MARINE ALGAL VEGETATION 139 



these depressions which fact will be mentioned again later on. 

 Some of the depressions are occupied by semi-littoral associations, 

 thus, Corallineta, Gigartineta, Chondreta, Monostrometa, Ceramieta, etc. 

 frequently occur, and, in addition, several of the species which be- 

 long to these associations are found growing in a scattered manner. 

 As mentioned above (p. 125), the vegetation in the pools is 

 highly heterogeneous, and, in the large pools it can frequently be 

 seen that the associations or their representatives arrange them- 

 selves in belts, here on a small scale in the same manner as they 

 do on a larger scale outside the pools. 



c. The Sublittoral Zone. 



The sublittoral vegetation covers the sea-bed from the limit of 

 low-tide down to a depth of 40 60 metres, and perhaps to an even 

 greater depth. Under normal circumstances it is not exposed during 

 low-tide; yet it may happen that the uppermost Laminarice, espe- 

 cially when small, become exposed during extreme low-water, but 

 this must be considered exceptional. What especially characterizes 

 this zone is the circumstance that the associations are always sub- 

 merged. The associations are certainly few in number, but most of 

 them have a remarkably wide horizontal distribution, and the vege- 

 tation, taken as a whole, is homogeneous. With the exception of 

 illumination the conditions of life must be supposed to be stable; 

 in any case, compared with the conditions of life in the littoral 

 zone, they are unstable to a very slight degree. In the littoral zone 

 the change of season is noticeable, but in the deep, under normal 

 circumstances, these changes are very slight. The character of the 

 vegetation is therefore nearly the same all the year round. 



The Laminariacecc vegetation, as previously mentioned, is do- 

 minant in this zone and stretches in a broad belt along the coasts. 

 Beyond the Laminaria-belt another narrower belt occurs composed 

 almost exclusively of Rhodophycece. 



14. The Community of Laminariacese. 



This community occurs especially on a rocky substratum, and 

 extends over large areas of the sea-bed from about the limit of low- 

 tide to a depth of about 30 metres. It forms a belt round the 

 coasts, and varies in breadth according to the conditions of depth 

 and the nature of the substratum. This belt is for the most part 



