MECHANICAL CONSTRUCTION OF PLANT-BODY 187 



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

 which is often aided by marginal deposits of sclerenchyma. lhe 

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



'■■■A--- ~fJM • •*** is? K» Lb-i VHf H 



.-•■N 



■ \..- 



^V 



Fig. 120. 



Fig. 121. 



Fig. 122. 



T7 , .«, Transverse section of the leaf of Pkormium tenax, New Zealand Flax. 

 Fig. 120. lransverse secuuu w ^ Deschampsia caespitosa. 



in positions opposite to the aqueous areas in Fig. 120. (t.v.u.) 



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 



