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 Thallophytes, 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 
pospore in the archegonia of ferns and liverworts, but the 
process is so obscure that to an ordinary observer the pollen 
grains and the ovule appear to be the real gametes, and were 
long supposed to be such. The fertilized germ cell in the 
embryo sac (251) corresponds to an odspore ; 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. 
