THE GREAT GROUPS OF ALG^E 



259 



and reddish-brown) making them very attractive. They 

 show the greatest variety of forms, branching filaments, 

 ribbons, and filmy plates prevailing, sometimes branching 

 very profusely and delicately, and resembling mosses of 

 fine texture (Figs. 222, 223, 224, 225, 226). The differen- 

 tiation of the thallus into root and stem and leaf-like struc- 

 tures is also common, as in the Brown Algse. 



174. Reproduction. — Eed Algae are very peculiar in both 

 their asexual and sexual reproduction. A sporangium pro- 

 duces just four asexual spores, but they have no cilia and 

 no power of motion. They 

 can not be called zoospores, 

 therefore, and as each spo- 



FiG. 227. A red alga ( Callitkamnion). show- 

 ing sporangium (.4), and the tetraspores 

 discharged (^).— After Thuret. 



Fig. 228. A red alga (XefnaUon) : A, 

 sexual branches, showing antheri- 

 dia (a), oogonium (o) with its trich- 

 ogyne (0, to which are attached two 

 spermatia (s) ; B, beginning of a 

 cystocarp (o), the trichogyne (t) still 

 showing ; C. an almost mature cys- 

 tocarj) (o\ with the disorganizing 

 trichogyne (0-— After Kny. 



rangium always produces just 

 four, they have been called 

 tetrasj)ores (Fig. 227). 



Eed Algge are also heterog- 

 amous, but the sexual process has been so much and so 

 variously modified that it is very poorly understood. The 

 antheridia (Fig. 228, A, a) develop sperms which, like the 

 tetraspores, have no cilia and no power of motion. To dis- 



