homologous Alternation of Generations in Plants . 349 
Taking first, as the most prominent example, the Fern, we 
shall enquire what is the meaning of the alternation as we see 
it there. The gametophyte or prothallus of Ferns shows in 
its more delicate structure and its habit, as well as in the way 
in which the sexual process is effected, an adaptation to moist 
conditions, under which it grows best, while its ultimate func- 
tion, that of sexual reproduction, cannot be carried out without 
the presence of external fluid water : it is, in fact, typically 
semi-aquatic in its nature, sharing its main characters with the 
Algae from which we have every reason to believe that the 
land-flora originated. The sporophyte, on the other hand, is 
fitted by its more robust texture as well as by its differentia- 
tion of tissues for successfully enduring exposure to the air 
under comparatively dry circumstances, while dry weather is 
important for the dispersal of the spores which it is the final 
function of the sporophyte to produce : thus the Fern, as we 
normally see it, is an organism with, so to speak, one foot in 
the water, the other on land. 
Calling in also such evidence from phylogeny as we can 
command, it will, I think, be generally accepted that the 
gametophyte is the older and pre-existent generation 1 ; it 
corresponds to the gametophyte as seen in the Liverworts, 
or in the green Algae, and if we trace the descent of the 
great archegoniate series from some green Algal forms, we 
may recognise that the gametophyte of the Ferns retains 
the chief Algal characters, as regards both its texture and its 
sexual process. The sporophyte, on the other hand, is the 
younger generation : among the present green Algae, which 
must undoubtedly have been in some measure related to 
the progenitors of the Archegoniatae, there is hardly any 
body strictly comparable to the sporophyte, nor is it to 
be expected that there should be, if as above stated the 
sporophyte is typically sub-aerial in its characters, while the 
green Algae are typically aquatic. A comparison of the 
successive families of the archegoniate series demonstrates 
1 This view was definitely stated by A. Braun, Ber. d. k. Akad. zu Berlin, 
1875, P- 2 97 - 
