MARINE ALGAL VEGETATION 121 



metum of Monostroma groenlandicum was here found in abundance, 

 often entirely covering the boulders and occurring on them, like 

 U. flacca, closely pressed to the stone-surface with a radiating ar- 

 rangement. In addition, an abundance of E. intestinalis f. compressa 

 and a few Ulothrix flacca were found. A little lower down, a Uro- 

 sporelum (U. Wormskioldii) occurred, which can scarcely be separated 

 from this association; it seemed to form a connecting link with 

 the semi-littoral communities. 



Cladophora gracilis occurred intermingled in the E. clathrata- 

 association, yet without playing any important role in the vegetation. 



A similar association, especially an Enteromorphetum of E. in- 

 testinalis, is, beyond doubt, very common in other countries. It 

 seems to be more poorly developed in Greenland (Rosenvinge, 

 63, p. 205) than in Iceland; in the Faeroes (Borgesen, 12, pp.714, 

 715), on the other hand, the Enteromorpha in testinalis- vegetation seems 

 to be more luxuriant than in Iceland. 



5. The Acrosiphonia-association. 



This association consists of decidedly filiform, much branched 

 algae, which grow very socially. During the period of desiccation 

 they retain the water as a sponge retains it. This circumstance is 

 mentioned with regard to some of these species by Rosenvinge 

 (63, p. 202), who says of Cladophora arcta (= Acrosiphonia incurva, 

 cf. Jonsson, 32, p. 43) that in Greenland it behaves during low- 

 tide like a sponge saturated with water. 1 



The principal species in this association are Acrosiphonia albe- 

 scens and A. incurva. They form individually, pure Acrosiphonieta 

 in the upper and lower littoral zones, where they cover flat stones 

 with a densely matted vegetation. On flat rocks the association may 

 be somewhat widely distributed, in other places it has more the 

 character of scattered Acrosiphonieta. The density of the vegetation 

 is due to the structure of these species. As the basal part of the 

 principal axis is too weak to carry the plant when its branch- 

 system has developed, lateral rhizoids, which slope downwards, are 

 developed at an early stage from the principal axis itself and from 

 the lower branches. These rhizoids often form creeping filaments 

 from which arise erect shoots, which can be detached from the 

 parent-plant and thus become independent individuals. The principal 



1 Possibly this is the case in an even higher degree with Callithamnion Ar- 

 buscula (ef. Borgesen, 12, p. 726). 



