74 



species of Ulothrix. Occasionally the zoogonidia do not escape, 

 but lose their cilia, become invested with a cell-wall, and . form 

 what is termed a " palmelloid condition " (fig. 20 F). The produc- 

 tion of zoogonidia usually commences near the apex of a filament 

 and progresses towards the base. 



Sexual reproduction is by the conjugation of isogamous gametes, 

 which are indistinguishable from the microzoogonidia. The gametes 

 are biciliated and usually escape from the gametangia in the 

 morning, conjugating in pairs with considerable rapidity. The 



Fig. 20. A and B, Ulothrix zonata (Web. et Mohr.) Krttz., from near Meaux Abbey, 

 E. Yorkshire (x500). C F, U. subtilis Kiitz., from near Mullion, Cornwall 

 (x500); F shows the "palmelloid condition"; a, aplanospore ; za, macro- 

 zoogonidium ; zi, micro/oogonidium. 



resulting zygospore invests itself with a firm cell-wall and germi- 

 nates after a more or less extended period of repose. On germina- 

 tion the contents break up into many zoospores each of which 

 forms a new filament. The gametes frequently germinate directly 

 without conjugation. 



The movements of the microzoogonidia and gametes are fre- 

 quently very strange, one cilium being kept more or less rigid and 

 its extreme apex used as a pivot, while the other cilium exhibits 

 violent movements causing a rapid lateral oscillation of the 

 body. 



