2.3 4 5 6 



Time in Hours 



Fig. 207 Loss of water, as represented by loss in weight, in 

 Pelvetia canaliculata during intertidal exposure. (After Isaac.) 



during periods of exposure is quite clearly an important causal 

 factor. On salt marshes it is undoubtedly responsible for the oc- 

 currence of gelatinous Myxophyceae at the higher levels where the 

 exposure periods are long. 



Among others Pringsheim (1923), Zaneveld (1937)3 Isaac (i933) 

 and Kanwisher (1957) have studied rate of water loss in intertidal 

 algae. Much of the work has centred around the principal fucoids 

 found on European shores. In Pelvetia canaliculata the main water 

 loss occurs in the first six hours of exposure (Fig. 207) whilst in 

 Fucus spiralis var. platycarpus, F. vesiculosus, F. serratus and Asco- 

 phyllum the maximum water loss can be spread over 18 hours (Fig. 

 208) : in F. vesiculosus as much as 90 per cent of the total initial 

 water can be lost in i| hours. Fucus spiralis var. platy carpus loses 

 its water the slowest of these last four species, and a definite in- 

 crease in the rate of water loss can be observed with the different 

 species as each occupies a successively lower zone on the shore, but 

 it must be noted that F. spiralis var. platycarpus ultimately loses a 

 rather higher percentage of water than the other three. Haas and 

 Hill (1933) have also shown that the higher the alga grows the 

 greater is the fat content (Table 15). 



387 



