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. Ectotrophic 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, 



