220 THE 



than the green or brown algae because of the complicated sexual 

 organs and life histories and the absence of motile stages repre- 

 sented by the zoospores and motile gametes of those groups. 

 The red algae resemble ColeocJicete (Sec. 222) in a number of 

 features, and it is possible that this type may be rather close to 

 the starting point of the group. The peculiar structure of the 

 female sexual organ (carpogonium), which is really anoogonium 

 with a receptive organ (trichogyne), is undoubtedly associated 

 with the loss of motility on the part of the sperms. But the 

 most remarkable peculiarity is the development from the ferti- 

 lized carpogonium of a tissue which produces carpospores. This 

 structure (from the fertilized carpogonium to the carpospores) 

 is a new phase in the life history of algae, and together with 

 protective envelopes, when present, constitutes the cystocarp. It 

 is probable that the asexual tetrasporic plants found in most 

 species of the red algae arise from the carpospores, and that the 

 sexual plants in these types are developed from the tetraspores. 

 The structure developing from the fertilized carpogonium and 

 ending with the carpospore, together with the tetrasporic plant, 

 when present, therefore forms an asexual phase in the life history, 

 alternating with the sexual plants. Such asexual phases are 

 called sporophytes (meaning spore-bearing plants), to distinguish 

 them from the sexual plants, called gametophytes (meaning 

 gamete-bearing plants), and their following one after another 

 in a life history constitutes an alternation of generations. Such 

 an alternation of generations is found in very few groups of 

 the thallophytes, but it is characteristic of the life histories 

 of all higher groups beginning with the liverworts and mosses 

 (Sec. 285). Its significance is discussed in Chapter xxvi. 



