THALLOPHYTA: ALGAE 39 



and modified to serve as a holdfast, but all the other cells are alike, 

 being shortly cylindrical. Each contains a central nucleus and a periph- 

 eral band-like chloroplast usually with many pyrenoids. The chloro- 

 plast may form either a complete or a partial band. Any cell in the 

 filament, except the basal one, may divide by the formation of a cross 

 wall between two daughter protoplasts, thus resulting in growth of the 

 filament. 



In asexual reproduction, the contents of any vegetative cell, except 

 the holdfast, may divide to form mostly 2, 4, 8, or 16 zoospores (Fig. 21 B). 

 These escape through a pore in the cell wall and swim by means of four 

 equal cilia attached apically. When discharged, the zoospores are 

 enclosed in a common membrane that soon disappears. As in the 

 vegetative cells of the Volvocales, each zoospore has a red eyespot and a 

 contractile vacuole. After a period of free swimming, a zoospore comes 

 to rest, withdraws its cilia, and secretes a cell wall. It then gives rise to 

 a new filament by repeated cell divisions. Sometimes aplanospores are 

 formed instead of zoospores. They frequently germinate within the 

 parent cell. 



Sexual reproduction takes place in Ulothrix by the conjugation of 

 isogametes (Fig. 27C). These originate from the vegetative cells 

 in the same way as the zoospores do, but are smaller, more numerous 

 (usually 32 or 64 in a cell), and have only two cilia. They escape 

 through a pore in the cell wall, enclosed in a common membrane that 

 soon breaks down. Following pairing and fusing of the gametes, the 

 resulting zygotes secrete a heavy wall and generally do not germinate 

 until the following spring. Then each produces 4 to 16 zoospores 

 (or sometimes aplanospores) that, in turn, give rise to new vegetative 

 filaments. The zygote is the only diploid cell in the life history. When 

 its nucleus divides, the chromosome number is reduced one-half. 

 Although isogamous, Ulothrix shows some degree of sexual differentiation 

 in that the gametes of one filament fuse only with those of another. 



Chaetophora. Some of the I lotrichales are branching filaments, 

 often with cells showing a differentiation in size. One such member is 

 Chaetophora, frequently found in standing water attached to submerged 

 objects. The cells of the branches become progressively smaller and 

 end in hair-like appendages that taper to a point. In a closely related 

 form, Draparnaldia, common in clear, cool streams, the cells of the main 

 filament are much larger than those of the branches (Fig. 28). Cell 

 structure and reproduction in both genera are much the same as in 

 Ulothrix. Stigeoclonium, another relative of Chaetophora, is differentiated 

 into a cushion-like basal portion from which arise a number of sparingly 

 branched upright filaments. When exposed to dry conditions, the cells 

 round off and separate, giving rise to a "palmella" stage. These cells 



