288 PLANT PHYSIOLOGY 



amount of salts and develop a very high osmotic pressure, up to 

 100 atmospheres or more. 



Besides this osmotic resistance to absorption, another type of 

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

 consists chiefly of solid particles of various sizes together with 

 more or less decayed organic matter of colloidal nature. Inor- 

 ganic colloids also are present. The soil water is distributed in 

 various degrees through these phases of the soil. Some of the 

 water fills the larger spaces in the soil and remains in a rather 

 mobile condition. This is called ''gravitational water." It 

 moves under the force of gravity, descending into the soil after a 

 rainfall. In the smaller soil spaces, water is retained by the force 

 of surface tension. This is called ''capillary water," and it is 

 held against the force of gravity. 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 space, or capil- 

 lary. This water, therefore, is absorbed without difficulty by 

 the root hairs, in general by the surface cells of the absorbing 

 zone of the root. Gravitational water, of course, is taken up 

 still more readily. 



Water directly surrounding the soil particles is retained by the 

 force of molecular attraction, or adsorption. This force is of a 

 considerable magnitude. Moreover, it increases rapidly as the 

 water film surrounding the soil particle grows thinner. Such a 

 film of water is available to the plant only with difficulty. Fin- 

 ally, 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 is unavailable to the 

 plant when the force with which it is held exceeds the suction 

 tension of the cell. 



The colloidal substances of the soil have the property of 

 swelling in water. They have a considerable water-holding 

 capacity. The more of these colloidal substances there 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 consist almost entirely of partly decayed 

 plant residues. 



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

 surrounding soil particles. The smaller these particles are and 



