Christmas mistletoe, and the dodder ([Figure 68]), but there are hundreds of others. Nothing in all the realm of plant life so perfectly fits the action to the word as plants of this type, flourishing when the host flourishes, dying when it dies. Producing flowers and seeds, and often, by an irony of fate, perfectly green leaves, they are nevertheless the most debased of all plants in their mode of life.
These successive steps in the degradation of food habits, are not always the clean-cut things they might be inferred to be from the foregoing. There are many intermediate stages; it may even prove to be the case that some plants are wholly autophytic at certain stages of their life, and slip partially into more devious practices at other stages. The whole affair is not yet thoroughly understood and may well be the result of competition, as it is quite conceivable that if the getting of food in normal ways became difficult or impossible plants may have had to resort to other methods.