No. 435.] ANTITHETIC VERSUS HOMOLOGOUS. 157 



As yet we have no evidence beyond mere hypothesis as to the 

 way in which the motile zoospores, arising; from the zygote of 

 the algal ancestors of the Bryophytes, gradually were replaced 

 by the tetrads of non-motile spores which characterize all Arche- 

 goniates. The same difficulty is met with in tracing the origin 

 of the sexual organs. In both respects there is much greater 

 difference between the humblest liverwort and the highest alga 

 than there is between the former and the most highly organized 

 fern. It must be confessed that the gap between all existing 

 Algae and Archegoniates is very great. 



When we compare the two series of Archegoniates, the case 

 is very different. The structure of the archegonium and spores 

 is identical throughout, and the early stages of the sporophyte 

 agree very closely, this being especially true of the more primi- 

 tive types of Pteridophytes. In these the young sporophyte 

 remains very much longer dependent upon the gametophyte and 

 the external organs which characterize the Pteridophytes, are 

 relatively late in making their appearance. Both of these facts 

 point to a nearer approach to the bryophytic type in the lower 

 Pteridophytes, a fact which is not readily explicable on the 

 assumption that they are in no way connected with the 

 Bryophytes. In a number of the lower Pteridophytes, c. g. t 

 Marattia Lycopodium, Botrychium, the young sporophyte may 

 remain attached to the gametophyte for months, or even years, 

 long after it has passed beyond the embryonic stage. 



If we compare the gametophyte and sporophyte of any typical 

 Archegoniate, we note a very significant difference in their rela- 

 tion to the water-supply. The gametophyte is always, to a 

 greater or less degree, an aquatic organism, never quite emanci- 

 pating itself from the life conditions of its algal ancestors. 1 he 

 sporophyte, on the contrary, at least in its earlier stages, is never 

 exposed directly to the water, although a few groups ,.t IVn 

 dophytes have developed, secondarily, aquatic s ^ ,P ' r '- U ^ 

 Almost from the first the sporophyte is pr " 

 massive absorbent organ, the foot, which 

 by the true roots of the Pteridophytes, a much more emeicm 

 means of obtaining water than is provided by the rhizoids o 

 the gametophyte; and the unlimited capacity for growth of 



mperscded 



