15 14 Proceedings of the Hoy at Irish A eadetny. 



interesting ecological papers by Miss S. M. Baker have lately appeared ('09, 10), 

 in which experiments, with a view to determine the causes which bring about 

 zonation, are described ; but the general question of algal associations is not 

 touched upon. The ecology of Ulva Lactuca has been discussed by me at 

 some length in a report to the Eoyal Commission on Sewage Disposal ('1 1 6). 

 The subject of plant-communities was not in view, but the presence of a 

 distinct mud-formation and an Ulva-association, though not stated in words, 

 was clearly demonstrated. N. H. Johnson has furnished two notes dealing 

 respectively with the mapping and nomenclature of marine vegetation 

 (11, 12). 



An account of the algal associations of a definite area in the British Isles 

 was thus urgently needed, and in the Clare Island investigations this study 

 was given a prominent place. It will be noted that in several of the works 

 referred to, the systematic account was published first, and the ecological 

 treatise appeared later. This order of investigation is essential. The flora 

 must be thoroughly mastered, before the groupings of the species can be 

 accurately studied. 



(2) The Areas and the Units of Vegetation. 

 A. The Regions. 



Kjellmann's term " region " has been generally adopted by algologists ; 

 but the limits of the three vertical divisions included under this term are 

 variously accepted. In the present account they are taken as under : — 

 The littoral region extends from the level of the highest marine vegeta- 

 tion to that of low-water mark at neap-tide. The sub-littoral begins from 

 the lower limit of the last-named, and extends down to the limit of sea- 

 weed vegetation (i.e. about 25 fathoms). The clittoral region follows, 

 and being devoid of algal vegetation, is of no importance in the present 

 paper. 



The reasons for so defining these regions are as follows : — With regard to 

 the upper limit of the littoral region, this, according to Kjellmann, com- 

 menced at the highest tide-level ; but, as several writers have pointed out, 

 the algae on exposed coasts often extend far above the # spring-tide limit. A 

 separate region — the supra-littoral — has been suggested by Lorenz. Borgesen, 

 however ('05), showed that no hard-and-fast line can be drawn between the 

 vegetation of the supra-littoral and littoral, and for this reason he did not 

 recognize the area as a definite region. In Clare Island the supra-littoral 

 vegetation, though often found, is of less general occurrence than, for 

 instance, in the Faeroes. It is largely due to the presence of spray (a factor 



