THE ROOTS 21 



tissue. Woody tissue possesses in a large degree both rigidity 

 and elasticity, while the cylindrical form is of all forms the one 

 giving greatest rigidity. The stem cylinder is often hollow, 

 but usually the axis is occupied by a core of spongy tissue the 

 pith. This by itself would be of little value as a supporting 

 structure and yet in young shoots it probably adds greatly to 

 the strength of the stem by preventing buckling of the cylinder. 

 In far the greater number of the plants there is also a cylinder 

 of bark which surrounds the woody part. This bark has a 

 double function. It adds greatly to the elasticity of the stem 

 through the layer of fibers the bast which lies directly over 

 the wood and which possesses great tensile strength. The 

 other function of the bark that of protection is subserved 

 by its outer layers which consist either of a smooth and tough 

 epidermis or thick layers of corky tissue, both of which are 

 highly resistant to mechanical injury. 



The Roots 



37. The stem of the plant is firmly anchored in the soil by 

 the roots. Continuing downward from the base of the stem 

 there is often a short, rapidly tapering tap root, while other 

 and longer roots pass out radially and usually at a small angle 

 downward. These master roots branch repeatedly, giving rise 

 ultimately to a vast number of minute rootlets which interlace 

 and penetrate the soil in all directions and through a space of 

 considerable radius. The tap root is often insignificant and 

 the plant is held erect by the combined bracing and guying 

 action of the lateral roots. Only the larger roots where they 

 unite with the stem possess any great degree of rigidity. The 

 more remote parts of the root system have little rigidity or 

 elasticity in comparison with the stem and branches. They 

 are, on the other hand, quite flexible and tough and capable of 

 resisting a considerable pull longitudinally. 



