94 SOILS AND PLANT LIFE 



side by cells, known as guard cells. When there is more 

 than enough water to unite with the carbon dioxide to 

 form starch, and when the sun is shining, these cells draw 

 apart and allow the water in the leaf to escape and the 

 carbon dioxide to enter. This escape of water, which is 

 in the form of invisible vapor, through the stomata 

 of the leaves is called transpiration. 



The roots of a plant must absorb from two hundred 

 and twenty-five to nine hundred and fifteen pounds of 

 water for every pound of dry matter, or food material 

 produced. In times of drouth, the stomata are partially 

 or nearly closed, so that transpiration becomes slower. 

 During the night, the stomata are closed, but the roots 

 absorb moisture in the darkness as well as during the day- 

 time. It follows that when moisture reaches the leaves 

 in the night, it can not escape, but accumulates both in 

 the leaves and back in the stem, making them turgid. 

 This explains why plants look fresh in the morning and 

 why com will break under the cultivator in the early 

 morning bvrt not at noon. 



We are able by a very simple experiment to determine 

 how much water escapes from the leaves. 



EXERCISE 32 



Object. — To determine how much water the leaves of a 

 plant throw off. 



Procedure. — Cover with melted paraffin the outside 

 and bottom of a flower pot containing a healthy plant. 

 Put a cork in one end of a glass tube and thrust the open 

 end into the soil in the pot. Cover the surface of the 

 soil with melted paraffin which has cooled enough to do 

 no injury to the stem of the plant. 



Notice that no water can now escape from the soil in 

 the flower pot except by passing out through the plant; 



