282 ABSORPTION OF WATER 



is another species in which absorption is soon retarded if the soil temperature 

 is relatively low. Cotton plants wilt when soil temperatures are lowered to 

 between 17 and 20° C. (Arndt, 1937). 



The mechanism whereby low soil temperatures cause a retardation in the 

 rate of absorption is undoubtedly a complex one. Among the factors involved 

 are probably: (i) increase in the viscosity of water, (2) decrease in the per- 

 meability of the cj'toplasmic membranes of the root cells to water, and (3) a 

 diminution in the physiological activity of the root cells. For obvious reasons 

 virtually no water is absorbed by roots from frozen soils. 



3. Aeration of the Soil. — In general absorption of water by the root sys- 

 tems of most species of plants proceeds more rapidly in well aerated soils than 

 in those which are not. Poorly aerated soils contain a lower concentration of 

 oxygen and a higher concentration of carbon dioxide than the atmosphere. 

 Reduction in the available oxygen supply, at least when severe, reduces the 

 rate of respiration of the roots. If this process is very greatly checked, the 

 rate of root growth and other metabolic processes within the root cells are 

 greatly disturbed. While the roots of most species of plants can survive for 

 short periods in soils practically devoid of oxygen (saturated soils, etc.), as a 

 result of the occurrence of anaerobic respiration (Chap. XXX), maintenance 

 of this process for any considerable length of time by the roots of most species 

 leads to a stunting or even death of the roots. The results of the various 

 physiological disturbances induced by lack of oxygen is a lower rate of absorp- 

 tion of water. Henderson (1934) has shown that there is a close correlation 

 between the rate of absorption of water by maize seedlings and the rate of 

 respiration of their roots. 



The retarding effect of poorly aerated soils upon absorption of water may 

 also be due in part to a toxic effect of carbon dioxide on the root cells. 



In contrast with other species of plants the roots of hydrophytes normally 

 grow in saturated soils, and absorb water readily from such soils. Some spe- 

 cies of hydrophytes have well developed intercellular air passages which lead 

 from the leaves down through the stems into the roots; oxj'gen luidoubtedly 

 moves to the roots through such channels. In other species of hydrophytes, 

 however, no such air conductive system is present. The roots of such species 

 are apparently able to carry on their processes at relatively low dissolved 

 oxygen concentrations. 



If a plant with a root system which has developed in the soil is trans- 

 planted so that its roots are immersed in a solution culture they will often 

 die within a relatively short time. Flooding the soil in which a plant is rooted 

 often has the same effect. In many species if the root system is allowed to 

 develop from the beginning under water it will grow thriftily in spite of the 



