MARINE ALGAL VEGETATION 123 



doubt, very common in adjacent lands. During low-tide the upper- 

 most free branches appear to be more exposed to desiccation and 

 possible death in Greenland than in Iceland; in the latter place it 

 occurs rather frequently. Both in Greenland (Rosen vinge, 63) and 

 in the Faeroes (Borgesen, 11, 12) the Acrosiphoma-association is 

 well represented. 



bb. The Shade-vegetation. 



To this vegetation are referred certain littoral associations which 

 occur exclusively in shady places. The shade is produced by the 

 light being subdued both by a covering of living plants and by 

 projecting blocks of stones. Thus the greater part of the under- 

 vegetation-associations of the Fucus- belt mentioned above belong 

 to the shade- vegetation. In addition, the shade-vegetation grows, 

 as already mentioned, on the under side of overhanging rocks — 

 on their in-sloping sides. The vegetation is divided into several 

 associations which have already been mentioned in connection 

 with the under- vegetation of the Fucus-belt (cf. p. 118). 



The frond differs in form and structure in the various species 

 of this vegetation. It is a feature common to the whole of the 

 shade-vegetation to be low and dense in growth. The frond of 

 Hildenbrandia is a crust which covers the rocks; the others are 

 finely branching; some, as Sphacelaria and Polysiphonia are 

 bush-like and are richly and finely branched; Rhodochorton has 

 a similar mode of branching, but to a considerably less degree. 

 These three species often propagate vegetatively by means of run- 

 ners, which increases the density of the vegetation. Plumaria is 

 closely and distichously branched; it is tightly adpressed to the 

 surface of the stones during the time of low-tide and retains much 

 water between its branches, considered from a biological point of 

 view it might almost be regarded as leaf-like during the period of 

 desiccation. 



6. The Hildenbrandia-association belongs almost exclu- 

 sively to the under-vegetation of the Fucus-belt, and of the shady 

 localities in the littoral zone (see above, p. 118). 



7. In shady places the Rhodochorton -association, the 

 Sphacelarietum britannici, and the Polysiphonietum ur- 

 ceolatse behave in a similar manner outside the Fucus-belt as 

 they do within it (see above). They may occur either as pure as,^ 



