THE INTAKE AND UTILIZATION OF WATER 21 



new soil throughout the growing season, leaving those regions from which they 

 have already drawn their water and nutrient supply. 



Furthermore, the absorbing capacity of the root hairs is not limited to that 

 portion of the soil with which they come in immediate contact. As the root 

 hair withdraws moisture from the water films about the soil particles, these films 

 become thinner than those about neighboring soil particles. Since surface 

 tension maintains an equilibrium between the amounts of water on contiguous 

 surfaces, water tends to distribute itself evenly. As a result it flows toward 

 those parts of the soil from which water has been withdrawn, hence, in the 

 direction of the root hairs. In this way the individual root hair is capable of 

 absorbing water which is a considerable distance away from it. 



The movement of the roots, which their growth in length brings about, is 

 likewise a movement in a definite direction, for the root tip is sensitive to differ- 

 ences in the amount of moisture present on opposite sides and responds to this 

 difference by bending toward that side where there is more moisture." By this 

 means roots grow toward those portions of the soil which have the optimum 

 water content. 



Adaptation of Roots to Moisture Conditions. — In addition to the factors 

 already discussed the adaptability of the root system to the condition of the 

 soil is important in enabling the plant to obtain a maximum supply of water. 

 A small water content of the soil, within certain limits, stimulates the roots to 

 greater development, resulting in a greatly increased absorbing surface. In 

 spite of this greater surface, however, the supply of water is often restricted 

 and the portion of the plant above ground is not capable of much development. 

 In one investigation the ratio of roots to tops for oats, grown in dry soil, was found 

 to be 1:7.4 and in wet soil 1:16.16.^'^^ In this second instance the roots 

 remained small because the optimum conditions for moisture were exceeded. 

 These figures give some indication of the correlation which exists between root 

 and shoot development though the relation may be quite different in other 

 plants and under other conditions. The accommodation of roots to soil con- 

 ditions varies with different species. Thus Weaver^^^ finds that 7 out of 10 species 

 investigated by him respond to changed environmental conditions. Pulling^"^ 

 states that characteristically shallow rooted plants such as Picea Mariana, 

 Larix laricina and Betula alba papyrifera, as well as the more deeply rooted 

 Pinus Strobus and P. Banksiana do not adapt themselves, while the shallow 

 rooted Picea canadensis and the deeply rooted Popidus balsamifera exhibit con- 

 siderable plasticity. In another place the root distribution of orchard trees is 

 discussed in greater detail and some of its relations to water supply are mentioned 

 in that connection. 



Factors Influencing Rate of Absorption. — The ability of plants to 

 absorb water depends upon the absorbing surface of their roots and on 

 the following external factors: the power of the soil to deliver water, the 

 temperature and aeration of the soil, its chemical properties and the 

 concentration of the soil solution. Other things being equal the higher 

 the temperature the greater the absorption until a certain optimum value 

 is reached; temperatures above this optimum retard water absorption. 



