160 



ROOTS 



creases. They retard absorption and may become so great as to 

 actually prevent it. The wilting of plants when the soil becomes 

 dry is not due to the fact that there is no water in the soil, but to 

 the fact that the roots can not pull the water, known as the un- 

 available water, away from the soil particles. It has been found 



71 



FIG 141. Lengthwise section through a root, showing the way the water 

 and mineral substances of the soil reach the vascular bundles, e, epidermis 

 with root hairs; c, cortex; a, endodermis; p, pericycle; x, xylem of vascular 

 bundle. The arrows indicate the way the water and dissolved substances 

 pass to the vascular bundles. After MacDougal. 



that most plants wilt when the soil moisture is reduced to 4.6 per 

 cent in medium fertile garden soil, 7.8 per cent in sandy loam, and 

 49.7 per cent in peaty soil. From these figures it is seen that the 

 amount of unavailable water depends much upon the kind of soil. 

 As shown in the table below, it also depends much upon the kind 

 of plant, for plants differ widely in their ability to pull water 

 away from the soil particles. 



UNAVAILABLE WATER FOR DIFFERENT PLANTS IN LOAM SOIL 



Plant. 



Cabbage 



Corn 



Oats 



Asparagus , 



Lettuce 



Cucumber 



Pondweed (a water plant) 



Unavailable water. 



Per cent. 



5.8 

 5.9 

 6.2 

 7.0 



8.5 

 10.8 



24.8 



