The Seniors of the Forest 281 
tissue. The oldest wood is at the centre of the 
trunk, and the newest is just beneath the bark 
(Fig. 78). 
But in many cone-bearers and notably in the 
white pine, the heart-wood undergoes little altera- 
Fic. 78.—Crosswise section of the trunk of a fir-tree, showing 
growth-rings. 
(From the Vegetable World.) 
tion as the tree matures, and it can resume the 
industry of former years, if necessary, and conduct 
water upward toward the thirsty leaves. Indeed 
so great is its versatility that it can make shift to 
fill, after a fashion, the offices of young wood and 
of bark, so that plant-fluids still ascend and 
descend slowly even in a girdled pine. 
