THE FOREST CANOPY 39 
mycelial hyphe of fungi in the form of fine filaments 
attach themselves to the root-fibrils of the tree, and 
in some way aid the processes of nutrition. Masses 
of united rootlets and fungal hyphz are called my- 
corrhiza. The edible portions of the subterranean 
fungus, called the truffle, are mycorrhiza attached to 
the roots of oak-trees. The materials absorbed by 
the roots, together with the substances elaborated by 
the leaf from the atmosphere, are manufactured by 
the protoplasm of the tree into more protoplasm and a 
host of other complex substances, but the most varied 
and most interesting, and the commonest of all, is 
wood, with which we are all familiar, and with which 
the forester is most concerned. 
Although there is considerable wood in the lmb- 
age, the bulk of good material is, or should be, in 
the boles or trunks of the trees. Beginning with the 
outside of the tree, there is, first, the dead outer bark, 
then the living inner bark, then the sap-wood and 
heart-wood, and finally a little pith in the center. 
The wood of the tree may not always be divided into 
sap-wood and heart-wood. It is often homogeneous 
throughout. The increase in the size of the tree 
trunk is produced by the addition of layers of wood. 
The cambium tissue, which is a zone of active cells 
between wood and bark, proliferates or grows in such 
a way that as a layer of bark is formed on the outside, 
