34 VEGETABLE PHYSIOLOGY 
into a continuous system which goes from the tegumentary 
region towards the interior of the stem, embracing the 
vascular bundles and attaching them to each other. 
Similar differentiation of the supporting system 1s 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 
