MECHANICAL PROBLEMS 99 
leaf, which at first sight often seems to spring 
from the stem some distance above the node. 
In reality, however, the lower part of the leaf 
forms a cylindrical sheath surrounding and 
supporting the succulent, elongating portion of 
the stem just above the actual node. The 
cylindrical leaf sheath is supplied with abun- 
dant sclerenchyma, which is arranged in a 
more complex way.than in the sunflower, but 
again in the strictest accordance with what 
we have discovered to be sound mechanical 
principles. 
It is the mechanical tissue which forms 
the economically valuable fibre yielded by 
many plants—such as hemp, flax, Jute and 
the like—and for commercial purposes it has 
to be separated by various processes from the 
softer tissues in which it lies imbedded. 
As a plant becomes larger, the crushing 
effect of the increasing weight of the foliage 
and branches begins to make special demands 
for additional mechanical tissue. This is 
most often provided for by a large increase 
in the tissues of the wood. In a cross section 
of such a plant as an old sunflower the wood 
is seen to have assumed the form of a hollow 
cylinder, variously buttressed and thickened 
towards the pith. 
In many of the perennial plants the character 
of the mechanical supporting tissue is less 
obvious, principally because it has to serve 
several purposes, and also because it is rela- 
tively so abundant that, if the expression 
