Taking the plant kingdom as a whole, it may be said that in the lowest plants only a gameto phyte existed. Presently a sporophyte began to appear, at first dependent upon the gametophyte, as in the mosses. hut presently attaining inde pendence and prominence, as in the ferns and seed plants. With the independence of the sporo phyte. the gametophyte became gradually re duced 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 hearing numerous spores, en eh one of which May form a new gametophyte, a single fertilized egg may result in many new plants. Flowerer, this may be but one of the incidental results of a differ entiation that is probably of far deeper biologi cal significance. Consult: Goebel, Outlines of Classification and Special Morphology of Plants, English translation by Garnsey and Balfour (Ox ford. ISST ) ; Vines, A Shulcuts' Text-book of Botai2y (New York, It:495) ; Coulter, Plant Struc tures (New York, 1900).
_VmoNn _AtnEALs. In the simplest case of alternation of generations among animals, the successive generations differ only slightly. Thus, in many butterflies having two broods (hiring 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 sea soma.
The next example shows a greater difference between alternating generations. In a certain nematode (Leptorlera nigrorcnosum), both males and females are found living in pools of water or in damp earth. These pair. and the fertilized eggs develop into larva that enter into the bodies of aquatic animals and develop there as para sites, not as male and female individuals. but as hermaphrodites. These lay self-fertilized eggs, which develop free as either males or females. Here a diceeions generation (A) alternates with an hermaphroditic generation (B). This kind of alternation of generations is called heterogong.
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 diweions, sexual individuals. 170 this class belong many eases of parthenogenetically repro ducing species; among flat-worms, trematodes; among crustacea, the Clado•era; and among in sects, aphids, such as Phylloxera, Chermes, etc. In most of these eases there is a marked differ ence in form between the individuals of the diwcious and the parthenogenetically reproducing generation. This class of eases is called hetero genesis. See Hoe-LOUSE.
In the fourth form of alternation, the ferti lized egg develops into a generation (_\) 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 di(W.i01.1.3 and sets free zygotes. from which generation A is produced. There are numerous examples of this class among animals, e.g., among coelenterates, the Hydrozoa and some Seyphozoa and Strohila ; among flat •orms, certain eestodes (Echinoeoecus) ; among Annelids, certain SyllicIfe and aquatic Oligo clueta; among tunicates, the Salpte and Hobo lid e. This class of alternation of generations has been called metagenesis. As an example of it, one of the Hydrozoa may be taken. The free swimming jellyfishes are ditrcious, and produce the male and female gametes, which unite in the water. The larvie which result from the development of the eggs settle down, become attached and develop into a hydroid, which pro duces a colony by budding. Certain buds are set free as jellyfishes. and these in turn set free the gametes. Thus the hydroid form (A) and the jelly form (B) alternate with each other.
In certain respects the gamete-producing gen eration of spermatophytes among plants (see above) resembles the maturation period preceding time formation of gametes in the higher animals; and it has been suggested that even in verte brates, including man, we have an alternation of generations: (1) the non-sexual generation beginning with fertilization and ending with the primary oiicyte or primary spermatoeyte; (2) the sexual generation beginning with the pri mary germ-cell and ending with the ripe germ eell (two-cell division).