82 OUR WOODLAND TREES. 
wood, its liber or inner bark, and its outer bark, 
with its beautifully arranged radiation of cellular 
tissue from centre to circumference, there has 
been continual increase, chiefly shown, as years 
have rolled on, in the woody consolidation of the 
trunk. The early office, and indeed the chief use 
of the pith, have consisted in ministering only to 
the early growth of the Tree. Through this column 
of tender succulent tissue the moisture absorbed 
by the rootlets has been first carried, passing 
from cell-wall to cell-wall, and contributing, by 
its wonderful changes, to the formation of wood 
and bark and leaf. Then, with the formation of 
the first year’s cylinder of wood, the alburnum or 
sap-wood, as this cylinder when young and soft is 
first called, itself acts as the chief medium for 
the conveyance of the sap from root to stem, 
branches, and leaves—the medullary rays, or 
radiating strips of cellular tissue, acting, in a 
measure, as the branch canals for the more even 
and perfect distribution of the sap. As woody 
layer is thus laid upon woody layer—each albur- 
num cylinder fulfilling in turn the office of nurse 
before becoming hardened into wood—there is 
