214 REPRODUCTION OF ULOTHRIX 



asexual reproductive cells or zoospores (Fig. 115, d), which can 

 swim after the manner of a Chlamydomonas for several hours, 

 and thus travel some distance away from the parent-filament. 

 During most of this time they seek out regions of the water which 

 are well, though not excessively, illuminated, but ultimately 

 their sensitiveness to light alters and they move towards darker 

 spots, where they come to rest on the surface of stones, submerged 

 parts of Flowering Plants, or other Algae. The zoospore flattens 

 out against the substratum and the cilia are withdrawn ; a cell- 

 wall is secreted (Fig. 115, e) and then, by gradual elongation 

 and division, there is produced a new filament (Fig. 115, / and a), 

 which soon breaks away from its attachment and becomes 

 free-floating. Such asexual reproduction is obviously very prolific. 



The gametes are formed and liberated in exactly the same way 

 as the zoospores, except that in the cells producing them division 

 into 16 or even 32 parts is not uncommon (Fig. 115, n, o). The 

 isogamous sexual cells (j), which differ from the zoospores only 

 in their small size and in having but two cilia, behave just as 

 in Chlamydomonas, those from different filaments ordinarily 

 fusing together in pairs (k, 1) to form a quadriciliate zygote (m) 

 which, soon after, comes to rest and secretes a thick wall (h). 

 The resulting zygospore remains in a dormant condition during 

 the hot season, and may be dispersed in the same way as in 

 Chlamydomonas (cf. p. 186) ; occasionally it forms a short 

 outgrowth which becomes attached to some substratum (g). 

 On germination, the contents divide into a small number of 

 parts each of which gives rise to a new thread (Fig. 115, i). It 

 appears that under certain conditions the gametes lose the ten- 

 dency to fuse with one another, and round off singly to produce 

 spores which in all respects resemble the zygospores, an indication 

 that their sexual character is not very pronounced. 



At certain times the cells of the t//0^n'#-filament develop 

 mucilaginous walls and commence to divide along successive 

 planes at right-angles to one another. Since the products 

 become rounded off and themselves secrete mucilaginous walls, 

 a condition is assumed closely resembling the Palmella-stages 

 of Chlamydomonas (Fig. 116, A). These may persist for some 

 considerable time, but sooner or later the cells are liberated as 

 zoospores which produce new filaments. At times of extreme 



