ABSORPTION OF WATER BY AERIAL ORGANS 283 



inadequate aeration of liquid water or saturated soils. Roots that develop 

 under water can often endure deficient aeration because they are structurally 

 and probably also physiologically different from the roots which develop on 

 individuals of the same species in a well aerated soil. The cortex of "water 

 roots," for example, usually has larger intercellular spaces than the cortex of 

 "soil roots" of the same species. 



4. Concciitration of the Soil Solution. — The concentration of the soil 

 solution of most soils is so slight as to have little or no influence on the dif- 

 fusion pressure deficit of the soil water, which in most soils, as long as they 

 are fairly moist, does not exceed i atmos. In alkaline or saline soils the con- 

 centration of the dissolved salts in the soil water is often sufficient to raise the 

 osmotic pressure of the soil solution to a very considerable value — sometimes 

 100 atmos., or even higher. Intensively cultivated soils, such as those of 

 greenhouses, truck gardens, or irrigated lands sometimes become heavily 

 charged with soluble salts due to the copious application of fertilizers, or to 

 salts brought into the soil by irrigation water. 



The diffusion pressure deficit of soils of high solute content, except when 

 relatively dry, i.e. below the field capacity, is essentially equal to the osmotic 

 pressure of the soil solution. In general therefore the rate of absorption of 

 water and hence of transpiration is impeded more or less in proportion as the 

 concentration of the soil water solution increases. In solution cultures with 

 an increase in the osmotic pressure of the solution there is a corresponding 

 decrease in the rate of absorption of water by plants. Plants may, within 

 limits, become adjusted to an increased concentration of the soil solution 

 inasmuch as the osmotic pressure of the cells of the plant may increase under 

 such conditions (Table 20). Hence a plant in which the absorption rate is 

 markedly checked when its roots are first brought into contact with a soil 

 solution of higher concentration than the one in which it had been growing, 

 may, after an interval of time, entirely or nearly regain its original rate of 

 absorption of water. Most species of plants can develop normally only when 

 the osmotic pressure of the soil solution does not exceed a few atmospheres. 

 The principal exceptions are species which are native to saline or alkali soils, 

 the so-called halophytes. 



Absorption of Water by the Aerial Portions of Plants. — Leaves and 

 other aerial parts of plants are frequently brought into contact with liquid 

 water when raindrops gather on their surfaces, or when dew condenses upon 

 them. Wetzel (1924) found that of a large number of species investigated 

 practically all absorbed some water directly through the epidermis of the 

 leaves. The turgidity of the leaves of a large proportion of the species studied 

 was restored from the wilted condition in 24 hours or less when they were 



