72 



AGRONOMY 



may usually be distinguished by the difference in the veining 

 of the leaves. In the monocotyledons the main veins usually 

 run parallel from base to apex or from midrib to margin, and 

 all the lesser veins are parallel and joined to one another at 

 the tips. This is called parallel venation. In the dicotyle- 

 dons the venation is known as reticulated or netted. Here the 

 small veins form an irregular network and the main veins 

 either spread out through the leaf, like the fingers on the 



FIG. 57. Two types of branched leaves 

 The left figure is pinnately branched ; the right, palmately branched 



hand, in the form known as palmate venation, or they branch 

 out from the midrib, forming the pinnate venation to be seen 

 in the elm, dandelion, and others. There are also two types of 

 branched leaves, the palmately and the pinnately branched, 

 corresponding to the two types of venation in dicotyledons. 



Internal structure of the leaf. Although the leaf blade is so 

 thin, it consists of several layers of cells which show consider- 

 able differentiation in structure. In a cross section there may 

 be distinguished an upper and lower layer of clear cells, form- 

 ing the epidermis, between which are the layers of green cells 



