THALLOPHYTA: ALGAE 



89 



The reduction of chromosomes occurs just after fertihzation, when the 

 fusion nucleus in the carpogonium divides. Thus the gonimoblasts, 

 carpospores, and sexual plants are hapioid. There is no true alternation 

 of generations. 



Batrachospermum. This is a widely distributed fresh-water alga, 

 growing in streams attached to rocks along the bottom. The plants 

 are blue-green, olive green, violet, or reddish. The variation in color is 

 primarily a result of differences in light 

 intensity, plants growing in shallow water 

 being greener than those in deeper water. 

 Bafrachospcrnmm is related to Nemalion 

 but differs from it in several respects. 

 The vegetative body consists of long 

 branching filaments of unUmited growth 

 bearing whorls of dwarf branching fila- 

 ments of limited growth (Fig. 75). The 

 long filaments consist of an axial row of 

 cells which, in the older portions of the 

 body, is covered by a layer of small- 

 celled filaments that form a sheath 

 around it. The cells of the sheath arise 

 from the basal cells of the dwarf fila- 

 ments. Growth occurs by means of an 

 apical cell. 



The sex organs are borne at the ends 

 of some of the dwarf filaments and re- 

 semble those of Nemalion (Fig. 76). 

 After coming in contact with the trichogyne, the spermatium remains 

 uninucleate instead of becoming binucleate. Moreover, following fer- 

 tilization, the cells at the base of the carpogonium send out loose fila- 

 ments that grow up around and invest the cystocarp while the carpo- 

 spores are being produced. 



The germinating carpospore gives rise to a branching filamentous body 

 that is much simpler than the gamete-producing plant. This plant, 

 which represents a juvenile stage in the life cycle, may multiply by mono- 

 spores, which are formed singly within sporangia at the ends of short 

 lateral branches. Finally a special branch appears that becomes a 

 gamete-producing plant. As in Nemalion, the chromosome number is 

 reduced one-half when the fusion nucleus divides in the fertilized 

 carpogonium. Conseciuently, the juvenile plant is not a sporophyte and 

 there is no alternation of generations. 



Polysiphonia. Polysiphonia is a widespread genus of about 150 species. 

 It is abundant along the Atlantic coast of North America but is less com- 



FiG. 75. Small portion of the 

 vegetative body of Batrachospermum, 

 showing dwarf filaments arising in 

 whorls from the cylindrical main axis, 

 X400. 



