46 THE BIOLOGY OF THE SEA-SHORE 



The vegetation of rock pools differs according to their 

 situation within tidal range, geological nature, size, depth, 

 configuration, temperature, illumination, and drainage. 

 Their species are perennial and more or less constant. In 

 upper pools encrusting forms are conspicuous (Cotton, 19 12). 

 Pools may become too warm for certain species, while others 

 are found in these situations beyond their normal limits, as 

 in the case of some tropical species. Skinner (1903) found 

 that the higher the elevation of the pool, the fewer the 

 species, though the greater may be the number of the 

 individuals ; the more gradual the slope, and the rougher 

 and more irregular the sides, the more abundant, however, 

 becomes the plant life. 



The Corallines and others are protected by a deposit 

 of carbonate of lime. The Lithothamnion Associations 

 which occur in Arctic, Temperate, and Tropic regions are 

 often known as " Coral Banks," and play an important part 

 in the formation of coral reefs. 



The tidal zone in the tropics is, as a rule, poor in vegeta- 

 tion, and often absolutely barren down to i metre below 

 low-water mark, except where the surf constantly breaks. 

 This is due to the killing of spores and seedlings by intense 

 heat ; calcareous species such as Lithothamnion ^ however, 

 may exist in enormous number. In other cases, in summer, 

 the tidal algae may become so dry as to break to powder 

 between the fingers, and yet on the return of tide are able 

 to resume their normal functions (Yendo, op. cit.). On 

 the other hand, many algae can survive temperatures below 

 3° C, but may succumb to the attrition of floating ice. 



Mention must also be made of Maritime Lichens. 

 Lichens occur on three forms of substratum, (i) Rocks 

 (saxicolous) ; (2) bark of trees, or wood (corticolous) ; and 

 (3) soil (terricolous). The shore forms naturally belong 

 chiefly to group (i). They have been studied in several 

 localities. Knowles (1913) described five belts for the Howth 

 coast : {a) a wide belt characterised by Ramalina and extend- 

 ing landward from just above high water, {b) the orange belt, 

 due to species of PAy^m and Placodium,{c) Lichina vegetation 



