278 ABSORPTION OF WATER 



Many authorities believe that the water in the cells in absorbing regions 

 of roots may often pass into a state of tension under conditions such as those 

 just described. If this occurs greater diffusion pressure deficits could develop 

 in the peripheral cell walls of young roots than would otherwise be possible. 

 However, the mechanism as just described will operate even if the water in 

 the root cells never passes into a state of tension. The osmotic pressure of the 

 root epidermal and root hair cells of most species for which measurements are 

 available is about 3—5 atmos. although higher values undoubtedly occur in 

 some species. Hence diffusion pressure deficits of this magnitude can develop 

 in the peripheral cells of roots even if the water in them is never under tension. 



Whenever the diffusion pressure of the water in the soil exceeds that of 

 the peripheral walls of the young root cells water will move from the soil into 

 the root. Since the osmotic pressure of the soil solution in most soils is only 

 a fraction of an atmosphere the diffusion pressure deficit of the absorbing 

 cells of a root does not have to be very great before water will enter them 

 from any soil with a water content equal to or greater than the fileld capacity. 



The absorption process which has just been described is often called 

 "passive absorption" because the entry of water into the roots is brought 

 about by conditions which originate in the top of the plant and the root cells 

 seemingly play only a subsidiary role. Although the general picture of this 

 mechanism of absorption which has just been presented is probably correct 

 in its essentials, it is almost certainly oversimplified. The influence of cer- 

 tain environmental factors upon absorption, particularly temperature and 

 oxygen, suggests that the metabolic activities of living cells in the absorption 

 zone of roots also play a role in this process. 



The mechanism of absorption just described undoubtedly accounts for the 

 intake of most of the water which enters the roots of plants but it is not the 

 only mechanism of absorption which is known to operate in plants. In many 

 species an internal pressure known as "root pressure" often develops in the 

 xylem (Chap. XV). The occurrence of sap exudation resulting from "root 

 pressure" can be strikingly demonstrated with some species by immersing the 

 root system of a decapitated plant in a potometer (Fig. 39). After a time a 

 dilute sap will begin to ooze from the cut stem, and the absorption of water 

 will be indicated by the movement of the meniscus on the capillary arm of the 

 potometer. If the volume of water exuded is measured it will be found to be 

 not sensibly different from the volume absorbed. In other words water is 

 being absorbed and is moving in an upward direction through the plants as a 

 result of processes which take place in the root cells. 



This type of absorption, in which the mechanism involved is localized 

 within the root system, is often called "active absorption." Root pressure 



