MECHANICAL CONSTRUCTION OF PLANT-BODY 155 



as to form strong ribs. The latter danger is met by arched venation, 

 which is often aided by marginal deposits of sclerenchyma. The 

 whole blade is held together by the upper and lower layers of epidermis, 



A 



FIG. 115. 



FIG. 113. A , Transverse section of the leaf of Phormium tenax, New Zealand Flax. 

 FIG. 114. B, ditto, Elymus arenarius. FIG. 115. C, ditto, Deschampsia caespitosa. 

 ( x 20.) Thin-walled tissue left clear : mechanical tissue dotted ; xylem cross- 

 hatched. Aqueous cells in B and C indicated diagrammatically. In A, aqueous 

 areas are outlined with dots. The involutions in B and C appear in positions opposite 

 to the aqueous areas in A. (F.O.B.) 



which, having a thickened outer wall, form a firm skin over the softer 

 mesophyll within. Sometimes the mesophyll may be itself sclerotic 

 in places, as it is in many Monocotyledons. 



The structural stiffening of the flattened blade against folding is 

 best illustrated in the leaves of Monocotyledons, for there the parallel 

 venation makes the transverse section appear simpler. All the main 



