PLANT AND ANIMAL ASSOCIATIONS 43 



many local variations in distribution and extent. Each of 

 Cotton's five formations enumerated above consists of a 

 number of associations, and these may be grouped in a 

 manner which varies considerably in different locaUties. 



It should also be pointed out that many species appear 

 periodically, some being but short lived, and conspicuous 

 only for a few weeks, while others require six or eight months 

 to complete a life cycle. Owing to these seasonal phases 

 the varying zones may present very different appearances 

 and offer different degrees of protection and food supply 

 to their animal inhabitants during the course of the year, 

 and thereby profoundly affect their life-histories and 

 distribution. 



The methods of attachment of algae to the substrata are 

 of some importance also, as the typical stout disc frequently 

 offers shelter to considerable animal communities. Algae 

 with a membranous thallus may be found on the smooth 

 faces of schists, quartzites, etc. Yendo (1914) found that 

 pure cultures of Glceopeltis can be obtained by laying 

 quartzite as a substratum while, for Laminariay andesitic and 

 basaltic stones are found most useful (by the seaweed 

 cultivators of Japan), as they are hard, yet have many minute 

 surface cavities which afford resting-places for the spores. 

 Soft stones — sandstones, conglomerates, claystones, tuffs, 

 etc. — are unsuitable ; only small algae can grow on them : if 

 large, the resistance offered to the water movements may 

 cause the plants to come off. One or two local phenomena 

 may be also noted, such as the extension of algae even far 

 above the spring-tide limit, in situations where much spray, 

 shade, and terrestrial moisture combine to render such a 

 condition possible, as in the Faroes. Sunlight is of course 

 essential for algal growth, and algae may be absent from 

 vertical cliffs with a northern exposure (Pearse, 1913-14). 



As is well known, botanists classify plant zones, associa- 

 tions, etc., by using the names of the dominant species in 

 each. Since algae are abundant on almost all rocky shores, 

 this nomenclature can be conveniently used in describing 

 almost all the various types encountered, though some 



