[Chap. XLVI UNDER-WATER ENVIRONMENTS 



The Land Habitat 



589 



The Water Habitat 



Plants and animals with high 

 water content living in a medinm of 

 air. 



The medium: the atmosphere 

 above soil always in motion, and its 

 molecnles always in rapid motion; 

 rarelv saturated with water \'apor; 

 compressible, hence rarely destruc- 

 tive when in motion; air and water 

 movement in soil restricted by soil 

 particles. 



Carbon dioxide: always available 

 above and below soil surface. 



Oxygen: always available above 

 soil surface, not alwavs available 

 below soil surface. 



Temperature: large daily and sea- 

 sonal fluctuations of air temperature; 

 smaller variations of soil tempera- 

 ture, except where relatively dry soil 

 surface is exposed to the sun, where 

 the daily and seasonal extremes com- 

 monly exceed tliose of the air. 



So/7; stability, penetrability, poros- 

 ity, gas content, and water content 

 important in root development. In- 

 organic salts in soil and dust avail- 



o 



able when in solution or in direct 

 contact; concentration sometimes 

 very low; in arid regions more con- 

 centrated and sometimes toxic. 



Light: usually abundant at upper 

 vegetation surface; decreased at 

 lower levels by taller plants. 



Plants and animals with high 

 water content living in a medium of 

 water. 



The medium: the hydrosphere rel- 

 ativelv quiescent, except surface 

 lavers of large bodies of water and 

 rapid streams; molecular motion 

 comparativelv slow; incompressible, 

 hence offers great resistance to move- 

 ment within it, and when in rapid 

 motion very destructive. 



Carbon dioxide: in solution, both 

 free and bound in bicarbonates; con- 

 centration variable, often high in 

 water containing organic matter and 

 in soil beneath water. 



0x1/ gen: in solution; concentration 

 highest in surface layer, often de- 

 ficient in deep water and in soil be- 

 neath water. 



Temperature: smaller daily and 

 seasonal fluctuations even in upper 

 layers; temperature very constant in 

 deeper water; temperatures of water 

 and underlying soil not very dif- 

 ferent. 



Soil: stability, porosity important. 

 Inorganic salts obtained from me- 

 dium by suspended plants, from soil 

 by rooted plants; salt concentration 

 sometimes very low; usually higher 

 in oceans than in moist soils; in 

 closed basins salts may accumulate 

 to toxic concentrations. 



Light: abundant at water surface 

 except for clouds, fogs, and shade of 

 plants above surface; rapidly de- 

 creases below surface. Rough sur- 

 faces and turbidity increase reflec- 

 tion, decrease light penetration. 



