ABSORPTION OF WATER 61 



(3) The condition of the water. Plants differ in their demands 

 for water. Plants with a very fine, fibrous root system, draw almost 

 entirely upon the slight films of moisture surrounding each soil par- 

 ticle. Plants with few roots require that the moisture be present more 

 abundantly. Corn seems to take a place rather between these ex- 

 tremes. The root system is not fine enough to absorb moisture from 

 a dry soil, and yet the plant will not thrive in a saturated stratum. 



FUNCTION OF WATER. 



In Plant Growth, Water has six distinct functions. 



(1) Water is an essential constituent of the plant. 



The most abundant constituent of growing farm crops is water. 

 In chemical combination with carbon, it enters into almost every com- 

 pound stored or used by the plant. 



(2) Water regulates the temperature of the plant. 



When there is danger of excessive heat injuring the plant, the 

 rapid evaporation of water from the leaves reduces the temperature. 

 This is proved in the corn field in July. The temperature may rise 

 very rapidly to extreme heat, but the moisture which is taken up by 

 the roots is continually evaporating from the leaves; this keeps the 

 whole plant cool. If the moisture supply be deficient, evaporation is 

 diminished and the temperature of the plant rises. 



(3) Water maintains turgidity. 



A cell which has absorbed water until it is exerting considerable 

 stretching force upon the cell walls is said to be turgid. The moisture 

 necessary to maintain the turgidity of the plant is obtained from the 

 soil by the root hairs. These hairs draw upon the capillary and 

 "hygroscopic" water within their reach. The root system receives this 

 moisture and passes it from cell to cell into the tubes of the central 

 cylinder. 



The moisture continues its upward course as sap. Just why sap 

 rises has never been entirely satisfactorily explained. It is probably 

 due to a combination of physical phenomena; among them root pres- 

 sure, capillarity, the "purriping" action of certain cells of the stem and 

 the higher concentration of the cell sap where transpiration is rapid. 

 The passage of moisture from these tubes to the cells is affected by 

 osmosis. This is the diffusion of liquids through a membrane in which 

 no openings are visible. 



Vapor is transpired, or evaporated through minute openings on the 

 surfaces of the leaves of a plant. These pores or stomata are sur- 

 rounded by guard cells which open or close according to the amount 

 of water stored in the plant. They help to regulate the degree of 



