THE ALGAE 91 



starting point of new growth. In many species this process, which occurs 

 mostly in early spring, is assisted by the action of a ring-like fold on the end 

 walls of the cells, somewhat like that in Oedogonhim, which expands laterally 

 and thus ruptures the cuticular sheath of the filament. 



Although the filaments of Spirogyra are multicellular they are not, 

 biologically speaking, individuals, but rather colonies of comparatively 

 independent cells. At sexual reproduction they behave like unicellular 

 types in that the conjugating units are used up and disappear as such. This 



Zygote 



Zygospore ^ , ^ • 



Gamete Gamete 



Spirogyra 

 plant 



Fig. 71. — Life-cycle of Spirogyra. 



limits the reproductive capacity of the plant in comparison with higher 

 forms in which a vegetative body persists and may reproduce many times 

 (Fig. 71). 



Siphonales 



The Siphonales are Chlorophyceae in which the thallus consists of a single 

 multinucleate cell or coenocyte, which often grows to form a structure of 

 macroscopic size. Asexual reproduction occurs in a few genera by the 

 formation of zoospores. The sexual reproduction in many of the marine 

 genera is by the fusion of unlike motile gametes ; that in the fresh-water 

 genera is oogamous. 



Some of these coenocytic Green Algae show great complexity. For 

 example, in the genus Caulerpa, there is a complex development of coenocyte 

 branches to form a thallus, which may be as much as 10 cm. in height, with a 

 definite external form. The thallus in Caulerpa is especially remarkable, 

 since the branches of the single coenocyte build up thalli that simulate in 

 appearance the differentiation into roots, stems and leaves found in the 

 higher land plants (Fig. 72). 



We shall consider two types of the Siphonales, Bryopsis and Vaucheria. 



Bryopsis plumosa 



The genus Bryopsis is distributed mainly in tropical seas, but one species, 

 B. plumosa, is commonly found in spring and early summer on the British 



