There are many other kinds of mosses than sphagnum, and their life histories differ in slight degrees from it. But they all agree in this, that the greenish, feathery little moss plant is a stage in its life history bearing male and female cells, the mating of which produces a spore-bearing contrivance. In most of the familiar green mosses this is a capsulelike body on a short stalk, usually well elevated above the green mass of plants. From this the spores are shed and develop into a protonema or “first thread” just as in ferns. Unlike them, and unlike sphagnum, the green mosses produce no thallus, and the young leaves of the moss are developed directly from this protonema.
LIFE HISTORY OF A MUSHROOM
The common mushroom that we eat is easily enough divided into a thick stalk, known as a stipe, and a broad hood called a pileus. The under side of the pileus is seen to be composed of thin plaits set closely together and radiating from the center toward the edge. These are known as gills. From among the gills the spores are shed when they are mature, usually foretold by the changing of the color of the gills from whitish to purplish and even to brown or blackish. The spores are then shed and ready for the next stage. From what we already know about ferns and mosses, it is clear that from these spores a mushroom cannot develop without the production of male and female cells and all the rest of that process of hidden marriage that characterizes all flowerless plants. But in most mushrooms no one has ever seen, nor have the most carefully conducted experiments ever demonstrated the germination of the spore. So far as we know at the present, many mushrooms may or may not produce their young through the germination of their spores in their native fields and meadows and the subsequent production of male and female reproductive organs. But if their spores do produce such organs, which all our knowledge of spores makes probable, it is, in a truer sense than in most cryptogams, a case of hidden marriage. The process of producing their young is thus a secret one that scientists have not yet been able to disclose. Of course it is a common practice of mushroom growers to purchase spawn from seedsmen which under favorable conditions will produce many young mushroom plants. This, however, is the production of young without mating of the sexes, a fairly common characteristic of many other plants which will be considered presently.
As we saw in the section devoted to Flowerless Plants in Chapter I, there are many other kinds of fungi than the familiar edible mushroom and their close relatives, the often deadly poisonous toadstools. The reproductive processes in these other fungi are fairly well understood, but they can hardly be included here. In the mold on bread, the yeast used in baking, the rust of wheat and the diseases of other plants and of animals, the individual organism is so minute that it can only be detected under the microscope. Their reproductive processes are, of course, on such a minute scale that they could be followed with profit only by those equipped to study them. They have been described in many botanical textbooks, and those interested in them should consult such books.
In recapitulating the reproductive processes in cryptogamous plants the thing that distinguishes them from all flowering plants is that they bear, in some stage of their life history, a spore. From this, in the great bulk of them, a mature plant never develops. Only by the production from the spore of some contrivance for bearing male and female cells, which may, as in some seaweeds, even be on different plants, can a mating of these be accomplished, and from this union will develop the mature plant. There are many modifications of this plan, but in nearly all of them the presence of water, for the free swimming of the male cell to its mate, is essential. Just as in flowering plants and in all the larger animals, however, the reproduction of young in cryptogams is a sexual process depending on the union of male and female. While in phanerogams that process may well be spoken of as visible marriage, with all the pageantry of insects and beautifully colored flowers, in cryptogams the process is not only a hidden marriage, its ways are sometimes so secret that, even in the common mushroom, the actual mating is conjectured rather than demonstrated.
THE PRODUCTION OF YOUNG PLANTS WITHOUT MATING
It is so generally true in all plants that a union of male and female is necessary for the production of young, and, as we have seen in most of them, the process is so uniformly successful that still another mode of producing them seems almost unnecessary. Yet in a surprisingly large number of plants new individuals, both of flowering and flowerless plants, are regularly produced without such a union and where sexuality has nothing to do with the increase.
In the life plant—a thick-leaved shrub from Mexico commonly grown in greenhouses—the leaves are wavy margined. From their edges, especially when injured, many tiny new plants will often start to grow. Even if the leaf is cut up into fairly small pieces many of these will develop young plants, and in various forms of the common rex begonia the leaves are usually cut into small pieces by gardeners for the production of young plants which always sprout from such pieces. It is useless to multiply such cases, as everyone knows of the production of young plants from the ends of strawberry runners, the cutting up of potatoes, the universal garden practice of making cuttings, and the sprouting of willows, all of which are effective by virtue of this faculty of plants to produce young quite without the intervention of different sexes. Not so well known are the cases of a liverwort, a small relative of the mosses, which, if chopped into fine pieces, each will develop into a new plant. We have already spoken of the spawn of mushrooms; and even on sphagnum moss, in addition to its sexual reproduction, it produces sterile branches that will root and, after separation from the old plant, form a new one.
Wherever this tendency is found, whether it be in a microscopic seaweed, some of which know no other means of reproduction, or in the showy begonia, it depends for its success upon a property of the ultimate unit of its structure, the cell. Sometimes, as in bacteria or the most minute seaweeds and in some other kinds, the whole plant consists of a single microscopic cell, when it is said to be a unicellular plant. All others, in which the grouping or modifications of the cell makes more complex structures, such as trees or shrubs and all the plants that grow, both flowering and flowerless, are called multicellular plants. Whether they be of one or many cells, these have the faculty of dividing, and by this division making two where one existed before the division. This division of cells is what happens in the normal growth of plants and it is this division, in more unusual ways, that results in the production of new plants without mating of the sexes. As cells are themselves microscopic, of course their division is equally so, and cannot be described in detail here. It has been many times described and pictured both in books on plants and animals, as it is the ultimate unit of the structure of both.
Plant life, then, seems to be better provided with means to renew itself than most animals, for, as we have seen, it has several methods to rely on. These may be divided into sexual, which includes both that in flowering plants with their visible mating and in flowerless plants with invisible mating, and asexual, literally without sex. In the latter are all those unicellular plants that reproduce themselves by simple division of the cell, and also those flowering plants that either naturally, as in life plant, or by the gardener’s art of making cuttings, produce new plants quite without the intervention of the sexes. Whether it be sexual or asexual, nature has more than fulfilled its obligation to the plant world in providing it opportunities for self-renewal. No matter what apparently unfavorable condition arises and often in spite of an almost unbelievable wastage of potential life stuff, the renewal goes on, or else there is the total disappearance of the species. So strong is this tendency to provide for renewal of their kind that many plants, if injured or cut by a mower, will almost in their last gasp hurriedly flower and set seeds, and we have already seen that the little liverwort, even if cut to pieces, also obeys that nearly universal law of nature: “Be fruitful and multiply.”