Bottom heat is always essential to the best success with cuttings. In out-door work this is supplied by the natural heat of the soil in spring and summer, and it is often intensified by burying hard-wooded cuttings bottom end up for a time before planting them. This operation of inverting cuttings is often practiced with grapes, particularly with the Delaware and others which root with some difficulty. The cuttings are tied in bundles and are buried in a sandy place, with the tops down, the butts being covered two or three inches with sand. They may be put in this position in the fall and allowed to remain until the ground begins to freeze hard, or they may be buried in spring and allowed to remain until May or June and then be regularly planted. In out-door cuttage, the cuttings which are of medium length, from six to eight inches, derive more bottom heat than the very long ones, such as were formerly used for the propagation of the grape. In in-door work bottom heat is obtained artificially. Cutting benches should have abundant piping beneath, and in the case of many tropical and sub-tropical species the bottom heat may be intensified by enclosing the benches below so that no heat can escape into the house. Doors can be placed in the partition alongside the walk to serve as ventilators if the heat should become too intense. In all cuttings, bottom or root growth should precede top growth, and this is aided by bottom heat. [Illustration: Fig. 43. Grape cutting.] As a rule, roots arise most readily from a joint, and it is therefore a common practice to cut off the base of the cutting just below a bud, as shown in the grape cutting, Fig. 43. Sometimes the cutting is severed at its point of attachment to the parent branch, and a small portion, or "heel," of that branch is allowed to remain on the cutting. This heel may be nothing more than the curved and hardened base of the cutting at its point of attachment, as in the cornus cutting, Fig. 44. Sometimes an entire section of the parent branch is removed with the cutting, as in the "mallet" cuttings of grapes, Fig. 45. Of course, comparatively few heel or mallet cuttings can be made from a plant, as only one cutting is obtained from a branch, and it is advisable, therefore, to "cut to buds" rather than "cut to heels;" yet there are many plants which demand a heel, if satisfactory results are to be obtained. The requirements of the different species in this direction can be learned only by experience, but it may be said that in general the hardest or closest wooded plants require a heel or a joint at the base. Willows, currants, basswoods, and others with like soft wood, emit roots readily between the buds, yet even in these cases propagators quite usually cut to buds. [Illustration: Fig. 44. Heel cutting of Cornus.]