THALLOPHYTA— ALG^ 



61 



to form a zygospore, which germinates and produces two new 

 plants. 



The Zygnemiae are, like the Desmids, furnished with peciiliar 

 chloroplastids. In Spirogyra these form a spiral band wliich 

 winds round the cell ; in Zygnema they form two stellate 

 plates ; other forms occur in other genera. 



CEdogoniwn {figs, 808, 809) is noteworthy among the 

 group on account of certain 



peculiarities of its reproduction. Fig. 809. 



The ordinary filanient produces 

 a munber of oogonia along its 

 length, the cells swelling and 

 becoming ovoid. Eaohproduces 

 a single oosphere. The anthe- 

 ridia may be on the same 

 filament as the oogonia or on a 

 different one. A cell of such 

 filament divides transversely 

 several times, and each cell so 

 formed becomes an antheri- 

 dium, giving rise either to a 

 single antherozoid or dividing 

 into two to form two mother- 

 cells, each of which produces 

 one. The oogonium admits 

 the antherozoid usually by a 

 perforation of the wall. Some- 

 times no antheridia are formed 

 directly on the parent filament. 



Instead, special cells produced Fig. 809. a,b. Escape o{ the zoogoniaia 

 , . -, . of an QLdogonium. c. One in free 



m the same way as antneridia motion, d. The same after it has be- 

 set free their contents in the come fixed, and has formed the attach- 

 ing disc. E. Escape of the whole pro- 

 form of a ciliated cell Jinown toplasm of a germ-plant of CBdogo- 

 j ^ T4. ;., ^^,,^1, nium in the form of a zoogoiiidiiim. 



as an anarospore. it is much ^jj^^. p^ingsheim. 



like an antherozoid, but differs 



from the latter in its behaviour. It becomes attached to the 

 wall of an oogonium and germinates, forming a very small 

 plant, of three or four cells, known as a dwarf male, the upper 

 cells of which are antheridia, and produce each a single 

 antherozoid. When the oosphere is fertilised it clothes itself 

 by a cell-wall, and the oospore so formed in due time 

 germinates and produces four zoospores (fig. 808 d). It thus 

 represents the sporophyte. 



