Chap. I] AQUATIC ENVIRONMENT 783 



In the horizontal differentiation of regions the physical condition of the 

 substratum also intervenes. Vegetation assumes a different character accord- 

 ing to whether the texture of the substratum be harder, more stony, or 

 softer, more muddy, or more sandy. The vegetation of marine benthos 

 consists chiefly of lithophytes, that of fresh-water benthos chiefly of plants 

 rooted in mud. 



The chemical nature of the substratum is important only in regard to the 

 smaller expanses of fresh water, the flora of which exhibits great differences 

 according to its poverty or richness in dissolved carbonate of calcium. The 

 flora of depressions in peat-moors is also peculiar. Finally, organic impu- 

 rities, of animal or vegetable origin, also exercise an important influence on 

 the composition of the flora. All these chemical influences are confined 

 to localities of small extent. 



3. PERIODIC PHENOMENA. 



The periodicity of aquatic vegetation is partly governed by factors other 

 than those that are operative in the case of terrestrial plants. Temperature 

 shows smaller fluctuations, is therefore less influential. Perennial marine 

 Algae have no winter rest ; for the most part their activity is chiefly vege- 

 tative in summer and reproductive in winter. But even in warm temperate 

 seas, such as the Mediterranean, the difference in temperature is no longer 

 effective. In the small bodies of inland water, owing to the greater varia- 

 tions in temperature and the easier formation of ice, the difference in the 

 seasons operates more strongly, but even in this case less so than on land. 

 Frequently seasonal variations in light cause a distinct periodicity, for 

 instance in southern seas. Finally, differences in the movements of the water 

 at different seasons of the year are not devoid of significance to the marine 

 benthos. 



4- SPECIAL CONSIDERATION OF THE FACTORS. 



The more important of the factors that have just been enumerated and 

 briefly characterized will be described somewhat more precisely in the 

 following paragraphs, in so far as they are of general significance. 



Salt. The salinity of sea water in inland seas is different from that in 

 oceans, and universally diminishes from the open sea towards the coast. 

 Richest in salt, owing to the great evaporation and the scanty supply of 

 fresh water, is the Red Sea, water of which has been found to contain as 

 much as 4-3 °/ o of salts. Very poor in salts is the Baltic, which, in places, 

 contains less than 1 V . 



