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 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 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 in the 



