266 SEEDLESS PLANTS 



separate existence, appears only as an obscure prothallium 

 that is usually as short lived as it is inconspicuous. ' 



375. Alternation of Generations in Seed Plants. — While 

 the alternation of generations is more conspicuous in 

 pteridophytes and bryophytes, it occurs also among the 

 algae, and is universal, though in a masked form, among 

 the spermatophytes. It is therefore very important to 

 have a clear idea of what it means, for the chief turning 

 points in the life history of all plants are connected with 

 it, and the natural relationships of the different groups 

 and their distribution according to those relationships 

 depend largely upon a comparison of the reproductive 

 processes in the various classes and orders. These studies 

 are too intricate and technical to be even outlined here ; 

 suffice it to say that some of the gymnosperms — pines, 

 yews, cycads, etc. — show striking similarities in their repro- 

 ductive processes to those of the higher pteridophytes, and 

 through them a repetition of the most salient features of 

 the alternation of generations in the highest seed plants 

 has been traced. Briefly stated, we may say that the 

 stamens of spermatophytes, and the pistils, or rather the 

 carpels, which we saw to be transformed leaves, represent 

 the sporophylls (Sec. 370) of the higher pteridophytes. 

 The pollen sacs and ovules are sporangia, bearing micro- 

 spores and megaspores (Sec. 370), represented respec- 

 tively by the pollen grains in the anther and the embryo 

 sac in the ovule. These go through a series of micro- 

 scopic changes in the body of the ovule analogous to the 

 production of the oospore in the archegonia of ferns and 

 liverworts, but the process is so obscure that to an ordinary 

 observer the pollen grains and ovule appear to be the real 

 gametes, and were supposed to be such, by the older bota- 

 nists. The fertilized germ cell in the embryo sac (Sec. 327) 

 corresponds to an oospore, the endosperm found in all 

 seeds (previous to its absorption by the cotyledons) is a 

 rudimentary gametophyte, and the embryo in the matured 

 seed, the undeveloped sporophyte, destined, after germina- 



