THE HOOT AND ITS FUNCTIONS 



47 



The fibro-vaseular cylinder occupies the core of the root, 

 furnishing mechanical strength and serving as a highway for con- 

 duction. As in other organs of the plant, it is composed of two 

 main tissues, the wood or xylem and the hast or phloevi. The 

 wood, which forms the central axis of this cylinder, is usually 

 star-shaped in cross section and is composed for the most part 

 of thick-walled and much elongated dead cells, the walls of which 

 have become woody. It provides rigidity for the root and 

 conducts upward the water and dissolved substances which 





Fig. 29. — Diffusion of a dissolved substance. Diagram representing the 

 outward diffusion of molecules which are being dissolved from the surface of a 

 soluble substance immersed in water. 



enter from the soil. Between the points of the star are patches 

 of bast, formed of thin-walled cells which transport manufactured 

 food upward or downward. 



In older roots the fibro-vascular cylinder, particularly as 

 to its woody portion, increases greatly in thickness through the 

 activity of a growing zone or cambium, just as does the stem; 

 and a corky bark is usually developed on the outside. 



Diffusion and Osmosis. — The most important function of 

 the root is to absorb from the soil the water and mineral sub- 

 stances needed for the plant's life and growth. This involves 

 the physical processes of diffusion and osmosis, a consideration 

 of which is necessary before we can understand clearly this 

 primary activity of the root. 



Diffusion.- — Diffusion may be defined as the tendency of any 

 substance, when it occurs as a gas or in solution, to become 



