Page:The Oak.djvu/126

108 tangential directions. These slight displacements from the vertical are chiefly due to the fact that the elements—fibers, tracheids, and vertical groups of wood-parenchyma—have not finished their growth in length when they pass over from the cambial condition; consequently the pointed ends of the elongating fibers, etc., push themselves between the ends of others which lie above and below them, and a slight tilting from the vertical results. This may be sufficient to produce a twisting of the stems and branches which is visible even to the unaided eye.

Another important point is that the length of the elements, as well as their diameters, vary at different periods in the life of the tree.

First as to the diameter. The fibers and tracheids developed in the autumn have a relatively smaller radial diameter than those formed earlier, and this, combined with the fact that those elements which develop in the spring have the relatively largest diameters, alone would suffice to mark the boundary between any two annual rings. But the same holds good for the vessels; those formed in the spring wood are very large compared with those formed later—the latter are also more sparely developed—whence the contrast at the boundary between the annual rings is intensified. With the diminution in relative diameter of the tracheids and fibers a corresponding increase in the thickness of their walls is connected—a phenomenon which again intensifies the contrast between adjacent annual rings.