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 



