STRUCTURE OF A ROOT 143 



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 distance of ten to twenty feet below the surface, until 

 they reach that part of the soil which is always moist with under- 

 ground water. 



Self-Testing Exercise 



A root system consists of ....;.. . (1), (2), and (3) 



roots. The end of the root is (4) to (5) and turns 



toward the center of the (6). Roots also respond to 



(7) in their environment and will penetrate many (8) 



into the (9) in order to get it. The effect of (10) 



on roots is seen by planting seeds in a (11) garden. 



PROBLEM III. HOW DOES THE STRUCTURE OF A ROOT 

 FIT IT FOR ITS WORK? 



Demonstration 4. The finer structure of a root. Use a prepared 

 slide or hand sections of bean roots stained with eosin or iodine and 

 place it under a microscope. 



How a root is built. If we study the diagram on page 144 and 

 compare it with what we see under the microscope, we find the root 1 

 is made up of cells, the walls of which are rather thin. Over the 

 lower end of the root is a collection of cells, most of which are dead, 

 arranged loosely so as to form a cap over the growing tip. This is 

 evidently an adaptation which protects the young and actively grow- 

 ing cells just under the root cap. In the body of the root a central 

 cylinder of wood can easily be distinguished from the surrounding 

 cortex. It is in the cortex of fleshy roots that foods are stored, as 

 in. the carrot or turnip. In a longitudinal section a series of tube- 

 like structures may be found within the central cylinder. These 

 structures are made up of cells which have grown together end to 

 end, the long axis of the cells running the length of the main root. 

 In their development these cells have grown together in such a 

 manner as to lose their small connecting ends, and now form con- 

 tinuous hollow tubes with rather strong walls. Other cells have 



1 Sections of tradescantia roots are excellent for demonstration of these 

 structures. 



