594 TEXTBOOK OF BOTANY 



compounds. Since so small a portion of it is transformed in biological 

 processes in lakes, its concentration is rather constant and uniform. 



Inorganic salts. The chemical elements essential to fresh-water plants 

 are the same as those essential to land plants. Rooted aquatics obtain 

 the salts containing these elements primarily from tlie underlying soil. 

 A much smaller amount enters the plants from the water surrounding 

 their green shoots. Suspended and floating plants are dependent on 

 the salts in solution in the water. The concentration of these inorganic 

 salts in lake water is lower on the average than in soil water. Extensive 

 sand areas in lakes may be as barren of rooted vegetation as similar 

 areas on land, even though the lake water contains sufficient salts for 

 the growth of large numbers of suspended and floating plants. 



Seasonal stratification of water. For a period of time in spring and 

 autumn the temperature of the water at difterent depths in lakes be- 

 comes nearlv uniform (about 4° C). This is the temperature at which 

 water reaches its greatest density. It becomes lighter whether cooled 

 below or warmed above 4° C. As these uniform temperatures develop, 

 convection currents are formed by the lighter water moving upward and 

 the heavier water moving downward. Wind storms at these seasons 

 result in mixing and stirring the water to great depths. These are the 

 spring and autumn "overturns." 



During the summer the surface water becomes warmer and lighter, 

 with the result that layers of warm water float on the colder and denser 

 water beneath. Winds then usually disturb only these upper layers of 

 warm water, and there is little vertical mixing of water. This condition 

 prevails until autumn. As the water at the surface is cooled to 4° C. 

 (39.2° F. ) convection currents carrv the water downward until the lake 

 becomes uniformly dense and the autumnal overturn occurs. 



With the coming of winter the density of the surface water is de- 

 creased again as it is cooled below 4° C. These upper layers of colder 

 and lighter water float on the denser water below. If ice is foiTned it 

 also floats, because its density is about nine-tenths that of water. Under 

 these conditions the water is quite stagnant, and there is little or no 

 mixing. 



During both summer and winter the water in deep lakes may become 

 stratified. In summer, when there are the greatest differences in tempera- 

 ture between the upper and lower layers of water, there is an inter- 

 mediate layer in which the records of temperature on a thermometer 

 change as much as 3° to 5° C. per foot as the thermometer is raised or 



