MARINE ALGAL VEGETATION 103 



from the highest point of the stone. Such a stone-surface is smooth 

 and dries more quickly and completely than the uneven face of 

 the rock. This vegetation appears to maintain itself well in spite 

 of being completely dried up, day after day, during low-tide, — 

 during the nocturnal low-tides, naturally, it is dried up to a much 

 lesser extent. As a rule, such a dried-up Ulothrix-vegeialion is so 

 tightly adpressed to the surface of the stone that it can only be 

 removed by being scraped away with a knife. Thus, owing to de- 

 siccation, it appears, from a biological point of view, to have as- 

 sumed a crustaceous form, which evidently diminishes the evapora- 

 tion from its surface. 



The other species which occur in great quantities in this as- 

 sociation, such as Urospora mirabilis and Monostroma groenlandi- 

 cum, act biologically in a similar manner to Ulothrix flacca. Both 

 these species are found fairly frequently growing among Ulothrix 

 flacca; and as I think that these species may in several respects 

 be comprehended in one biological unity, I consider them members 

 of the same association, although they both occur in pure growths 

 (Urosporetum, Monostrometum). 



While Ulothrix flacra and Urospora mirabilis are commonly 

 distributed, the distribution of Monostroma groenlandicum is more 

 restricted, for this species can be reckoned as commonly distributed 

 only in E. Iceland. 



Of the species which are rare or of local occurrence, and which 

 are reckoned in this association, the following may be mentioned: 

 Ulothrix pseudo flacca, Ulothrix consociata var. islandica and Urospora 

 Hartzii. In addition, Enteromorpha intestinalis f. micrococca and 

 Rhizoclonium occur here as they do in the Prasio/a-community. 



Of these species Ulothrix consociata var. islandica requires to 

 be described most fully. It is very social, and grows in cushion- 

 like masses higher up, as a rule, than the other species of the as- 

 sociation. It appears to be protected from desiccation both by its 

 manner of growth and by the thickness of its cell-walls. 



Codiolum gregarium I have found to be of very social growth, 

 covering comparative^ large stones as pure Codioleta. It is most 

 nearly related to this association. 



The structure of the frond in this association displays various 

 peculiarities, which must be considered as being beneficial to the 

 plant during the period of desiccation; for instance, the interior 

 of the frond of Monostroma groenlandicum is filled with a gelatinous 



