108 THE OAK 



tangential directions. These slight displacements from 

 the vertical are chiefly due to the fact that the elements 

 fibres, 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 fibres, &c., 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 

 unaided 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 fibres and tracheids 

 developed in the autumn have a relatively smaller 

 radial diameter than those formed earlier, and this, com- 

 bined with the fact that thoso 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 developed whence the contrast at the 

 boundary between the annual rings is intensified. With 

 the diminution in relative diameter of the tracheids and 

 fibres a corresponding increase in the thickness of their 

 walls is connected a phenomenon which again intensi- 

 fies the contrast between adjacent annual rings. 



