316 



Ulotrichales 



Each cell possesses one nucleus, and a single parietal chloroplast with 

 one or, more rarely, two pyrenoids. 



Asexual reproduction takes place by means of large ovoid zoogonidia, 

 which are biciliated, without stigma, and with a large parietal chloroplast. 

 They arise singly from the cells of the thallus, more especially from the 

 terminal cells of the branches, and they generally escape from the mother- 

 cell by a circular orifice. On corning to rest the zoogonidia germinate 



Fig. 208. Coleochsete pulvinata A. Br. J, young zoogonidangium (?) ; 2 and 5, young antheridia 

 (a) and oogonium (o) ; 4, oogonium (o) just before the opening of the trichogyne and empty 

 antheridium (a) ; 5, oogonium (o) after opening ; 6, zygote still with distinct male (sk) and 

 female (ek) nuclei ; 7, zygote which has become surrounded by the ' spermocarp ' ; 8, the 

 divisions of the hypnozygote (or oospore after a period of rest). In the vegetative cells in 

 fig. 7 : k, nucleus ; chr, chloroplast ; py, pyrenoid. (After Oltmanns.) 



directly into new plants; those of G. Nitellarum penetrate between the 

 lamellae of the wall of Nitella. The first division of the quiescent zoogonidium 

 results in a dorsal and a ventral cell and the disc is developed from the 

 ventral cell. 



The sexual reproduction is of a high type. The sexual organs are 

 oogonia and antheridia, and the thalli may be either monoecious or dioecious. 

 The development of the sexual organs was worked out by Pringsheim ('60) 



