HOW THE PLANT LIVES 115 



stomata, are especially abundant on the under 

 surfaces of most leaves. With changes in the 

 water content of the plant, these stomata open 

 or close, to a degree facilitating or inhibiting 

 transpiration. Like evaporation, transpiration is 

 hastened by higher temperatures, dry air, wind, 

 and the movements of the plant. On a very 

 hot day, or with insufficient soil moisture, a 

 plant may wilt, due to the fact that all of the 

 facilities for checking transpiration fail to keep 

 the balance between root absorption and tran- 

 spiration. The plant gives off more water than 

 it takes up; therefore, it wilts. 



189. The absorptive activity of the roots gives 

 rise to a sap- or root -pressure which tends to 

 force the current upward. In fact, the lifting 

 power of transpiration, osmosis, root- pressure, 

 and other forces cause the crude sap to ascend 

 through the woody bundles of the plant; and by 

 means of these bundles absorbed solutions are 

 carried upward through all parts of root and 

 stem, and through the leaf -stalk, veins and vein- 

 lets to all parts of the leaf. 



26. Soluble salts from the soil 



190. Along with the soil water absorbed by 

 the roots, minute quantities of the various min- 

 eral salts necessary for plant growth are taken 

 iii. These salts are in solution. In the plant, 



