WATER-RELATION BETWEEN PLANT AND SOIL. 9 



may be the nature of the forces producing the migration of water from 

 soil to plant (it seems that osmotic pressure does not play a prime and 

 direct role in this movement, though the opposite has been frequently 

 supposed),^ any decrease in the water content of the soil layer adjacent 

 to the absorbing surface must result in an increased attraction of this 

 layer for water. The more water is removed the thinner must be the 

 water films about and between the soil grains. Concomitant with this 

 increased attraction must occur an increased resistance to water move- 

 ment from soil to plant and, at the same time, must also occur an 

 increased tendency for water to move into the layer in question from 

 more distant soil layers. If the rate of this latter movement equals 

 the rate of removal of water at the absorbing surface, the increase in 

 resistance to absorption just postulated can not occur and the absorbing 

 surface is then drawing moisture from the more distant layers. On 

 the other hand, as soon as the rate of migration of water from more 

 distant soil to the active layer adjacent to the roots becomes less than 

 that of water entrance into the absorbing surfaces, then the active 

 soil layer (next to the root surfaces) must become drier and an increased 

 resistance to absorption must develop. Thus the power of the soil 

 to supply moisture to roots, depending directly upon the water- 

 attracting power of the adjacent soil layer, must depend indirectly 

 upon the resistance offered by more distant layers to the replacement 

 of the water which has been absorbed. Of course, the latter resistance 

 is a function of the nature and arrangement of these more distant 

 portions of the soil and of their moisture content. It is not to be 

 forgotten that the moisture content of the soil layer adjacent to the 

 absorbing surfaces is largely dependent, at any given time, upon the 

 rate of absorption itself during past time periods. It should also be 

 mentioned here that the tendency of the roots to absorb water and 

 that of the air above the soil to remove water by evaporation may 

 create a tension in the capillary water films for considerable distances 

 through the soil, and this tension acts to retard root absorption, 

 without appreciable drying of the soil. 



It is universally patent that transpiration accounts for practically 

 all of the water requirement of ordinary plants in their vegetative 

 phases and it is no less well known that the transpiration rate varies 

 in magnitude from a minimum in the night to a much greater maximum 

 in the day. It follows, then, as is also well known, that the water 

 requirement of the plant as a whole varies through a similar range of 

 magnitudes. 



^For brief reference to this matter, which should some time be investigated, see : 

 Dixon, 1909. 

 Renner, O., Versuche zur Mechanik der Wasserversorgung. 2. Ueber Wurzeltatigkeit (Vor- 



laufige Mitteilung). Ber. deutsch. Bot. Ges. 30 : 642-8. 1912. 

 Livingston, 1906, page 21. 

 Also see: Livingston, B. E., The relation of the cell sap in plants to arid habitats. Plant World 



U:159etseg. 1911. 



