THE TREE ITS SHOOT-SYSTEM. 107 



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 ver- 

 tical 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 en- 

 tirely obliterated. The markings, thickening, and want 

 of contents are as in the case of tracheids, the chief dif- 

 ference 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 diam- 

 eter 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 suc- 

 cessive 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 posi- 

 tion, but may lean over both in the radial and in the 



