MARINE ALGAL VEGETATION 119 



The following species of the under-vegetation occur in such 

 quantities that tliey form associations of considerable extent which 

 are usually pure, but may sometimes be mixed: — 



Hildenbrandia rosea. 1 Gigartina mamillosa. 



Rhodochorton Rothii. Ceramium rubrum. 



Sphacelaria britannica. Callithamnion Arbuscula. 

 Polysiphonia urceolata. 



The Hildenbrandia-association is most extensively distributed, 

 and is found everywhere along the coasts. It usually occurs as a 

 pure association, though, in many places, Verrucaria mucosa is inter- 

 mingled with it. Hildenbrandia may also occur at the same height 

 outside the Focus-covering, but is then — if the light is not sub- 

 dued by other plants or projecting rocks — usually of a darker 

 colour. 



The Rhodochorton-association often covers extensive areas, 

 like a dense, red carpet of felt. Most often it is pure, and appears, 

 as far as its littoral distribution is concerned, to be confined mainly 

 to the shady parts of the littoral zone. A few of the other shade- 

 species may be found intermingled with it in small numbers, which 

 fact is of minor importance, whereas the occurrence of the epiphyte 

 Pleurocapsa amethystea is of great importance, as this species is 

 found only on Rhodochorton Rothii. 



R. Rothii does not occur below 7 the Fucus-be\t of the littoral 

 zone until it occurs again at a considerable depth on Laminaria 

 stems; thus it is found in two distant belts: the shade-belt of the 

 littoral zone and the Laminaria-belt. 



A Sphacelarietum (S. britannica) occurs almost quite pure 

 in many places over rather large surfaces; it forms a dense covering 

 on the rock as does R. Rothii. Sphacelaria radicans, Polysiphonia 

 urceolata and Rhodochorton Rothii occur intermingled. 



Polysiphonietum. P. urceolata occurs in a similar manner, 

 covering rather large surfaces of rock. In places where the species 

 forms extensive mats, it is usually very low in growth, yet always 

 considerably higher than Rhodochorton Rothii. A thin layer of fine 

 clay may often be seen to have accumulated between its basal parts. 



A Gigartinetum occurs as under-vegetation, especially in more 

 exposed places, and is then generally found in the lowest part of 

 the Fucus-belt or in the Fucus in flatus-association; it is a direct 

 continuation upwards of the Gtgarfr'na-association below. 



1 Arranged in accordance with the frequency, the most frequent coming first. 



