THE TREE ITS SHOOT-SYSTEM 111 



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 phloem (also called secondary cortex) ; the 

 chief differences in the process being (1) that much 



FIG. 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 ; b, that between the 

 zone shown and the next youngest ring. In the annual 

 ring shown the spring wood begins with large vessels, 

 o 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, 

 i, are shown. (Hartig.) 



less phloem than xylem is formed; (2) that the 

 elements do not become lignified ; and (3) that the 

 disturbances in the arrangement of the elements are 

 more profound from the continued pressure exerted 

 upon 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 



