Hydt 'odictyacese 



217 



The normal method of reproduction is by the formation of new coenobia 

 by the close apposition of biciliated zoogonidia which have become quiescent. 

 Every coenocyte in the colony is capable of becoming a zoogonidangium, and 

 since vegetative cell-division does not occur, the number of zoogonidia pro- 

 duced in it determines the number of coenocytes in the daughter-coenobium. 

 In Euastropsis, however, several bi-cellular coanobia arise in each mother-cell. 

 This type of reproduction by the apposition of quiescent zoogonidia to form 

 new coanobia at once distinguishes the Hydrodictyacese from all other families 



Fig. 143. A, Pediastrum hiter/rum Nag. x 475. P>, P. tricornutum Borge, x 475. C and D, 

 P. Tetras (Ehrenb.) Kalfs, x 475. E, P. duple. r Me.yen, x 475 ; //, hypnospore. FH, 

 P. Boryainim (Turp.) Menegh. ; G, showing escape of zoogonidia into vesicles; H, young 

 ccenobium formed by apposition of quiescent zoogonidia ; F, x 475 ; G and H, x 220 (after 

 Kerner). 7, two marginal cells of P. glandulifera Bennett, x 475. J L, zoogonidia and 

 gametes of P. Boryanum (after Askenasy) ; J, zoogonidia and K, gamete, x 500 ; L, conju- 

 gation of gametes to form zygospores (z) ; gametes x 730, zygospores x 220. 



of the Protococcales. From the coenobic forms of the Autosporacese they are 

 distinguished by the coenocytic character of the coenobium, and by the pro- 

 duction of zoogonidia, although it must not be forgotten that the autocolonies 

 of the Autosporacese arise by the apposition of non-motile gonidia which may 

 very likely be zoogonidia which have lost the power of swarming. This 

 seems very probable since in Pediastrum itself the gonidia are sometimes 

 non-motile and do not ' swarm,' but simply arrange themselves in position to 

 form the daughter-coenobium. Aplanospores, which are resting-spores (hypno- 

 spores), occur in the genus Pediastrum. Isoplanogametes are also known to 

 occur, fusing in pairs to form zygotes the germination of which is indirect. 



