124 H. JONSSON 



sociations, and are then dominant here and there in patches, or 

 the species may be found intermingled with each other. Besides 

 these associations a Plumarietum, consisting exclusively of Plu- 

 maria elegans, is found in many places. This association is darker 

 in colour than the other red-algae associations of shady places; it 

 occurs frequently in S. and SW. Iceland. Intermixed in it occur 

 Callithamnion scopulorum and Chantransia virgatula. 



Rhodochorton Rothii is sometimes found high up in the littoral 

 zone on flat rock-surfaces exposed to the light and to the beat of 

 the waves, but then it grows in small globular cushions (f. globosa). 

 Consequently, this globetum of Rhodochorton Rothii does not be- 

 long exclusively to the shade-vegetation. 



On flat surfaces in the shade the usual arrangement is that 

 the decided shade-associations occur at the bottom, where the light 

 is feeblest, e. g. the Sphacelarietum, Rhochortonetum, Polysiphonietum, 

 Plumarietum; at the top, where the illumination is stronger, light- 

 loving species occur, e.g. Pylaiella, Uloihrix , or others. Near Rey- 

 kjavik a vertical section of such a surface showed uppermost, at 

 the edge, Pylaiella littoralis, next Rhodochorton Rothii f. globosa, and 

 lowest of all Plumaria elegans. 



In a grotto in the Vestmannaeyjar, where the illumination was 

 very feeble, Enteromorpha intestinalis f. micrococca occurred on the 

 roof, Ceramium acanthonotum grew rather high up on the walls, 

 and Plumaria elegans, together with Delesseria alata, formed a belt 

 lower down the walls. 



A vegetation corresponding to the shade-vegetation appears to 

 occur in Greenland where Hildenbrandia rosea, Ralfsia clavata and 

 Verrucaria mucosa form the undergrowth in the Fucus -belt and 

 in other places (Rosenvinge, 63, pp. 198 and 203). Rhodochorton 

 Rothii and Sphaceiaria britannica also appear to grow in a similar 

 manner in Greenland (Rosenvinge, 63, p. 205). 



A comparison with the Faeroes shows some difference. The 

 vegetation in the grottoes in the Faeroes (Borgesen, 12, p. 739), 

 however, resembles in its main features the Iceland shade-vegetation, 

 and, in addition, a similar vegetation is beyond doubt to be found 

 in fissures and clefts of the rocks in the Faeroes, but the mode of 

 occurrence of the species is not the same. Thus, in the Faeroes (Bor- 

 gesen, 12, p. 711), the Hildenbrandia-association appears to have a 

 much wider distribution upwards and to occur in fully illuminated 

 localities. Rhodochorton Rothii seems to occur in a similar manner 



