MARINE ALGAL VEGETATION 105 



rule, distinctly separated from the Porp/iz/ra-association which exists 

 below, but sometimes the two belts are concurrent, even over rather 

 large areas. 



The Porphyra-association is formed by Porphyra umbili- 

 calis f. typica. It might be justifiable to incorporate this association 

 in the fian^za-association, but I prefer to reckon it as a distinct 

 association; partly on account of the difference in the form of its 

 thallus, and partly because Bangia is confined to its narrow belt, 

 while Porphyra occurs also in other associations in and below the 

 Fucus-beli. 



The Porp/?yra-association occurs on a rocky substratum. On 

 vertical rock-faces it is very luxuriant, and then forms a continuous 

 belt below the a/i</za-association. Where the littoral zone has large 

 fragments of rock, that is to say consists of a talus of debris, this 

 species grows higher than any others upon the fragments at the 

 water's edge. In such places no continuous belt is formed, but the 

 species occurs in small scattered patches on the upper part of the 

 blocks of stone. This patchy distribution of the vegetation may 

 extend horizontally over either a small or a wide area, according 

 to the form of the coast. The vertical height of the belt is always 

 inconsiderable. 



During the period of drought Porphyra hangs down on the 

 vertical rock-faces, the thallus being repeatedly folded, according 

 to its length; the individuals in the upper rows of the association 

 often, to some extent, covering those below and thus, in some ways, 

 recalling the behaviour of the above-mentioned filiform alga?; on 

 the blocks of stone, also, the thallus is folded up in accordance 

 with its length, and rests on the surface of the stone. The folding 

 apparently serves to reduce evaporation during low-tide, as the 

 evaporating surface is thereby diminished; the folding may well be 

 of importance also in counteracting the effect of the light. As the 

 folds of the thallus are produced while the water is subsiding, it 

 is probable that some of this is retained between them, especially 

 on a flat substratum ; on a sloping or vertical substratum, however, 

 the quantity of included water would be very small. This circum- 

 stance is probably of some importance, and has also been empha- 

 sized by Borgesen (12). 



The Porp/iz/ra-association seems to maintain its freshness for 

 a considerable time, and I saw it with its usual dark purple colour 

 in several places in E. Iceland in the summer; but round about 



