STRUCTURE AND GROWTH OF ROOTS 137 



plant needs, but also they may be poorer in some of the 

 excretions of the plant which appear to be harmful to the 

 plant itself. 



The root-hairs adhere very closely to the soil grains. 

 It is almost as though they were glued to the grains. This 

 is shown by the fact that the hairs all break off when you 

 pull the root up, or else they come up, carrying the grains 

 sticking to them. This close relation to the grains appears 

 to be an advantage to the plant, for the water that is closest 

 to the grains, the water that forms a sort of invisible 

 iilm around them, is that which is richest in the solutes 

 used by the plant as food materials. 



You remember that substances pass out from the roots 

 as well as enter them. One of the substances which passes 

 out from the root-hairs is carbon dioxide. This substance 

 may exist as a solute in water as well as a gas. You have 

 already heard about it as a gas. You know that leaves 

 absorb it as a gas and use it in photosynthesis. You now 

 find that roots excrete it dissolved in water. It is one of 

 the wastes resulting from the changes which go on inside 

 the plant, as well as one of the substances used in build- 

 ing up food. This carbon dioxide excreted by the roots as 

 waste is of aid in the work of absorption. In the presence 

 of carbon dioxide some of the substances which the plant 

 needs will dissolve in the soil water much more readily 

 than if carbon dioxide is not present. 



E. Cortex and Stele. — ■ The internal structure of a root 

 is better explained by a picture than by words. (See 

 Figure 47.) 



The cortex is composed of thin-walled cells. From cell 

 to cell of the cortex osmosis readily occurs ; the walls are 



