Chap. II] 



MARINE VEGETATION 



789 



accordingly provided with root-like fixing organs that penetrate the soil 

 (Fig. 481). 



The number of epiphytic Algae is large. They are frequently identical 

 with lithophytes and exhibit no special adaptations (Fig. 477,^). Hemi- 

 parasitic forms also are frequent among the Florideae (Fig. 477. e). 



The macrophytic algae are almost exclusively denizens of the pJiotic region, 

 and t/ie phanerogams are exclusively so. The photic region may be sub- 

 divided into two belts : the e merging belt and tJie submerged belt. 



The emerging belt extends landwards from low-tide mark above high- 

 tide mark to a distance proportionate to the strength of the surf. Its 

 flora is characteristic and depends on the conditions that prevail — intensity 

 of light, violent movement, alternation of water and air. It is also again 

 subdivided into belts of unequal duration of emersion. The plants forming 

 the lowest belt are attached just above the low-tide mark, so that they 

 always remain with the greater part of their members submerged ; here 

 the conditions are the most favourable, and the 

 largest plants occur. The highest belt, on the 

 contrary, involves the risk of desiccation and 

 accordingly has a scanty vegetation. Dense low 

 growth, strong thickening of the membranes, 

 and squarrose close branching are character- 

 istic of most of the emerging plants, which are 

 exclusively Algae. 



The submerged belt includes all the marine 

 Phanerogamae and the main mass of the marine 

 Algae. Here also belts of depth may be distin- 

 guished ; but these depend on the reduction in the 

 amount of light with increasing depth, therefore 

 on a factor not operative in the emerging belt. 



Frequently the green Algae predominate 

 chiefly in the upper part of the belt, the brown 



Algae in the middle part, and the red Algae in the lowest part ; yet the 

 connexion between the colour of Algae and the depth of the habitat is 

 by no means so general and so pronounced as older authors assumed, 

 especially Oersted, who even founded on it a subdivision into regions. 

 The Phaeophyceae appear in the upper parts of the belt, the Chlorophyceae 

 are frequently predominant in its middle parts, whereas the Rhodophyceae 

 usually preponderate when the light is feeblest. The Rhodophyceae 

 also are more prevalent with increasing depth, because they are very 

 sensitive to light, and when exposed to strong light near the surface they 

 undergo discoloration, which does not, however, necessarily prejudice their 

 vital activity. 



Light again takes a foremost place in determining the horizontal topo- 



Fig. 478. Cymbella cistula, 

 Hemp. (Diatomaceae). Long- 

 stalked colony. Enlarged. After 

 W. Smith. 



