336 
STRUCTURE AND LIFE HISTORIES 
ing as one continuous stem. In all grafting the scion 
maintains essentially its true nature, seldom, if ever, being 
affected by the characteristics of the stock, which only 
serves as a channel for the passage of water and food ele- 
ments to the scion, and receiving in return from the scion 
the elaborated carbohydrate and other food (Figs. 243 
and 404) . Scion and stock therefore represent a case of 
symbiosis artificially brought about. In some cases 
branches of the same tree rub against each other until 
the bark is worn through, bringing the cambial layers in 
contact, and resulting in a "natural" graft. 
310. Mycorrhizas. The roots of many plants (espe- 
cially of woody plants) enter into intimate association 
with the mycelia of various fungi growing in the soil. 
FIG. 244. Ecto trophic micorhizas. At left, micorhizal mantle on root 
of hickory (Gary a ovata), in cross-section; at right, root- tip of an oak 
(Quercus), covered by fungus mantle. (After W. B. McDougall.) 
The mycelia either form a mantle or jacket at or near 
the surface of the young roots (ectotrophic, Fig. 244), or 
they penetrate through the cell-walls into the cell-cavities 
(endotrophic, Fig. 245). Recent careful studies seem to 
demonstrate that the ectotrophic mycorrhizas, common 
on the roots of many kinds of trees (hickory, oaks, birch, 
sugar-maple, larch, beech, hornbeam), "are, in reality, 
