284 



Ulotrichales 



Asexual reproduction takes place by aplanospores, akinetes and zoogonidia. 

 Aplanospores occur in a number of the smaller species of Ulothrix, in Uronema 

 and in Binuclearia (G. S. W., '04). They are more rarely found in the 

 larger Ulothrix zonata (fig. 183 B and C). Wille (12) regards the aplano- 

 spores of Stichococcus flaccidus, which are formed singly within the vegetative 

 cells, as reduced zoogonidia ; they germinate at once, dividing either cruciately 

 or tetrahedrally, and ultimately form a palmella-state. A similar palmella- 

 state may sometimes be observed in Ulothrix subtilis (G. S. W., '04). In 

 some instances they increase in size and become globular hypnospores with 

 slightly asperulate walls. Akinetes are frequently formed in various species 

 of Ulothrix, Stichococcus and Geminella. In Ulothrix idiospora G. S. West ('09) 

 they are thick- walled and scrobiculate (fig. 91 A and B). 



Fig. 181. Ulothrix zonata (Web. & Mohr) Kiitz. A, vegetative filament showing parietal 

 ring-like chloroplasts with pyrenoids (py). B and G, aplanospores. All x 800. 



Zoogonidia occur in Ulothrix, Stichococcus and Uronema. In Stichococcus 

 they are biciliated, but in Uronema they are quadriciliated ; in both genera 

 only one zoogonidium arises from a mother-cell. In the genus Ulothrix, in 

 which the different species range from 4/tt in diameter (in the narrowest 

 form of U. subtilis) to 70 /n (in the larger montane forms of U. zonata), the 

 zoogonidia vary very much. From 1 to 32, or even more, may arise in a 

 mother-cell. In the narrower species they arise singly or less often in 

 pairs, and are furnished with either two or four cilia. In the large species 

 U. zonata, in which zoogonidium-formation was very carefully studied by 

 Dodel ('76), every gradation in size is met with between macrozoogonidia 



