AMONG PLANTS. Alternation of generations is found in all forms of plants excepting the low est. though it is not very evident in the highest plants. One may get some coneeption of alter nation of generation. in plants by comparing it with the very different alternation of forms which occurs in the life history of a moth or butterfly. lm the plant, however, instead of having a series of forms which pass into one another, our plant larva forms an egg which produces the mature form. If in the life his tory of a butterfly the larva should lay eggs and thus produce the mature forms, we should have something resembling the alternation of genera tions in plants. One of these generations has sex organs, and hence is called the gametophyte; while the other generation has no sex organs, and is known as the sporophyte. Both genera Lions produce spores, hut in a very different way. By means of its sex organs the gametophyte produces spores in a sexual way, that is, by the fusion of two sex cells, and such spores are coiled in general oiispores, or fertilized eggs; while the sporophyte by ordinary cell division produces spores which are called asexual spores, meaning spores which have not been formed by sex organs. In the life history of the plant. the sexual spore of the gametophyte gives rise to the sporophyte, while the asexual spore of the sporophyte gives rise, in turn, to the gameto phyte, and so the alternation continues.
Alternation of generations is first manifested among the lowest plants (the thallophytes), hut it is not eompletely and clearlc- established until the liverworts and mosses (bryophytes) are reached. The phenomenon is still more apparent among the ferns and their allies (Pteri dophytes) : but among the seed plants (Sperma tophytes), while evident to the laboratory student, it is well-nigh invisible to the ordinary observer. It is to mosses and ferns, therefore, that one must go for the clearest examples of alternation of generations.
In an ordinary moss the gametophyte consists of the well-known leafy moss plant, which bears sex organs at the tips of its main stein or branches. By means of these sex organs a fer tilized egg (cilispore) is formed. When the fertilized egg germinates, it produces the sporo phyte. which in this ease consists of a more or less elongated stalk (seta) bearing at its sunnnit a capsule or spore ease. The leafless sporophyte
is anchored in the leafy gametophyte by means of an organ called the foot.. This peculiar sporo phyte of the moss is commonly spoken of as the fruit, and when it appears upon the leafy plants these are said to be "in fruit." The spores formed in the spore eases are asexual, and upon germination produce new leafy plants (gameto phytes).
In the case of the ordinary ferns, which belong to the great group Pteridophytes, the same phe nomenon may be observed, hut with a striking difference. In the mosses the prominent leafy plant is the gametophyte, while in the fern the conspicuous leafy plant is the sporophyte. The gametophyte of the fern is a simple tint body (prothallium) resembling a minute Upon this prothallimn the sex organs are devel oped and the fertilized eggs are formed. From these fertilized eggs the comparatively large leafy fern body arises. This leafy body (the sporophyte) produces, usually upon the under side of the leaves, numerous asexual spores, which upon germination give rise again to sim ple gametophytes.
Among certain Club mosses and other Pterido phytes the sporophyte produces two kinds of asexual spores. The most apparent differences between these spores is that of size, and hence they are called "microspores" (small spores) and "megaspores" (large spores). The micro spore upon germination produces a male gameto phyte, i.e., a gametophyte which bears only male organs. The megaspore upon germination pro duces a female gametophyte, i.e., a gametophyte which bears only female organs. This differen tiation of spores is spoken of as "heterospoi7,.." and all the higher plants are heterosporons. With the appearance of heterospory the alternation of generations passes out of the reach of ordinary observation, since the gametophytes are so much reduced as seldom to leave the spores which produce them. In a seed plant, for example, the whole visible body of the tree. shrub, or herb is a sporophyte; the pollen grains are the small asexual spores or microspores, while the so-called embryo sac in the ovule is the large asexual spore or megaspore. The male gametophyte con sists of hut two or three cells, which form within the pollen grain. The female gametophyte con sists of more numerous cells, but they are entirely confined within the megaspore walls and hence never leave the ovule.