108 Plant Physiology 



64. Leaf venation. Each leaf receives a definite quota 

 of bundles, and these and their subdivisions continued into 

 petiole and lamina constitute the so-called venation sys- 

 tem. In the case of monocotylous plants the veins are 

 usually parallel from the leaf stalk, or from the mid-vein, 

 so that they are often designated parallel-veined plants. 



In the leaves of the dicotylous type the bundle systems, 

 branch repeatedly, and also form a complete reticulum. 



[Photograph by H. M. Benedict.] 



FIG. 35. Minute venation of the leaf of Vitis riparia ; leaves of differ- 

 ent ages. 



In any event the leaves are well provided with fibrovascu- 

 lar tissue, easily demonstrated by macroscopic or micro- 

 scopic observation. As a matter of fact the bundles extend 

 to the most remote parts, and in dicotylous plants espe- 

 cially the leaf is divided up into a complete network, with 

 the areas between the vascular tissue being seldom larger 

 than 1-3 mm. in diameter (Fig. 35). The ultimate sub- 

 divisions of the bundles consist of tracheids and elongate 

 parenchyma cells (meristem). Sometimes the bundles 

 end abruptly or blindly. As the leaf grows each area 

 subtended by veinlets becomes larger, and this increase 



