FROM ROOT TO LEAF 79 



ture spread evenly through the soil. As the moisture spreads 

 it carries with it particles which the plant uses in making 

 food. Thus, in part, the fertility of the soil is renewed. 



The movements through the wall of the root-hair are 

 not all inward. Substances pass out of it into the soil as 

 well as into it out of the soil. Substances which pass out 

 of living bodies are called excretions. It is known that 

 these plant excretions affect the fertility of the soil. This 

 is a matter of much importance in agriculture. It has 

 been found that plant excretions may have injurious effects 

 upon other plants of the same kind, and this may be one 

 of the important causes of decrease in the fertility of soil. 



From the root-hair the water and the soil substances dis- 

 solved in it move into the young root. Only the young 

 roots do this work of absorption. Upon leaving the root- 

 hair, the water enters first a tissue of thin-walled cells 

 called the cortex. (Study Figure 2g.) The cortex is much 

 softer than the central part of the root which it surrounds: 

 This central part is called the stele (ste'le). In many 

 young roots you can easily separate the cortex from the 

 stele with your fingers. The stele is composed of several 

 different kinds of tissue. The movement of substances in 

 the plant is principally through the cells of the stele. The 

 water which has come from the soil moves through the 

 cortex and enters that part of the stele called the xylem. 

 (See Figures 2Q and 30.) The cells of the xylem are much 

 longer than they are broad. Through them the water 

 travels toward the leaves. 



These xylem cells have thick walls. They give rigidity 

 to the plant. The trunks of trees are principally com- 

 posed of xylem. As you probably have already guessed, 

 the common name for xylem is wood. 



