57 



The exact affinities of many of the Siphonese are still 

 obscure, and it is by no means impossible that the group 

 has had a multiple origin, i.e. all the members of the 

 order may not neces- 

 sarily be genetically 

 related, but there may 

 have been a develop- 

 ment of this peculiar 

 type from several an- 

 cestral forms. While 

 the lowest of the or- 

 der show much in 

 common with the 

 Protococcacese, and 

 may, perhaps, have 

 arisen from them, 

 others like the com- 

 mon genus Vaucheria 

 (Fig. 12) are struct- 

 urally more like 

 some of the Confer- 

 vacese. There are a 

 number of genera 



among the latter 

 where the elongated 

 cells are multinucle- 

 ate and there is a 

 partial suppression 



FIG. 12. Vaucheria sessilis, one of the 

 fresh-water Siphoneae ; A, plant with 

 unopened antheridium, an, and oogo- 

 nium, og ; B, an older plant with the 

 antheridium empty, and the oogonium 

 containing the resting-spore, sp ; C, the 

 end of a filament with a zoosporangium ; 

 D, zoospore showing the pairs of cilia 

 corresponding to the individual nuclei 

 in its outer part; E, a germinating 

 zoospore. 



of the division walls, 



nuclear division and cell division being quite indepen- 

 dent of each other. By the complete suppression of the 

 division walls in forms like these, it is conceivable that 



