70 THE OAK. 



vessels only. As the trace passes through the cortex to 

 the leaf the actual number of both xylem- and phloem- 

 elements diminishes ; hence it comes about that the 

 bundles in the leaves consist to a relatively large extent 

 of spiral vessels in the xylem and of sieve-tubes in the 

 phloem. As the bundles leave the midrib and larger 

 veins the true continuous vessels disappear altogether, 

 and only spindle-shaped tracheids with reticulated or 

 spiral thickenings occur, fitting obliquely at their point- 

 ed ends, and which are shorter and shorter as we ap- 

 proach the ends of the bundles. 



The phloem also is at length reduced to little more 

 than one or two sieve-tubes, the segments of which are 

 shorter and shorter as we near the end. The shorten- 

 ing of the elements is in evident correlation with the 

 early cessation of growth in length of the parts of the 

 leaf, and the diminution of the number of elements 

 with the decreased supply of fluids, etc., on the one 

 hand, and the smaller weight and strains to be sup- 

 ported on the other. 



We may sum up the changes in structure towards 

 the ends of the vascular bundles thus : The thickening 

 of the walls is less, and the elements become narrower 

 and shorter ; the xylem becomes simplified by the loss 

 of fibers and vessels, until finally only delicate tracheids 

 are left (Fig. 21), the thickenings of which are at length 

 not spirals or nets for the most part, but irregular pit- 

 tings. Moreover, they are nearly isolated. Neverthe- 

 less, the inner elements can be distinguished as primary 



