10ft 



A TEXTBOOK OF THEORETICAL BOTANY 



period of several months before germination. It germinates directly to 

 produce a new filament without the intervention of zoospores (Fig. 82). 



Oospore 



Zoospore 



Anlherozoid 



Vaucheno Plant 

 Fig. 82. — Life-cycle of Vaucheria. 



Siphonocladiales 



The Siphonocladiales are Chlorophyceae in which each branch of the 

 thallus is composed of one or more coenocytes. Though in the simpler 

 members the thallus consists of a single coenocyte and therefore may be 

 comparable with that of the Siphonales, the group as a whole shows a pro- 

 gressive complexity, brought about partly by the development of septa 

 which cut the large coenocytes into smaller ones and partly by the develop- 

 ment of the branching system. 



Asexual reproduction is by means of zoosporangia in which are developed 

 zoospores, or by aplanospores. Sexual reproduction is by the fusion of 

 isogametes. 



Most of the species are marine and are more characteristic of the warmer 

 temperate and tropical seas than of this country. Many are found in the 

 Mediterranean while others occur particularly in the West Indies. A few 

 genera are found in fresh water, A number of species are calcified, fossil 

 types of which have been described. (See Volume III.) 



We shall consider only one type, Cladophora, which is among the more 

 advanced members of the group. In fact recent classifications have tended 

 to favour separating it as an independent order, the Cladophorales, while 

 relegating all the other families to the Siphonales, from which they have most 

 probably arisen. 



