MARINE ALGAL VEGETATION 113 



Generally this species forms a pure association, hut yet, some- 

 times, it and Ascophyllum can grow intermixed. 1 It is always sub- 

 merged at high-water, but seems well able to withstand desiccation 

 at low-water, when however the upper side of the frond dries up 

 considerably, especially on rocks which face the sun, and may then 

 and this is true of Fucus inflatns also be bent upwards, or 

 even slightly distorted, at the apex, though never to the same extent as 

 is the case with Fucus spiralis. When such a branch is lifted up, 

 it can be seen that the under side is moist, even on a warm sum- 

 mer's day. During low-tide the Fucus plants lie prostrate on the rock, 

 one plant overlying the other, or one branch of the frond covering 

 the other, and in this way some water is retained amongst the plants. 



Naturally, this applies also to the members of the other asso- 

 ciations in the Fuczzs-belt. Those individuals which hang freely from 

 the rocks are exposed more than others to desiccation. 



The Ascophyllum -association differs from the other asso- 

 ciations in its light-brown colour, and, in addition, by the occur- 

 rence of Polysiphonia fastigiata, which grows very luxuriantly on 

 it in S. and SW. Iceland; it is often somewhat singular to see the 

 light-yellow Ascophyllum-be\t with the numerous dark reddish-brown 

 patches of Polysiphonia. 



So far as the behaviour of the species during exposure to the 

 beat of the waves is concerned I must refer to the above-mentioned 

 remarks, and can only add that at times it may also occur in the 

 most exposed places, but no longer in the uppermost belt. Here 

 it gives place to the surf-forms of Fucus inflatus (f. exposita, f. den- 

 droides), which form a narrow belt at the upper boundary of the 

 Fucus-belt. Ascophyllum occurs, then, between this and the real Fucus 

 inflatus-belt, which retains its usual position. In a few places, how- 

 ever, on the most exposed points, Fucus inflatus only is found. 



The Fucus in flatus-association. When exposed, this spe- 

 cies behaves conversely to Fucus uesiculosus and grows most luxu- 

 riantly on exposed coasts. Thus, it behaves in Iceland as it does 

 in Greenland (Rosenvinge, 63) and in the Fseroes (Borgesen, 

 12). In Iceland, as in other places, the species varies extremely 

 and must be considered very capable of adapting itself to varying 

 circumstances, especially as regards exposure. The association can 

 in accordance with the exposure - be divided into three belts: - 

 The Surf-belt, the Wave-belt, and the Calm-belt. 



1 The epiphyte-vegetation is mentioned subsequently. 



The Botany of Iceland. I. 8 



