352 ECOLOGICAL FACTORS, ETC. 



and treated artificially to diflFerent periods of exposure. As a result 

 she came to the conclusion that the essential control of zonation 

 was height (modified by exposure), substrate and sunshine. It is 

 difficult, however, to see how the effects of exposure can be 

 separated from those of actual height, and there would appear to 

 be no good reason why exposure was not the principal determining 

 factor. Grubb (1936) has suggested that submergence and emerg- 

 ence are the most important factors in determining the occurrence 

 of algal zones, but it would appear, however, that all these factors 

 really operate indirectly through the degree of desiccation that the 

 different species can tolerate. Since this is bound up with their 

 physiological economy it may be expected to have more significance 

 than just simply height or exposure per se, because the real control 

 must be related to the physiology of the plant. Gail (1920) has 

 declared that it is the desiccation of young plants which prevents 

 the appearance of algae outside their usual zones, and it is a re- 

 markable fact that sporelings of fucoids are usually very strictly 

 confined from an early stage to the zones occupied by the adult 

 plants. As sporelings of the fucoids are not readily identified 

 specifically when young, field experiments with young plants 

 become extremely difficult, if not impossible, to perform. Berthold 

 (1882) was so much impressed by the importance of this factor that 

 he divided the rocky shore into five zones based on the degree of 

 desiccation. It has been concluded that species growing high up on 

 the shore have a power of resisting desiccation which is not 

 possessed by those growing lower down, and also that those 

 species which resist desiccation best possess the slowest growth in 

 contrast to the others which do not resist desiccation and grow 

 more rapidly. Fig. 202 A compares the distribution of the principal 

 fucoids from various areas in relation to the tidal levels, and it has 

 been suggested that the demarcation between the Fucus spiralis f. 

 platycarpus and Ascophyllutn zones is probably caused by desicca- 

 tion, whereas the determination of the other limits may be partially 

 or wholly explained by one or more of the following factors : 



(a) Bottom structure. Boulders are essential for the attachment 

 of Ascophyllutn but smaller stones will suffice for the other species. 



(b) Water movements, although the evidence here is somewhat 

 conflicting. 



(c) Light. 



