Wounds upon plants begin to heal by the formation of loose cellular matter which gives rise to a mass of tissue known as a _callus_. This tissue eventually covers the entire wound, if complete healing results. As a rule, the first apparent change in a cutting is the formation of a callus upon the lower end, and it is commonly supposed that this process must be well progressed before roots can form. But roots do not necessarily arise from the callus, and in many plants they appear to bear no relation to it in position. In willows, for instance, roots arise from the bark quite irregularly and at some distance from the callus. Yet, as a matter of practice, best results are obtained from callused cuttings, particularly if the cuttings are made from mature wood, but this is probably due quite as much to the fact that considerable time is required for the formation of the adventitious buds which give rise to the roots as from any connection between the callusing and rooting processes themselves. Hard-wood cuttings give better results when kept dormant for some time after they are cut. They are usually made in the fall, and stored during the winter in sand, sawdust or moss in a cool cellar, or buried in a sandy and well-drained place. This, at least, is the practice with hard-wood cuttings of deciduous plants. Hard-wood evergreen cuttings, when taken in the fall, are usually set at once, as their foliage will not allow them to be buried with safety; but in this case, the cuttings are kept "quiet" or dormant for a time, to allow callusing to progress. If cuttings are buried so deep that they cannot sprout, callusing may be hastened by placing them in a mild temperature. Single-eye grape cuttings are sometimes packed between layers of sand in a barrel and the barrel is set under a forcing-house bench where the temperature is about 50°. Eight or ten inches of sand is usually placed over the top layer. In this manner, cuttings which have been obtained in winter or spring can be callused before planting time. [Illustration: Fig. 45. Mallet cutting of grape.] It is a singular fact that the lower end of the cutting, as it stood upon the parent plant, produces roots and the upper end produces leaves and shoots, even if the cutting is inverted. And if the cutting is divided into several parts, each part will still exhibit this same differentiation of function. This is true even of root-cuttings, and of other cuttings which possess no buds. The reasons for this localization of function are not yet clearly understood, although the phenomenon has lately been the subject of study. Upon this fact depends the hastening of the rooting process in inverted cuttings by the direct application of heat to the bottoms, and it likewise indicates that care must be taken to plant cuttings in approximately their natural direction if straight and handsome plants are desired.