14 PLANT STKUCTURES 



reproductive cells are not distinctly differentiated, but that 

 the same cell may be nutritive at one time and reproductive 

 at another. 



I In suitable conditions certain cells of the filament will 

 be observed organizing within themselves new cells by 

 internal division (Fig. 2, (7, a, b). The method of forma- 

 tion at once suggests that the new cells are asexual spores, 

 and the mother cell which produces them is acting as a 

 sporangium. The spores escape into the water through an 

 opening formed in the wall of the mother cell, and each is 

 observed to have four cilia at the pointed end, by means of 

 which it swims, and hence it is a zoospore or swarm spore. 

 After swimming about for a time, the zoospores " settle 

 down," lose their cilia, and begin to develop a new filament 

 like that from which they came (Fig. 2, D). 



Other cells of the same filament also act as mother cells, 

 but the cells which they produce are more numerous, hence 

 smaller in size than the zoospores, and they have but two 

 cilia (Fig. 2, (7, c). They also escape into the water and 

 swim about, except in size and in number of cilia resem- 

 bling the zoospores. In general they seem to be unable to 

 act as the zoospores in the formation of new filaments, but 

 occasionally one of them forms a filament much smaller 

 than the ordinary one (Fig. 2, E). This indicates that 

 they may be zoospores reduced in size, and unable to act as 

 the larger ones. The important fact, however, is that 

 these smaller swimming cells come together in pairs, each 

 pair fusing into one cell (Fig. 2, C, d, e). The cells thus 

 formed have the power of producing new filaments more or 

 less directly. 



It is evident that this is a sexual act, that the cell pro- 

 duced by fusion is a sexual spore, that the two cells which 

 fuse are gametes, and that the mother cell which produces 

 them acts as a gametangium. Cases of this kind suggest 

 that the gametes or sex cells have been derived from zoo- 

 spores, and that asexual spores have given rise to sex cells. 



