ANATOMY OF TISSUES. 143 
cross-section, there are one central and from five to seven 
peripherical wood rings, each having its own axis. In the 
centre of each woody growth les a large-celled pith. In the 
wood are large porous ducts, and outside the cambium a narrow 
bast rind. This latter becomes contorted through the pressure 
exerted by the growing wood portions. Of the monocotyledons, 
only a few have secondary growth. Some of the Liliaceae which 
have trunk-like stems, Dracaena, Cardyline, Yucca, and Aloe 
form in the periphery of their central cylinder a cambium ring 
from which are cut off toward the centre isolated xylem bundles 
with parenchyma between them. Toward the outer surface 
phloem is developed and later a bark-like covering. 
FIGURE SHOWING NorMAL SECONDARY GROWTH OF CONIFER Woop. 
