28o ABSORPTION OF WATER 



sorption" is, with rare exceptions, relatively insignificant. A potted herbaceous 

 plant, for example, which will exude only a few cubic centimeters of sap 

 per day when decapitated may lose as much as fifty to a hundred cubic centi- 

 meters of water per day by transpiration, practically all of which represents 

 "passive absorption." 



Factors Influencing the Rate of Absorption of Water. — Any factor 

 which influences the diffusion pressure of the water in the peripheral walls of 

 the young roots or the diff^usion pressure of the water in the soil will influence 

 the rate of absorption of water. Furthermore the root system of a plant is 

 more or less continually growing through the soil and when the water con- 

 tent of a soil is less than the field capacity absorption of water at any appre- 

 ciable rate can occur only if growth of roots through the soil continues. 

 Factors which influence the rate of root growth may therefore also have im- 

 portant effects on the amount of water which can be absorbed. 



For reasons which should be clear from the preceding discussion the rate 

 of absorption of water is greatly influenced by the rate of transpiration. 

 Hence any factor which influences the rate of transpiration will indirectly 

 influence the rate of absorption of water. Contrariwise, as already shown in 

 the discussion of transpiration, any factor which influences the rate of absorp- 

 tion will also influence the rate of transpiration. The more important soil 

 factors which influence the rate of absorption of water will now be discussed 

 briefly. 



1. Available Soil Water. — This term is generally used in a loose way to 

 refer to that fraction of the soil water which can be absorbed by plants. 

 For most practical purposes the available soil water represents that portion 

 in excess of the wilting percentage. Within limits a more rapid rate of 

 absorption of water is possible when the available soil water content is high 

 than when it is low. This is reflected in the more rapid rate of transpiration 

 which may be observed in a plant when the soil is well provided M'ith water 

 as compared with the same or a similar plant growing in a soil deficient in 

 water. As shown in Table 27 the greater the percentage of water in a soil, up 

 to a certain value, the greater the rate of transpiration. The retarding effect of 

 relatively high soil water contents upon the rate of absorption of water is due 

 to the accompanying decrease in soil aeration. 



2. Soil Temperature. — The influence of soil temperature upon the rate 

 of absorption of water varies greatly with the species. When plants whose 

 roots had developed in solution cultures were transferred from a temperature 

 of about 20° C. to a temperature of 0° C. the rate of absorption was greatly 

 retarded in some species, but scarcely affected in others (Doring, 1935). In 



