CHAP, xxxvi OECOLOGICAL FACTORS 151 



change in colour and assume that which is complementary to the light 

 acting on them, and by this means assimilate more vigorously. 



Temperature of water. Submerged aquatic plants are exposed to 

 far smaller extremes and fluctuations of temperature, both diurnal and 

 annual, than are land-plants, because water has a high specific heat and 

 is a bad conductor of heat : annual changes of temperature descend only to 

 relatively small depths except in shallow water. Many aquatic plants 

 hibernate in their green state, because no considerable degree of cold reaches 

 them, and most of them are perennials. Their optimum for growth is 

 generally low ; certain species, including Hydrurus (an alga belonging to 

 the Phaeoflagellata), thrive only in very cold water. The disappearance 

 of many algae in summer may be due to their optimum temperature 

 having been exceeded. Arctic fresh-water lakes are usually very poor 

 in organisms a circumstance that may possibly be ascribed to the 

 temperature of the water. Algae are frequently very sensitive to sudden 

 changes in temperature, 1 in salinity or in other conditions. Each species 

 has its own peculiarities. 



High temperatures are encountered only in hot springs, where the 

 plants growing are almost exclusively Oscillarieae and other Cyano- 

 phyceae, which may be representatives of the vegetation that was the 

 first to appear on Earth. 



The temperature of water decreases as the distance below the surface 

 increases, but at different rates in fresh and salt water. In standing 

 fresh water, at the bottom of lakes which are so deep that the annual 

 variations of temperature in the upper strata cannot affect the strata 

 at the bottom, the temperature is about 4 C., because fresh water attains 

 its maximum density at this point. Strata of water lying above this 

 may thus be much colder in winter. The temperature at the bed through- 

 out the year in Swiss lakes is about 5 C., but is always below 4 C. in 

 Baltic lakes, which are frozen at the surface during winter. In the sea, 

 on the contrary, strata of water are colder the deeper they lie ; moreover, 

 warm, or cold and salt, currents may be intercalated between them. The 

 influence of temperature upon the distribution of aquatic Spermophyta 

 is proved by an observation made by Magnin, 2 who discovered that they 

 descend to a depth of n metres in the warmer Jura lakes, but only 

 down to 6 metres in deep and cold lakes. 



Temperature affects the amount of gas dissolved in water ; the colder 

 this is the richer is it in oxygen and carbonic acid, and the more favourable 

 may be the conditions for nutrition and consequently for growth. This 

 is possibly the cause of the luxuriant development of algal vegetation 

 in arctic seas. 



Nutritive and other substances dissolved in water. Water con- 

 tains in solution many substances which vary according to the kinds of 

 rock or soils with which it has entered into contact. Calcic carbonate is 

 commonly present, being dissolved by the contained carbonic acid ; and, 

 as many aquatic plants (Characeae, species of Potamogeton, and mosses) 

 seize upon the carbon dioxide contained in the calcium hydrogen carbonate, 

 incrustations of chalk are excreted on their surfaces, and may lead to 

 deposits of lime on lake-beds. 3 



1 Oltmanns, 1892. * Magnin, 1894. C. Wesenberg-Lund, 1901. 



