CRYPTOGAMS 355 



analogies in the reproductive processes of the two groups 

 that connect them together as successive links in one continu- 

 ous chain of vegetable life. It is therefore very important 

 to have a clear understanding of the nature and meaning of 

 these processes, for the chief turning points in the life his- 

 tory of the different groups of plants are connected with 

 them, their natural relationships to each other, and their 

 distribution according to their respective places in the evolu- 

 tionary scale, being determined largely by a comparison of 

 their modes of continuing the life of the group. 



414. Alternation of generations in seed plants. — This 

 process, so conspicuous among Bryophytes and Pterido- 

 phytes, and not unknown among ThaDophytes, is universal 

 among seed plants (Spermatophytes) also, though in so 

 masked a form that it is not easy to recognize without a 

 more detailed study than would be practicable within the 

 limits of a book like this. Briefly, we may say that the 

 stamens of spermatophytes, and the pistils, or rather the 

 carpels, which we have seen to be transformed leaves (298), 

 represent the sporophylls (406) of the higher pteridophytes. 

 The pollen sacs and ovules are sporangia, bearing micro- 

 spores and megaspores (409), represented respectively by 

 the pollen grains in the anther and the embryo sac in the 

 ovule. These go through a series of microscopic 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 the ovule ippear to be the real gametes, and were 

 long supposed to be such. The fertilized germ cell in the 

 embryo sac (251) corresponds to an oospore ; the embryo sac 

 with the endosperm found in all seeds (previous to its absorp- 

 tion by the cotyledons) is a rudimentary gametophyte; and 

 the embryo in the matured seed is the undeveloped sporo- 

 phyte, destined, after germination and further growth, to 

 produce a new generation with its recurrent cycle of alternat- 

 ing phases. 



