apparent among the ferns and their allies (Pteridophytes); but among the seed plants (Spermatophytes), 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 stem or branches. By means of these sex organs a fertilized egg (oöspore) is formed. When the fertilized egg germinates, it produces the sporophyte, which in this case consists of a more or less elongated stalk (seta) bearing at its summit a capsule or spore case. The leafless sporophyte is anchored in the leafy gametophyte by means of an organ called the foot. This peculiar sporophyte 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 cases are asexual, and upon germination produce new leafy plants (gametophytes).
In the case of the ordinary ferns, which belong to the great group Pteridophytes. the same phenomenon may be observed, but 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 flat body (prothallium) resembling a minute liverwort. Upon this prothallium the sex organs are developed 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 simple gametophytes.
Among certain Club mosses and other Pteridophytes 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 microspore upon germination produces a male gametophyte, i.e., a gametophyte which bears only male organs. The megaspore upon germination produces a female gametophyte, i.e., a gametophyte which bears only female organs. This differentiation of spores is spoken of as "heterospory," and all the higher plants are heterosporous. 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 consists of but two or three cells, which form within the pollen grain. The female gametophyte consists of more numerous cells, but they are entirely confined within the megaspore walls and hence never leave the ovule.
Taking the plant kingdom as a whole, it may be said that in the lowest plants only a gametophyte existed. Presently a sporophyte began to appear, at first dependent upon the gametophyte, as in the mosses, but presently attaining independence and prominence, as in the ferns and seed plants. With the independence of the sporophyte, the gametophyte became gradually reduced in size, until in the highest plants it is visible only under the special manipulation of the laboratory. The significance of alternation of generations in the plant kingdom is by no means clear. One of its results, however, is to multiply the product of a single fertilized egg. If there were no alternation of generations, one fertilized egg would result in a single new plant. By the interposition of a sporophyte bearing numerous spores, each one of which may form a new gametophyte, a single fertilized egg may result in many new plants. However, this may be but one of the incidental results of a differentiation that is probably of far deeper biological significance. Consult: Goebel, Outlines of Classification and Special Morphology of Plants, English translation by Garnsey and Balfour (Oxford, 1887); Vines, A Students' Text-book of Botany (New York, 1895); Coulter, Plant Structures (New York, 1900).
Among Animals. In the simplest case of alternation of generations among animals, the successive generations differ only slightly. Thus, in many butterflies having two broods during the year, the spring brood is unlike the fall brood: for example, in our "spring azure" the spring brood is of a violet color, and the fall brood is dark; both are sexual, but the spring and fall forms alternate in the series of generations. This form of alternation of generations is called seasonal.
The next example shows a greater difference between alternating generations. In a certain nematode (Leptodera nigrovenosum), both males and females are found living in pools of water or in damp earth. These pair, and the fertilized eggs develop into larvæ that enter into the bodies of aquatic animals and develop there as parasites, not as male and female individuals, but as hermaphrodites. These lay self-fertilized eggs, which develop free as either males or females. Here a diœcious generation (A) alternates with an hermaphroditic generation (B). This kind of alternation of generations is called heterogony.
In a third form of alternation of generations eggs are produced, but the fertilization of the egg is omitted from alternate or even several successive generations; these are then followed by diœcious, sexual individuals. To this class belong many cases of parthenogenetically reproducing species; among flat-worms, trematodes; among Crustacea, the Cladocera; and among insects, aphids, such as Phylloxera, Chermes, etc. In most of these cases there is a marked difference in form between the individuals of the diœcious and the parthenogenetically reproducing generation. This class of cases is called heterogenesis. See Hop-Louse.
In the fourth form of alternation, the fertilized egg develops into a generation (A) having a characteristic form, and capable of setting free neither eggs nor spermatozoa, but capable of forming buds. These buds develop into a new and different form of individual (generation B), which is diœcious and sets free zygotes, from which generation A is produced. There are numerous examples of this class among animals, e.g., among cœlenterates, the Hydrozoa and some Scyphozoa and Strobila; among flat-worms, certain cestodes (Echinococcus); among Annelids, certain Syllidæ and aquatic Oligochæta; among tunicates, the Salpæ and Dolio-
