THE TREE—ITS SHOOT-SYSTEM.
are more numerous, are not confined to the radial walls, and they are not quite circular, but have an oval shape with a slit-like aperture to the border, the long axis of the slit being nearly transverse to the long axis of the tracheid.
In the conversion of cambium cells into vessels the chief point to note is that the vessel is essentially a vertical row of superposed tracheids—each of which has been developed from a cambium cell as just described—the oblique separating walls of which become almost entirely obliterated. The markings, thickening, and want of contents are as in the case of tracheids, the chief difference being the more pronounced growth in diameter of the vessel segments, especially those formed in the spring wood.
It will readily be understood that the growth in diameter of these vessel elements exerts a disturbing effect on the radial arrangement of the other elements of the wood, and the displacements and compression of the latter are considerable and various, so that, at length, very little trace of the original order is observable. It not unfrequently happens, however, that many successive rows of the fibers or tracheids are formed in the outer parts of the annual ring, and in such cases the original radial series can be detected.
There are several other points also to be noted in the development of secondary wood. In the first place, the various elements do not maintain an exact vertical position, but may lean over both in the radial and in the