Clare Island Survey — Marine A Igae. 1 5 23 



ordinary spring-tide (e.g., below the lace school on Glare Island). With an 

 alga such as Enteromorpha intestinalis, which is not dependent on salt water, 

 this is only to be expected. By the end of summer much of the growth may 

 be killed through drought, and the lower part of the association then appears 

 as a pure white band of dead tissue. 



In rock-pools above high- water mark an Enteromorpha vegetation is 

 found on exposed coasts which may be regarded as merely a pool extension 

 of the community just described. The pools in question usually have 

 a large admixture of rain-water, and during the less sunny parts of the 

 year, they contain a vigorous growth of Enteromorpha, and occasionally 

 Hildenbrandtia and Cladophora sericea. 



Fucaceae Association. (Exposed Coasts.) 



The conspicuous growth of the Fucaceae that is found on all rocky shores 

 of the British Isles is well represented in the Survey area. The large size of 

 the plants concerned, and the readiness with which the species can be recog- 

 nized, render the Fucaceae association one of the most useful and instructive 

 for ecological study. With few other genera can the effect of wave-action on 

 an exposed coast be studied with so much ease, and with few other associations 

 can the differences in composition and in plant-form be so readily traced. On 

 sheltered coasts the study is more difficult. Though familiar in a general way 

 to all British algologists, no definite account of the Fucaceae associations of our 

 islands has been published, and the study of the vegetation as it exists in 

 Co. Mayo was not without value in bringing to light several interesting 

 features. 



The composition of the association as it occurs in the British Isles is as 

 follows, the species occurring in the order given from above downwards : — 



Pelvetia canaliculata. 



Fucus spiralis. 



Ascophyllum nodosum (moderate shelter necessary). 



Fucus vesiculosus. 



F. serratus. 



F. ceranoides (admixture of fresh water necessary). 



An additional species, F. inflatus, was recorded by Borgesen for the 

 Shetland Isles, and Mr. E. M. Holmes has lately received a specimen of this 

 boreal species from Lewis, so possibly it is a regular constituent of the 

 association in the north of Scotland. Besides the ordinary forms and 

 varieties, some very remarkable modifications occasionally occur. (See p. 80.) 



Fully exposed Coasts. — Turning to the Fucaceae association of Clare Island, 



