294 



Ulotrichales 



the Chaetophoreas the cell which produces zoogonidia does not as a rule 

 change its external form, and therefore the zoogonidangium may differ in no 

 way from the vegetative cell in appearance (Trichodiscus is rather excep- 

 tional in this respect); but in the 

 Microthamnieae the zoogonidangia are 

 usually inflated and of larger size than 

 the vegetative cells. The zoogonidia 

 germinate directly into new plants, and 

 in the Chaetophoreas there is often a 

 simultaneous germination of large 

 numbers of zoogonidia which have con- 

 gregated on becoming quiescent (fig. 

 189 G). 



Aplanospores and akinetes (both of 

 which are hypnospores) are known in 

 several genera. They are especially 

 frequent in species of Draparnaldia, in 

 which almost all the cells of a tuft of 

 branches take part in spore-formation, 

 one globular resting-spore being formed 

 in each cell. They have a thick cell- 

 wall, generally brown in colour, and 

 often asperulate. The walls of the 

 mother-cells rapidly undergo dissolution 

 and the branches thus exhibit a monili- 

 form appearance. In other genera, 

 especially of the Microthamnieae, several 

 aplanospores are sometimes formed in 

 each mother-cell. 



A gamogenesis of biciliated iso- 

 gametes occurs in Stigeoclonium (fig. 

 1 89 F), Draparnaldia, Chxtophora and 

 Trichodiscus among the Chaetophorese, 

 arid in several genera of the Micro- 

 thamnieae. The gametes are scarcely 

 to be distinguished from the biciliated 



microzoogonidia and it is not unlikely in some genera of the Chaetophoreae 

 that the microzoogonidia are facultative gametes. In one instance both 

 micro- and macrogametes have been observed. The gametangia may not be 

 different in outward aspect from the vegetative cells or they may be much 

 inflated, as in Trichodiscus. 



It seems highly probable that the Algae included in the Ulvelleae and 



Fig. 188. A and B, Chsetophora incrassata 

 (Huds.) Hazen. A, entire thallus, nat. 

 size ; B, portion of branch-system, x 500. 

 (7, three thalli of Ch. elegans (Both) Ag., 

 showing attachment to aquatic macro- 

 phytes, nat. size. 



