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In the vegetable kingdom a parallelism may be traced between the development of the individual and the evolution of the type, somewhat similar to that observed in the animal.
The lowest plants, like the lowest animals, are unicellular, or at most colonies of a few similar cells. The first step in advance consists in the aggregation of these into definite groups, the plant-body, some of whose members serve one office, some another. Among the lowest plants with such a body are the filamentous algæ. Other species, little higher in the scale, have their cells arranged in flat, often more or less branched fronds. A spore from either of these reproduces the form which bore it, and which persists through the life of the plant.
In many of the liverworts, which stand higher in the series, a spore in germinating produces a thalloid plant, which, after vegetating for a considerable time, bears sexual organs. By their mutual action these give rise to a body smaller than the first, which lives but a short time, and in which the spores (reproductive bodies calculated for dissemination) are formed. This body, the sporogonium, though it maintains its connection with the parent plant, is in reality a distinct organism. In the life-history of the liverworts there are, therefore, two well-marked phases, one of which invariably takes its origin from the other: in other words, these plants possess an alternation of generations, a thalloid sexual form alternating with a non-sexual form of even a slighter degree of organization.
The spore of a mass in germination produces a confervoid organism very similar at first to the filamentous algæ, but which, in its active vegetative life, ultimately buds into a plant-body having stem and leaves, as in the higher plants. This is the sexual generation of the mass; and from the union of its reproductive cells a sporogonium results, which lives but a short time, and ends by bearing the non
In each of these groups of plants the sexual generation is the most highly developed, and is the most conspicuous, part of the plant; the non-sexual sporogonium being apparently an outgrowth from it. In the more highly organized vascular cryptogams there is a similar alternation of generations ; but the non-sexual plant (bearing the spores) very much exceeds the sexual plant, which is commonly thalloid and of slight development, never reaching the proportions of the larger liverworts, and in some of the groups being developed only sufficiently to bear the reproductive organs.
Two examples may illustrate this. In the ferns
the spore gives rise to a small prothallus, very similar to the thalloid liverwort, but seldom exceeding a quarter of an inch in any of its dimensions, and living but a short time before its vegetation is cut short by the early development of sexual organs, from the interaction of which there results a perennial plant, the fern, which in some tropical species reaches the proportions of a tree. This, the nonsexual generation, again bears spores, which in germinating develop prothalli.
In the second group of vascular cryptogams chosen for illustration (the rhizocarpex), there are two sorts of spores, — large ones, the macrospores, and small ones, the microspores. In germination they produce microscopic prothalli, which in some species result merely from the segmentation of the spores, with scarcely any increase in their size; so that the sexual generation is barely large enough to burst the wall of the spore, after doing which its career is soon terminated by the precocious development of reproductive organs. The prothalli from macrospores are uniformly female : those from microspores are constantly male. On the other hand, the nonsexual generation which results from the process of fertilization is of considerable size and of complex structure.
The flowering plants, to which an observer's attention is most frequently turned, show the same alternation of generations, but attended by a still greater precocity, and consequently an inferior development, of the sexual generation. As a result of the different mode of fertilization in these plants, the macrospore remains attached to the tissues in which it is produced, not only until the process of fertilization has been accomplished, but even until the resulting non-sexual plant has passed through the earlier stages of its embryonic development.
Pines and other gymnosperms afford a connecting link between the higher cryptogams and the more highly specialized flowering plants. In their flowers the macro- and micro-spores are represented respectively by a large cell of the ovule (the embryo sac) and the pollen grain. Each of these produces a cellular prothallus in what may be called its germination ; but the sexual plants (the endosperm and pollen tube) never assume large proportions. Even in some species in which the macrospore and its prothallus are larger than in ferns, the latter is not noticed as a distinct plant, because of its concealment in the ovule.
The great mass of flowering plants (monocotyledons and dicotyledons) show even a slighiter development of the female prothallus, which consists of only a few naked cells (embryo vesicle, antipodal cells, etc.) enclosed in the embryo sac, or macrospore, and a similar incompleteness in the male prothallus, which is reduced to a few nuclei in the pollen grain (microspore), or its tube.
From this brief account, it will be seen that the simple permanent condition of the algae and fungi
persists in all of the higher plants as a more and more fugitive stage in their existence; its arrested development being correlated with the increasing precocity in the performance of the sexual function with which it is charged. In the mosses and liverworts it is still most important; but in the vascular cryptogams it successively descends from a structure of small size, but still self-supporting, to one of no significance, except for its reproductive office, which it executes at the expense of food stored in it by the correspondingly important non-sexual generation. Finally, in the flowering plants, even its individual existence is apparently merged in that of the alternating stage, a part of which, the flower, has even appropriated its title of the sexual organ, so completely is its nature as a separate plant concealed.