EXTERNAL CHARACTERISTICS 



205 



should be noted that parallel-veined leaves have net veins 

 also ; that is, the very small, invisible veins which branch 

 off from the principal ones are of this character. 



Much of the beauty of the individual leaf is due to its 

 venation. Sometimes the veins have all the delicacy and 

 symmetry of beautiful lace work, as may be seen by hold- 

 ing the leaf up to 

 the light. The large 

 veins send off smaller 

 branches and these 

 send off still smaller 

 ones, until the small- 

 est veinlets are not 

 visible. With the 

 utmost precision, 

 each tiny portion of 

 the blade is equally 

 penetrated by the 

 veins, and all to- 

 gether they form a 

 perfect framework 

 and conducting sys- 

 tem. 



The principal vein 

 is called, as you know, the midrib, but often there is no 

 principal vein. There may be a number of principal veins 

 of about equal strength. In such cases the principal 

 veins are called nerves, the leaf being described as three- 

 nerved or five-nerved or whatever the number may be. 

 Such leaves occur among both the net- and the parallel- 

 veined types of leaves. 



One of the commonest types of net-veined leaves is the 



FIG. 68. The two principal types of leaf venation. 

 The figure at the left represents a leaf of Solo- 

 mon's seal, a monocotyledon ; the one at the right 

 represents a leaf of willow, a dicotyledon. 



