GREEN ALGAE 



395 



out (p. 140) how, as a set off against it, the more complicated forms 

 of the Siphoneae have acquired additional mechanical . strength, 

 either by internal cellulose ties, as in Caulerpa (Fig. 333), or by matting 

 of their branches together, as in Codium, or by cementing those 

 branches together, as in Halimeda. But these are concessions to 

 an essentially weak construction. It is only possible to carry it to 

 any considerable size when living in water, and all the larger forms 

 are marine. Vaucheria is an exception : for though many of its 

 species float in water, some live on moist soil, exposed to the air. 

 But the members of the genus consist only of simple or branched 

 filaments, and when living aerially they lie procumbent. 



FIG. 333. 



Part of a transverse section of Caulerpa, showing the thick outer wall, and the 

 reticulate rods of cellulose, which act as ties, and give added rigidity. F. O.B. 

 (xSO.) 



The cytoplasm of these plants contains many small chloroplasts, 

 and numerous nuclei usually lying internally to them. Centrally 

 is a large vacuole. In Vaucheria the product of photo-synthesis 

 appears as oil, but others of the Siphoneae may contain starch, which 

 is commonly present in Green Algae. The general physiology of a 

 coenocytic, or as it has been called a non-cellular plant, is probably 

 like that of any ordinary green plant. The difference lies in the 

 mechanical construction. 



Vegetative propagation is carried out in various Siphonaceous 

 Algae by non-motile, or by motile cells, produced in large numbers 

 in special branches, and liberated into the water. Vaucheria is an 

 exception, in that the whole contents of such a branch-ending, which 

 are previously shut off by a septum, are discharged as a single ciliated 

 zoospore, large enough to be seen with the naked eye. The escape is 

 effected in the early morning ; after a period of movement the 

 zoospore comes to rest, and germinates directly into a new plant 



