THALLOPHYTA-ALG^ 



61 



to form a zygospore, which gerinmates and produces two new 

 plants. 



The Zygneniiae are, Hke the Desniids, furnished with pecuhar 

 chloroplastids. In Spirogyra these form a spiral band which 

 winds roimd the cell ; in Zijgnema they form two stellate 

 plates ; other forms occur in other genera. 



(Edogoniuin {figs. 808, 809) is noteworthy among the 

 group on account of certain 



peculiarities of its reproduction. Fig. 809. 



The ordinary filament produces 

 a number of oogonia along its 

 length, the cells swelling and 

 becoming ovoid. Each produces 

 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, giwig 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 

 dnectly on the parent filament. 

 Instead, special cells produced 

 in the same way as antheridia 

 set free their contents in the 

 form of a ciliated cell known 

 as an androspore. It is much 

 like an antherozoid, but differs 

 from the latter in its behaviom-. 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 

 ceUs of which are antheridia, and produce each a single 

 antherozoid. ^\Tien 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. 



Fig. 809. 

 of an 



A, B. Escape of tlie zoogoiiidia 

 CEdo'joniiim. c. Oue in free 

 motion. D. The same after it has be- 

 come fixed, and has formed the attacli- 

 iug disc. E. Escape of the whole pro- 

 toplasm of a germ-plant of (Edogo- 

 niuin in the form of a zoogouidium. 

 After Printrsheim. ' 



