34 VEGETABLE PHYSIOLOGY 



into a continuous system which goes from the teguinentary 

 region towards the interior of the stem, embracing the 

 vascular bundles and attaching them to each other. 



Similar differentiation of the supporting system is found 

 in many leaves, in which it subserves the same purposes. 

 In many cases the veins afford sufficient protection against 

 tearing or rupture in consequence of violent winds. The 

 methods of their arrangement in many cases subserve 

 this purpose very completely. In other leaves of tough 

 leathery habit the delicate tissue of the mesophyll is fre- 

 quently protected from crushing by isolated thick-walled 

 cells of curious shape which extend from one epidermis 

 to the other. Others show bands of sclerenchyma sup- 

 plementing the veins and not infrequently enclosing them 

 and reaching the epidermis on each side. 



The supporting tissue is frequently known as the stereome 

 of the plant. It forms, as we have seen, the most promi- 

 nent part of the endo-skeleton. 



The cells of which the masses of sclerenchyma are 

 composed have been ascertained to possess almost as much 

 power of withstanding longitudinal strain as the finest 

 steel, and they are much more ductile than either this 

 metal or wrought iron. Their arrangement in the different 

 ways described has a very distinct relationship to the 

 character of the strain they have to resist. In such 

 structures as hollow stems where the ring of bundles is but 

 slender, as they are still required to resist lateral bending, 

 the additional supporting tissue is situated near the periphery 

 of the stem, and the latter is often still further strengthened 

 by being furnished with ridges or flanges. An instance of an 

 almost converse character is afforded by a young root. In its 

 growth, while it must possess sufficient rigidity to enable it to 

 penetrate the soil, it must be capable of frequent bending to 

 enable it to avoid obstacles. This is most advantageously 

 provided for by a solid central core of tissue, surrounded by 

 more succulent material. The transporting tissue of which 

 the centre is composed, which ultimately becomes a hard 



