70 THE OAK 



off below ; changes in structure also appear in the 

 leaves. 



The first noticeable change is the diminution in the 

 number of wood fibres and the presence of narrow 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 pointed 

 ends, and which are shorter and shorter as we approach 

 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 shortening 

 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, &c., on the one hand, and 

 the smaller weight and strains to be supported 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 fibres and vessels, until finally only delicate tracheids 



