MEAN HTS. WASH 



LWN 



UPPER LIMITS 

 yc Fucus serratus 

 / P Himanthalia 



LOWER LIMITS 

 Laminaria 



B 



-T- 



D 



—r 

 C 



Fig. 192 Mean heights of wash at different levels on the shore at 

 Port St. Mary (Isle of Man), and at points of varying exposure 

 (B, D, F, G), and their effects on the limits of certain algae. (After 



Southward.) 



alized scheme. The final answer can only come with further study, 

 and here the recent introduction of the aqualung will make a de- 

 tailed study of this belt really possible. 



The levels of the upper belts in the mid-littoral and of those in 

 the supra-littoral in any locality are not entirely dependent upon 

 the height of the spring tides. On an exposed coast the shore is open 

 to considerable wave action and a heavy spray rises against the 

 rocks to a height sometimes of many feet. As a result of this wave 

 action both upper and lower limits are elevated several feet above 

 their normal. This elevation is known as the *splash zone'. On a 

 very exposed coast with big waves the splash zone can be sub- 



332 



