MECHANICAL PROBLEMS 93 
tree from being pulled out of the ground or 
blown over, whilst its leaves must successfully 
resist tearing and the many other disruptive 
influences to which they are exposed. 
Fic. 138.—Diagram of transverse section of part of an old 
stem of sunflower. C, cambium; E, epidermis; LP, 
hard lignified parenchyma; P, pith; Scl: sclerenchyma; 
W, wood. 
An ordinary herbaceous stem, like that of 
a sunflower, affords an excellent illustration 
of the development of mechanical tissue and of 
its disposition so as to secure the maximum of 
efficiency with the least expenditurc of material 
