THE TREE—ITS SHOOT-SYSTEM 1st 
During the whole time of the activity of the 
cambium ring and the formation of wood on its interior, 
it must not be forgotten that the outer rows of cambial 
cells are passing over into the tissue known as bast or 
secondary phloém (also called secondary cortex); the 
chief differences in the process being (1) that much 
Fic. 29.—A small piece of one annual ring of old oak wood 
(magnified twenty diameters). a, boundary of the autumn 
wood of the preceding (older) ring; 0, that between the 
zone shown and the next youngest ring. In the annual 
ring shown the spring wood begins with large vessels, 
ce and d, some with tyloses, d, in them, and passes 
gradually into autumn wood, with smaller vessels, e, e, and 
more tracheids and fibres,g. Only small medullary rays, 
7, are shown. (Hartig.) 
less phloém than xylem is formed; (2) that the 
elements do not become lignified; and (8) that the 
disturbances in the arrangement of the elements are 
more profound from the continued pressure exerted 
upor them between the resistant wood and the elastic 
periderm and bark, on the one hand, and the increased 
extension tangentially which it undergoes as the 
