152 MINUTE STRUCTURE OF THE STEM. 



has lately shown that even in stems with the periderra free from 

 lenticels, provision for exchange of gases is secured by certain 

 intercellular spaces at or near the points where the medullary 

 rays come to the periphery of the stem. 



424. Grafting. If the cambium tissue of a young shoot is 

 retained for a time in close apposition with that of a nearly 

 related plant, union of the two parts may take place, and the 

 wound may heal by the natural process before described. Suc- 

 cess in this operation depends upon selection of suitable stock 

 and scion, choice of the proper season, freshness of the cut sur- 

 faces, and, generally, exclusion of air from the wound. The 

 methods of bringing the surfaces of the stock and scion together 

 in this operation of grafting are innumerable, but for the pres- 

 ent purpose may be referred to two principal types : (1) that in 

 which the scion, wholly separated from the plant on which it 

 grew as a branch, is placed in some sort of a cleft of the plant 

 which is thenceforth to furnish it with nourishment ; (2) that in 

 which the scion is still retained in its connection with the parent 

 plant, but is bent over and a freshly cut surface kept in contact 

 with a cut surface of another plant, until the scion has fairly 

 become attached by organic union. When this is accomplished, 

 it is cut off from the parent plant. This type of grafting, in its 

 many varieties, is known as " approach grafting." It takes place 

 in nature, as shown in the following paragraph. 



425. Two branches of one plant may become united when, 

 after removal of a section of bark from each, the two denuded 

 surfaces are kept in apposition for a time. Such unions of axial 

 organs are not rare. Occasionally they may take place between 

 two shoots at a point near the root, so that the trunk will ulti- 

 mately consist of a single deeply grooved stem. The union may 

 be between two plants of the same species, or even between 

 plants of different species. The attrition of two branches which 

 have grown against one another may suffice to wear off the bark 

 on both down to the cambium, and then, if their exposed surfaces 

 are held together for a while, union will follow. Such natural 

 grafts are met with frequently at the borders of forests. 



426. In the kindred operation of budding, a bud with a little 

 of the tissue behind it is placed in a cleft in the bark of the 

 stock, so that the cambium layer of the two ma} 7 come into close 

 contact. 



427. The stem may be invaded by parasitic roots at any part, 

 and its subsequent development seriously affected thereby. Such 

 invasions often give rise to swellings, distortions, etc., by which 



