254 



PLANTS AND MAN 



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a tension while the lower surface is subjected to a compression 

 strain. As in stems the greatest strain comes nearest the surfaces 

 and the least in the center. Thus the supporting strands found in 

 the midribs and veins of the leaves occur just under the epidermis; 



Fig. 178. — Supporting tissues in leaves are distributed to afford support where 

 it is needed most; supporting tissues in dotted areas. 



this often consists of collenchyma cells (fig. 178). In many Mono- 

 cot leaves — for example the grasses — the veins have groups of 

 sclerenchyma cells distributed just beneath the surface cells. 

 Monocot stems do not produce wood, since there are not the 

 numbers of tracheids or of wood fibers 

 found in conifers and Dicots. In 

 Monocot stems the scattered vascular 

 bundles are surrounded by large 

 amounts of pith parenchyma, a con- 

 trast to the compact mass of vascular 

 and supporting tissues forming the sap 

 wood and heart wood of a Dicot stem. 

 The supporting cells are found within 

 the vascular bundles so that each of 

 these acts like one of the rods used in 

 making reinforced concrete. The paren- 

 chyma of the stem — which can be com- 

 pared to the mass of the concrete — 

 resists compression while the skeletal 

 strands of the bundles act like the rods 

 in resisting tension strains imposed by lateral bending. Strands 

 of sclerenchyma fibers may completely surround each vascular 

 bundle, or may be disposed on their outer and inner edges (fig. 

 179). Such sclerenchyma fibers are usually lignified so that they 



Fig. 179. — Arrangement 

 of supporting tissues in 

 Monocot stems often in- 

 volves strands of fibers sur- 

 rounding each vascular 

 bundle. 



