THE TREE—ITS SHOOT-SYSTEM 107 
pine, except that they are more numerous, are not 
confined to the radial walls, and they are not quite 
circular, but have an oval shape witha 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 succes- 
sive rows of the fibres 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 inthe 
