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 phloém-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 phloém. 
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 phloém also is at length reduced to little more 
than one or two sieve-tubes, the segments of which are 
shorter and shorter aswe nearthe 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 
