144 TEXTBOOK OF PLANT PHYSIOLOGY 



is very high. In such places a special flora develops, the so- 

 called "halophytes." These plants show the faculty of accumu- 

 lating an enormous amount of salts and develop a very high 

 osmotic pressure, up to 100 atmospheres or more. 



Besides this osmotic resistance to suction, another type of 

 resistance due to adsorption is found in the soil. The soil consists 

 chiefly of sand particles of different size together with more or less 

 decayed organic matter of a colloidal nature. Inorganic col- 

 loids also are present. The water, moistening the soil, is con- 

 nected in various degrees with these principal parts of the soil. 

 Some of the water fills the larger spaces in the soil and remains in a 

 rather mobile condition. This is called ground water. It obeys 

 the force of gravity, descending into the soil after a rainfall. In 

 the smaller soil capillaries, the water is retained by the forces of 

 surface tension. It does not follow the force of gravity. This 

 water may rise in the soil, though slowly, above the level of the 

 ground water. It is called capillary water. The force by which 

 capillary water is retained in the soil is small, usually a fraction of 

 an atmosphere, depending on the diameter of the capillary. This 

 water, therefore, is absorbed by the root hairs without difficulty, 

 usually by the surface cells of the absorbing zone of the root. 

 Ground water, of course, is absorbed still more readily. 



Water directly surrounding the soil particles is in a different 

 state. This water is retained by the force of molecular attraction, 

 or adsorption. This force is of a considerable magnitude. More- 

 over, it increases rapidly as the water film surrounding the seil 

 particle grows thinner. Such a film of water is available to the 

 plant only with difficulty. Finally, air-dry soil contains, from 0.5 

 per cent in coarse sand, to 14 per cent in heavy clay, of so-called 

 "hygroscopic" water. Hygroscopic water is retained by the soil 

 particles with a force reaching as high as 1,000 atmospheres and 

 may be unavailable to the plant. 



The colloidal substances of the soil have the property of swell- 

 ing in water. They develop considerable water-holding capacity. 

 The more of these colloidal substances are present in the soil, the 

 more water is bound to them. The amount of this so-called 

 "imbibitional" water is especially great in peat soils, which con- 

 sist almost entirely of partially decayed plant residues. 



In the soil the root hair has to compete for water with the 



