ALG^E 



53 



are always biciliate, and are set free in the water where 

 they unite in pairs to form a single cell (zygote) with 

 four cilia (Fig. 8, D), which either at once grows into 

 a new plant, or first 

 passes into a resting 

 stage (spore), which 

 then gives rise to new 

 individuals by first form- 

 ing one or more zoo- 

 spores. 



In the higher mem- 

 bers of the order, the so- 

 called oogamous forms, 

 there is a sharp separa- 

 tion of the sexual cells, 

 the female cell becoming 

 here a large passive cell, 

 the egg-cell, usually 

 borne in a specially 

 modified and enlarged 

 cell called the oogonium 

 (Fig. 9, of/). In the 

 form figured, the egg 

 closely resembles in its 

 formation and structure 

 the large zoospores, with 

 which it agrees except 

 in the absence of cilia, 

 and there is no question that here also the gametes are 

 modifications of originally non-sexual zoospores. The 

 male gametes (spermatozoids) in these oogamous Con- 

 fervacese are also borne in special cells (anther idia) (Fig. 



DC. 



FIG. 9. (Confervacese). -- A, B, por- 

 tions of two female plants of QEdo- 

 gonium ; og, the oogonium ; in A. the 

 egg-cell has not yet been fertilized, 

 in B, the fertilized egg has become 

 transformed into a thick-walled rest- 

 ing-spore ; the spermatozoid enters 

 through the pore at the top ; C, part 

 of a male plant of the same species, 

 showing the antheridium, an; D, a 

 zoospore or motile non-sexual repro- 

 ductive cell ; E, one-celled plant de- 

 rived from a zoospore ; F, the lower 

 part of an older plant showing the 

 root-like outgrowths (/) of the basal 

 cell. 



