290 PHYSIOLOGY, SYMBIOSIS, ETC. 



which rarely become completely dry: these are resistant up to 

 concentrations of 2-2 times sea water. 



C. Algae of the littoral belt: these are usually completely 

 exposed and can resist a concentration of 3-0 times that of sea water. 



The behaviour of these algae is summarized in Table IX. 



It was found that most of the Rhodophyceae possess a cell sap 

 which has an osmotic pressure approaching that of the maximum 

 hypertonic resistance likely to be encountered in their habitat, but 

 this correlation between cell sap and external medium is not so 

 evident in the case of the Chlorophyceae and Phaeophyceae. In 

 their resistance to desiccation the algae fall into the same three 

 ecological groups as can be seen from Table X. 



In group I when the filament is dried up for only a very short time 

 by means of filter paper the cells die or collapse so quickly that they 

 do not even recover when put back into sea water. Those of group 2 

 are less susceptible and those of group 3 hardly susceptible to this 

 treatment. 



A study of the chlorophyll relations in all three algal groups by 

 Seybold and Egle (1938) revealed the fact that only in the Chloro- 

 phyceae are both chlorophylls a and h present, their proportions 

 being the same as those in the submerged flowering plants. In the 

 Rhodophyceae, Phaeophyceae, Cyanophyceae and Bacillario- 

 phyceae only chlorophyll a is present, so that if the absence of 

 chlorophyll h is considered to represent a primitive character, the 

 Chlorophyceae would have to be regarded as the most recent 

 group (cf. p. 265). On the other hand it is equally possible that in 

 these groups the second component has been lost, possibly due to 

 the introduction of the extra colouring pigment or to some other 

 factor. In not one of these groups does there appear to be any 

 relation between depth and quantity of chlorophyll and carotene 

 present, the actual amount being determined rather by the genetic 

 constitution. The quantity of pigment per dry or fresh weight is 

 hss in the Rhodophyceae and Phaeophyceae than it is in the 

 Chlorophyceae, but the fact that members of the first two groups 

 assimilate carbon dioxide as rapidly, weight for weight, as those of 

 the Chlorophyceae indicates that their carbon assimilatory appar- 

 atus cannot be deficient. It is also evident that the green algae 

 exhibit a far greater range in the amount of pigment present than 

 do the red and brown algae (cf. fig. 187). 



