The Leaf 



117 



even the smallest one, consists of two pipes running 

 side by side, one of which brings up the sap while the 

 other carries away the starch as soon as it is made. 

 Hold up any ordinary leaf such as a pear leaf to the 

 light and notice how the veins branch and rebranch 

 until you can hardly see them. In all our British trees 

 the veins form a network very similar to this one, 



Fig. 59. Leaf of Birch, showing pinnate venation. 



although, as you will see directly, the arrangement of 

 the larger veins may vary. Now compare this leaf with 

 an iris or grass leaf. In these last the veins are all 

 arranged side by side and do not branch at all. This 

 kind of veining is called parallel and is found in many 

 tropical leaves. If you look at the veins of birch or 

 lime and currant or maple leaves you will find that 



