702 PLANT GROWTH AND PLANT COMMUNITIES 



cyclic changes in moisture content. The amplitude of such diurnal 

 variations is determined by the intensity and duration of the periods of 

 high transpiration and by the nature of the S = f(P) function for both 

 the soil and the plant tissues. Moisture readjustments in soils are 

 known to occur rather slowly, particularly at higher values of P, as a 

 consequence of the well-established hysteresis effect that exists in the 

 C = f(P) relationship. Less quantitative information is available con- 

 cerning hysteresis in the C = f(P) function for plant tissues, but data 

 obtained by Slatyer ( 1957 ) and reproduced in Figure 5 indicate that 

 the phenomenon occurs in plants. If complete moisture recovery does 

 not take place during the readjustment period of the diurnal cycle, 

 progressive moisture depletion occurs in the region adjacent to the 

 root, to the point where the liquid flow approaches zero. 



From the preceding discussion it is clear that the ability of a soil 

 to supply water to a living plant is not determined solely by its mois- 

 ture content (Gingrich and Russell, 1957). It is not to be expected, 

 therefore, that any single-valued relationship should exist between 

 soil-moisture content and plant response to moisture. Rather, the rela- 

 tion between water uptake and soil-moisture conditions will be rate- 

 dependent as well as potential-dependent, and will be strongly affected 

 by the dynamic characteristics of the soil-plant-water system. 



In a recent quantitative assessment of the role of water movement 

 on its availability to plants, Gardner ( 1960 ) has calculated the distri- 

 bution of potentials as a function of time and the radial distance away 

 from a cylindrical absorbing root of unit length. A single root of unit 

 length was analyzed in an infinite, two-dimensional, isotropic medium. 

 Assuming cylindrical symmetry, the flow equation may be written as: 



8C 1 8 8G 



— = (rD ^) 



St r 8r 8r 



where C, D, r, and t represent the volumetric moisture content, diffu- 

 sivity, radial distance, and time, respectively. The curves shown in 

 Figure 6 are for t =: 1 day and for a flux of 0.1 ml. per cm. per day in a 

 Pachappa sandy loam having initial moisture potentials of — 5 bars and 

 — 15 bars. The relations shown in Figure 6 were from the following 

 solution of the preceding equation: 



F 4Dt 



-aP= . (In .577) 



47rk r^ 



where P, F, k, D, r, and t have the previously assigned meanings and t 

 was taken as one day. Gardner also calculated the difference in poten- 



