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 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 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. 
