GROWTH OF ROOTS 



67 



Plant mustard or radish seeds in a pocket garden, place it on one 

 edge, and allow the seeds to germinate until the root has grown to a 

 length of about half an inch. Then turn it at right angles to the 

 first position and allow it to remain for one day undisturbed. The 

 root tips will be found to have turned in response to the change in 

 position, and to point down again. That part of the root near 

 the growing point is the one most sensitive to the change. Thi? 

 experiment indicates that the roots are influenced to grow 

 downward by the force of gravity and that the growing point is 

 most responsive to this stimulus. Roots are positively geotropic. 



Root of a radish in a pocket garden that is turned in different positions at 

 intervals of a day or more. 



Water a Factor which determines the Course taken by Roots. — 

 Water, as well as the force of gravity, has much to do with the direction 

 taken hy roots. If radish seeds germinate on the under side of a 

 moist sponge suspended in the air, their roots will turn against 

 gravity and cling to the wet surface of the sponge. Water is always 

 found below the surface of the ground, but sometimes at a great 

 depth. Most trees and all grasses have a greater area of surface 

 exposed by the roots than by the branches. The roots of alfalfa and 

 sugar beets, in our Western States, often penetrate the soil for a dis- 

 tance of ten to twenty feet below the surface, until they reach that 

 part of the soil which is always moist with underground water. 



The Fine Structure of a Root.^ — Let us now examine a root with 

 the aid of the compound microscope, in order to see how it is 

 fitted for the work of taking in and circulating soil water. We 

 find the root to be made up of cells, the walls of which are rather 

 thin. Over the lower end of the root is found a collection of cells, 

 most of which are dead, loosely arranged so as to form a cap over 



^ Sections of tradescantia roots are excellent for demonstration of these structures. 



