MARINE ALGAL VEGETATION 129 



however, been subdivided. Reinke (58, p. 10) divides the littoral 

 region near Kiel into an upper zone which is always laid bare at 

 low-water, and a lower zone at a depth of 2—4 metres. So far as 

 I can judge from the species enumerated, Reinke's upper zone 

 corresponds approximately with the previously mentioned upper 

 littoral zone, while the lower zone corresponds only in part with 

 the semi-littoral zone here described. Gran (24, p. 11) records under 

 the term "sublittoral vegetation" transitional formations which evi- 

 dently, for the most part belong to the semi-littoral communities. 



On the west coast of Sweden (Kylin, 45) the border-line be- 

 tween the littoral and sublittoral algal vegetations is, on the coast 

 of Bohuslan, at a depth of 3 — 4 metres, and at Halland at a depth 

 of about 5 metres. The upper part of the littoral zone seems to 

 correspond to some extent with the above-mentioned upper littoral 

 zone, while the lower part has something in common with the 

 semi-littoral zone. 



Where there is no tide, the boundary between an upper and 

 a lower littoral zone is not, I think, so sharply defined. 



8. The Monostro ma-association. 



This vegetation is composed of relatively large, membranaceous, 

 green algae. Monostroma fuscum and M. Grevillei form associations 

 in a rather considerable number of places in protected localities in 

 the fjords, below the limit of low-tide, and to a depth of at least 

 4 — 6 metres. There, the association is most sharply defined, although 

 it is fairly frequently found intermingled with other semi-littoral 

 associations. In the lower littoral zone also, an abundant Mono- 

 stroma- vegetation frequently occurs, both on a rocky substratum, 

 and also very often as epiphytic vegetation in other associations, 

 as, for example, the f/a/osaccz'on-association, the Co/'a/Zzna-association, 

 the Polysiphonia urceolata- association and others. . In water-filled 

 depressions in both the upper and lower littoral zone the Mono- 

 stroma species often occur abundantly. 



Monostroma Grevillei var. arctica seems to belong more imme- 

 diately to the littoral zone. In many places it is found entirely laid 

 bare, sometimes on a clayey substratum, when it often attaches 

 itself to small individuals of Mytilns edulis, and sometimes where 

 the substratum of the littoral zone is pebbly. During April and May 

 this vegetation is rather characteristic, because then, the species 

 occurs for the greater part in the inflated stage. The bladders vary 



The Botany of Iceland. I. 9 



