352 



A TEXT-BOOK OF BOTANY. 



Leaf Venation. The foliage leaves of higher plants are 

 traversed by vascular bundles, which enter the blade through the 

 petiole and diverge at the base, or, as in the case of Dicotyledons, 

 branch in various ways ; and it will be seen that the form of the 

 leaves corresponds to the distribution of the bundles. These 

 bundles are known as veins or nerves, and they have two func- 

 tions, namely, (i) that of a mechanical support, and (2) that of 

 carrying nutritive materials to and from the leaves. 



The mode of venation in Monocotyledons and Dicotyledons 

 differs somewhat, but it will be found that in a number of instances 



FIG. 202. Leaf venation: A, parallel- veined leaf of Solomon's seal (Smilacina race- 

 mosa); B, pinnately-reticulate leaf of chestnut; C, palmately-veined leaf of Menispermum 

 canadense. 



the venation of leaves of plants belonging to one of these great 

 groups will resemble that of the leaves of certain plants in the 

 other group. However, there are certain general types belonging 

 to each group (Fig. 202). 



VENATION IN MONOCOTYLEDONS. An examination of the 

 leaf of lily-of-the-valley shows that the primary veins run more 

 or less parallel to the apex with short though distinct anastomoses. 

 Such a leaf is said to be PARALLEL-VEINED or NERVED. It will 

 moreover be noticed that the distribution of the veins in this 

 manner produces a lamina with an even, or entire margin, and 



