108 THE OAK. 



tangential directions. These slight displacements from 

 the vertical are chiefly due to the fact that the elements 

 fibers, tracheids, and vertical groups of wood-paren- 

 chyma 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 un- 

 aided 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 de- 

 veloped whence the contrast at the boundary between 

 the annual rings is intensified. With the diminution in 

 relative diameter of the tracheids and fibers a correspond- 

 ing increase in the thickness of their walls is connected 

 a phenomenon which again intensifies the contrast 

 between adjacent annual rings. 



