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. 



L.R 



FIG. 13s. 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 expenditure of material 



