56 



PLANT MORPHOLOGY 



formation of zoospores. Thus, as in Ulva, the life cycle of Cladophora 

 involves a distinct alternation of generations of the isomorphic type. 



Sphaeroplea. This is a fresh-water alga that grows in wet meadows 

 and occasionally in pools. Although widely distributed, it is not com- 

 mon. The vegetative body consists of an unbranched filament with very 

 long cylindrical cells, each containing numerous nuclei and chloroplasts 

 (Fig. 44). The chloroplasts, some of which have pyrenoids, are parietally 

 placed and grouped into wide annular bands of cytoplasm separated by 



W m m 



ii.!.iJU4^j5Sjp*.,uiu...r,i 





r 



-^ — ■ I n'* 





Fig. 44. Sphaeroplea annulina, X400. A, portion of a vegetative cell with ring-like 

 bands of cytoplasm containing many small nuclei, chloroplasts, and pyrenoids; B, anther- 

 idia producing sperms; C, portion of an oogonium with nianj- eggs ready for fertilization. 



wide vacuoles. The vegetative cells of Sphaeroplea do not produce any 

 zoospores. Sexual reproduction is heterogamous, the two kinds of sex 

 organs usually being borne in different filaments. Any vegetative cell, 

 without undergoing a change in shape, may become an antheridium or an 

 oogonium. The antheridium produces a large number of small biciliate 

 sperms, while the oogonium gives rise to many large nonmotile eggs. 

 The eggs are at first multinucleate, but later all the nuclei degenerate 

 except one. The sperms escape through small pores in the cell wall, 

 enter the oogonium through similar pores, and there fertilization takes 

 place. Each zygote becomes thick-walled and, after undergoing a long 

 resting period, gives rise usually to four biciliate zoospores. Each of 

 these forms a new filament. The reduction of chromosomes occurs when 

 the zygote germinates, and so the spores and vegetative filaments are 

 haploid. 



