THE SEEDLING AND YOUNG PLANT. 49 



those primary medullary rays which happen to have 

 been cut running between the bundles. 



If we now trace the vascular bundles of the leaf- 

 trace in the other direction that is, up into the leaf 

 their course is simple enough, as shown in Figs. 10 and 

 11. The five bundles run through the midrib and the 

 stronger lateral ribs to the tips and edges of the leaf, 

 first breaking up into several strands in the petiole and 

 midrib, and then becoming finer and finer as they give 

 off the lateral strands. The median bundle does little 

 more than run directly through the leaf as the midrib, 

 becoming finer and finer as it nears the apex. The two 

 lateral median bundles behave in a somewhat curious 

 way. We have already seen how large and flat they are 

 at the leaf insertion (Fig. 10). Soon after entering the 

 petiole they break up into several strands, two of which 

 converge and take a course along the dorsal side of the 

 midrib, thus nearly completing a cylinder of bundles 

 inclosing a pith ; moreover, the xylem portions of these 

 bundles are all turned inward towards the pith. 



The lateral bundles, coming obliquely into the leaf 

 insertion, pass up the midrib side by side with the 

 above, and, like them, break up into parallel strands. 

 Before entering the midrib they give off small bundles 

 (fst in Fig. 10) to the pair of minute stipules which 

 flank the petiole. As the strands pass along the mid- 

 ribs and chief lateral ribs they interosculate in various 

 degrees, and give off smaller side branches into the 

 mesophyll of the leaf (see Chapter VI). 



