116 H. JONSSON 



which grow exclusively, or by preference, on the Fucacece, and those 

 which occur as frequently on other substrata and must consequently 

 be considered chance visitors. 



Only one single species, Polysiphonia fastigiata, is exclusively 

 confined to the Fucus-belt (Fig. 7). It is well known that this species 

 grows only on Ascophyllum nodosum, into the frond of which it puts 

 its rhizoids, 1 although its distribution does not coincide entirely with 

 that of the latter. While Ascophyllnm is common everywhere along 

 the coast of Iceland, Polysiphonia fastigiata is common only in S. 

 and SW. Iceland; it has been found, also, in a single place in NW. 

 Iceland. Thus it keeps to the warmer parts of the sea off the coasts 

 of Iceland, but even within this area it may be absent from coasts 

 where the sea-water mixes abundantly with the fresh water. It did 

 not occur for instance at Borgarnes, nor on the nearest islands, 

 although Ascophyllum occurred in great quantities together with 

 Fucus vesiculosus and Fucus inflatus. There the sea-water is freely 

 mixed with water from the large glacier-torrent of Hvita in Borgar- 

 fjorSur. Polysiphonia fastigiata does not seem able to endure such 

 water, but further out, along the fjord, where the water becomes 

 more saline, it grows in the ordinary manner. 



Of those species which grow by preference on Fucaceo3, Ela- 

 chista fucicola and Ulvella fucicola may be mentioned. The first of 

 these is the most important and often occurs in wonderful abun- 

 dance, Ulvella forms a much less important part of the vegetation, 

 more especially on account of its minute size. 



Besides these two, other species frequently occur which just as 

 often, or even more often, grow on other substrata. Of these Py- 

 laiella littoralis plays a very important part in the vegetation, espe- 

 cially in the spring; then it sometimes occurs in such quantities 

 that it almost covers large stretches of the coast. Ulothrix flacca 

 also frequently occurs in great abundance, as may also be the case 

 with Isthmoplea sphcerophora. In spring and early summer Ecto- 

 carpns tomentosus and Ectocarpus fasciculatus are found growing to- 

 gether socially on Fucus inflatus (both of them in S. and SW. Ice- 

 land). Ectocarpus confervoides is also a rather frequent epiphyte. In 

 addition, Monostroma and Enteromorpha intestinalis may be mentioned, 

 on which, again, epiphytes can grow (as for instance Chantransia- 

 species); also Ulothrix pseudo flacca, Acrosiphonia, Ceramium rubrum, 

 Ralfsia verrucosa, Conchocelis rosea, Porphyra umbilicalis and others. 



1 Gertrud Tobler-Wolff, in Beihefte z. Botan. Gentralbl., Bd. 24, 2. Abt., 1909. 



