Grafts et al. — 180— Water in Plants 



the ecological viewpoint water retention or the prevention of loss is of the 

 greatest importance. From this viewpoint it becomes obvious that the pri- 

 mary function of the above mechanisms and processes in the plant is water 

 conservation. Opening of stomata is necessary for the gaseous exchange 

 essential to photosynthesis ; stomatal closure is just as necessary for the 

 conservation of water, particularly in arid regions. While stomatal open- 

 ing is of general occurrence and most plants photosynthesize, differences in 

 time and extent of stomatal closure are extreme. Many important ecologi- 

 cal relations of plants are related to this process as it functions in water 

 conservation and the adaptation of plants to the supply of water in their 

 environments. 



Under conditions of rapid absorption and low transpiration, water may 

 be lost from plants by guttation. These conditions may prevail in fields of 

 young grain and in lawns (Curtis, 1944) during the night and early morn- 

 ing hours in the spring; they have been observed in vegetable and forage 

 crops and they are prevalent under the humid environment of the tropics. 

 They are favored by warm soils, cool saturated atmosphere and adequate 

 but not excessive soil moisture. 



In contrast to the above situation, it is widely recognized that most 

 plants live the greater part of their lives in a state of negative water bal- 

 ance; that is, transpiration often exceeds absorption and pressure in the 

 xylem is subatmospheric. Transpiration is termed stomatal, cuticular, or 

 lenticular, depending upon the structures through which the water vapor 

 passes, and all three types may occur at the same time. 



Stomatal transpiration is normally the predominant form in leafy plants 

 and it consists of -Z ) evaporation of water from wet cell walls of the stomatal 

 chambers, 2) diffusion from the humid atmosphere of these chambers 

 through the stomata into the external atmosphere, and 3) movement of the 

 vapor from the surface of the leaf where under still conditions it tends to 

 form a molecular cloud. 



Laws of Evaporation : — Although many publications describe the loss 

 of water from plants, soils, and free water surfaces (Leick, 1939), few 

 formulae have been presented that express the nature of this loss in meas- 

 urable terms. Such formulae as exist apply only to relatively simple sys- 

 tems and they prescribe the most rigid experimental conditions. 



Even the simplest case — the evaporation of water from a free water 

 surface — is sufficiently complex so that only a few conditions governing the 

 loss can be exactly expressed mathematically. Nevertheless, certain general 

 principles of evaporation are demonstrable, and apply not only to loss from 

 free water surfaces but, with modification, to transpiration as well. 



Because of the kinetic movement of liquid molecules and the statistical 

 rules governing energy distribution among them, there is a certain escape 

 of high energy individuals from any gas-liquid interface. Since this escape 

 represents a loss of kinetic energy from the hquid cooling occurs, and, if 

 the evaporation rate is to be maintained, heat must flow into it. 



Molecules entering the gas phase partake of the nature of gas mole- 

 cules — that is, they have unordered motion. Consequently, molecular 

 movement at the interface is omnidirectional and condensation as well as 

 evaporation is taking place. When the gas and liquid are at the same tem- 

 perature and in a restricted space, an equilibrium attains ; the gas is saturated 

 and the net loss or gain of water by the liquid is zero. Although evapora- 

 tion and condensation under these conditions have real and equal values. 



