THE TREE ITS SUOOT-SYSTEJI. 



Ill 



During the whole time of the activity of the cam- 

 bium 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 preced- 

 ing (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, c and d, some with tyloses, <?, in them, and passes 

 gradually into autumn wood, with smaller vessels, <?, e, and more 

 tracheids and fibers, g. Only small medullary rays, ', are shown. 

 (Hartig.) 



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

 ments do not become lignified ; and (3) that the dis- 

 turbances in the arrangement of the elements are more 

 profound from the continued pressure exerted upon 

 them between the resistant wood and the elastic peri- 

 derm and bark, on the one hand, and the increased ex- 

 tension tangentially which it undergoes as the thicken- 



