If leaves did not perform this most important function to perfection, all animals, including man as well, would perish, and it would almost seem that their obligation to us and the plant world might stop there as long as their success in reaching the light is so overwhelming. But there are no union hours of labor, no regulation as to the kind of work leaves may perform, and some actually reach out for new tasks to do, and do them. In one, our common pitcher plant, the leaf, as is implied by the name, is formed into a slender hollow pitcher, wide at the mouth, but narrow at the base. Inside the pitcher are slender downward-pointing hairs so arranged that an insect may crawl in, but never out. The lower down the luckless insect gets the more certain is its death, and, to clinch matters, there is a tiny pool at the bottom where it is not only drowned, but, due to the composition of the mixture in the pool, digested. Only a very few plants can do this; only a minute fraction of the world’s vegetation can digest animal matter. Some experiments on the pitcher plant, which grows in bogs, show that it will digest bits of beefsteak dropped into the liquid at the base of the pitcher.
In the East Indies and in Africa there is a pitcher plant—in fact, scores of varieties of them—which grows up on the branches of trees. In this case the pitcher may be as long as some of our American kinds, often twelve to eighteen inches, and many of them are attached to a slender leafstalk two to three feet long, by which they hang suspended. Insects, literally by the thousands, are caught in these gaudy traps, for many of the pitchers are beautifully colored, and near the opening they secrete a sweetish liquid that lures their prey. They are, in fact, such curious and handsome plants that they are commonly grown in greenhouse collections.
Nature sometimes finds still other ways of using strange and curious-shaped leaves, and in our American bogs is a group of plants, also insect digesters, still more unusual than the pitcher plants. In bright sunny places in open bogs one may often find small reddish, glistening plants, called sundews, usually only a few inches tall, covered with sticky hairs. In fact, the glistening is due to the secretion of the sticky substance, a tiny drop of which may be found at the end of each hair. Flying insects are caught in these leaves, and, as a fly on fly paper, the greater the struggle the more involved does the insect become among the sticky threads. Once caught by such a plant, escape is practically impossible.
Lying in ambush for chance insects, as these sundews and pitcher plants do, may seem nearly the limit of what is to be expected of leaves. Merely to be always on the job, with a plentiful supply of insect digester, might seem to be all that could well be expected from what, after all, are only modified leaves. But nature’s devices are infinite, and there are still other ways to accomplish the apparently impossible. In a small section of the southeastern States there grows a plant that not only lies passively in wait for insects, but actually captures them. This flycatcher, known as Venus’s flytrap, has two valves to the leaf blade, supported on a stout broad stalk so arranged that their fringed surfaces face each other. If an insect—and many do—alights between these valves, they close together rapidly and the prisoner is hopelessly caught by the interlocking marginal bristles that fringe each valve. In this case there are glands on the face of the valves, against which the live insect is tightly pressed, and which secrete a digestive fluid. When nothing remains the valves slowly open and are ready for the next victim. They may be made to close by slight irritation with a lead pencil, and it is the impact of the insect that releases one of the most curious examples of movement in leaves known to us. There are a few other plants in different parts of the world that by still other modifications of their leaves catch and digest insects, but none of them are to be considered as “insect eaters,” or other names implying that they have definite designs on the life of passing insects. The process is sufficiently remarkable, the success of the operation so sure, that there is nothing gained by attributing to such plants, as many have done in the past, malignant characters that are possibly confined only to man. The whole wonderful process is more reasonably explained by realizing that all these insectivorous plants are so by virtue of necessity, that many of them are bog plants, which are often hard put to it to get suitable food, and that the extraordinary change of shape and function is but one more contribution of leaves to the economy of nature.
In dry or desert regions, where the conservation of moisture is essential to plant growth, water storage by leaves is nearly as great an aid to the plant as we have seen it to be in the stems of cactus, South African spurges, etc. Our common century plant, whose leaves are, in some kinds, a hundred times thicker than in ordinary foliage leaves, is a good example of leaves adapted to water storage. In our southwestern deserts hundreds of species of plants can exist only by virtue of the fact that their leaves are so changed in their form or structure that they serve as reservoirs for water storage. This may be accomplished by thickening, or it is more often contrived by a thick coating of hairs. The surfaces of thousands of different kinds of leaves are clothed with hairs either on the upper or lower side, or sometimes on both sides. In many cases they are quite obviously protection from too rapid drying out of the leaf. In others, as in the nettle, the hairs secrete a stinging substance which seems to insure the plant against grazing animals.
Leaves, then, are for something more than to provide the beautiful foliage which is their most spectacular accomplishment. So varied is this in its beauty, from plain green leaves to the wonderful coloring found in begonias, coleus, and many other garden plants, that the sheer beauty of the panorama of foliage is likely to blind us to the more important uses of leaves. First of all must we consider them the factories, in which night and day are produced the food of all plants and most animals. Then in certain cases we have seen that, by every ingenious device known to nature, they perform other special work, such as helping the plant to climb where that is necessary, catching or even capturing insects and digesting them when that peculiar service is demanded of them, and, finally, serving as storage reservoirs in regions where water is scarce. Probably no part of the plant works so unceasingly each season at its varied tasks. In the autumn, dropping to the forest floor, its decomposition furnishes still other food for the plant, and, to crown all, this busy life and by no means unprofitable death leaves behind it, as a promise for the continuance of the work, a snugly protected leaf bud which will repeat the process the next season.
THE FLOWER
While the plant’s and, consequently, our debt to the leaf is seen to be tremendous, it cannot be ignored that, if plants produced nothing but leaves, the end of all plant life would come with the death from old age or disease of the present generation of plants. Except for those kinds that reproduce themselves by division or extension of their rootstocks, which bear buds, there would be no provision for increase. As only a comparatively small number of plants can reproduce by this method, it is obvious that something more must be provided to secure new generations of plants. Flowers, and the fruits and seeds which inevitably follow them, do this. All plants, with some exceptions to be noted later, produce flowers at some time in their life. In the case of the century plant, only once, after which they die. But except for ferns, mushrooms, seaweed, yeast, bacteria, and some other forms of so-called flowerless plants, a flower or blossom is to be found at some stage in the life of all plants.
If we examine the leaves of a goldenrod, we find that they are large below and diminish in size toward the top. Just below and among the flower clusters they are so much reduced in size and often changed in color that they cease to be ordinary foliage leaves, and are known as bracts. The occurrence of bracts is nearly universal in flowering plants, and they form not only an apparently transitional stage between leaves and flowers, but an actual one.
In a complete and perfect flower there are, at the bottom of it, a row of green leaflike sheaths which