PHYSIOLOGY 281 



the concentration of CI" in the medium is much the closer. In 

 nature, Valonia derives its salts from a fluid which is much more 

 alkahne than its sap, but unlike most other marine plants it does not 

 appear to be able to accumulate bromide from sea water. The 

 determining factors for these two species appear to be: 



(i) Exposure to surf; a feature of the environment which 

 operates mechanically and also through variations in pH, oxygen 

 concentration and temperature. V. macrophysa is tolerant of 

 considerable variations in the last three factors but it is not tolerant 

 of the mechanical effects, whereas V. ventricosa responds in the 

 reverse manner. 



(2) Physical character of the substratum. 



(3) Composition of the fluid medium, especially in respect of 

 sodium, potassium and chloride ions. 



(4) Illumination. 



Rhodophyceae 



Some depth studies in relation to photosynthesis by Tshudy 

 (1934) may usefully be considered here, whilst further references 

 to this particular problem will be found on pp. 293 and 357. 

 Nearly all studies of photosynthesis in the algae regard Englemann's 

 theory that the colour of the alga is complementary to that of the 

 incident light as the basis for the investigation. For example, in 

 the green algae the greatest assimilation takes place in the red region 

 of the spectrum, whilst in the red algae it takes place in the green 

 region. In 1909 Hanson, considering only those Rhodophyceae 

 which grow at considerable depths, suggested that the chlorophyll 

 utilized the energy that was absorbed by the phycoerythrin, in 

 which case the red colouring matter was simply acting as a passive 

 colour screen. This theory, however, still left unsolved the problem 

 of the function of the coloured pigment in those red algae which 

 always grow in the littoral belt, though it was of course possible 

 that those algae grew in such situations merely because they could 

 survive the competition. In 1920, Moore, Whitley and Webster 

 showed that the Rhodophyceae assimilated less rapidly than the 

 Chlorophyceae in bright sunlight and more rapidly than the Chloro- 

 phyceae in diffused light, and so they argued that phycoerythrin 

 does not act as a passive colour screen but takes an active part in the 

 process of assimilation. However, their results are partially 



