28o 



PHYSIOLOGY, SYMBIOSIS, ETC. 



they are perhaps simply ecological forms. They do, however, differ 

 from each other biochemically in their K/Na ratio even though the 

 Cl~ content is about the same in both species, but this is not 

 necessarily of taxonomic significance. It is pointed out that the 

 vesicle should not be regarded as an enormous single cell but as a 

 fluid enclosed within a coenocytic wall composed of living cells. 

 In contrast to the observations of many workers it was found that 

 the enclosed fluid or sap of a Valonia plant in contact with sea 

 water does not have the fixity of composition that has been ascribed 

 to it. The sap of both species can, for most purposes, be regarded 

 as a mixed solution of sodium and potassium chlorides, V. macro- 



per lit 



I I I 



12 3 4 5 



10 



Size 



T5G 



rm. 



20 



Fig. 179. Size of vesicles of Valonia ventricosa in relation to sap concentration. 

 (After Steward and Martin.) 



physa being poorer in potassium and richer in sodium than V. 

 ventricosa. The K+ and Na+ content is definitely influenced by 

 illumination; a bright light, for example, induces a high K+ and 

 low Na+ content, whilst increased mechanical protection also 

 raises the potassium and sodium chloride content, the former more 

 so than the latter. The total salt concentration is also affected by the 

 size of the vesicle as may be seen by a study of fig. 179. 



Another interesting observation was that when the chloride 

 concentration of sea water is increased, but not otherwise, the sap 

 will respond to small increments of K+ in the medium. Previously 

 it had always been thought that changes in concentration of K+ in 

 the sap were related to the concentration of hydroxyl (0H~) ions 

 in the medium, whereas it is now evident that the relationship with 



