Page:The Oak.djvu/88

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70
THE OAK.

vessels only. As the trace passes through the cortex to the leaf the actual number of both xylem- and phloëm- 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 phloëm. 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 phloëm 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, etc., 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 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 pittings. Moreover, they are nearly isolated. Nevertheless, the inner elements can be distinguished as primary