Callus - formation is not confined to the basal end of a cutting; it has nothing to do with position, but is a reaction to the wound stimuli, independent of light, gravitation, etc. As time goes on, however, the internal organisation of the erect cutting usually reacts on the callus at either end, and roots only rise from the lower one, while shoot-buds may form in the upper one, though it is possible to bring about the formation of buds from the lower end also.
The essential facts regarding wounds and healing by occlusion are given in Marshall Ward, Timber and some of its Diseases, 1889, chapters viii. and ix., and in Laslett, Timber and Timber Trees, 1894, chapters iv. and v. More detailed treatment will be found in Frank, Krankh. d. Pflanzen, B. I. cap. 2, where the special literature is collected. The reader may also consult Hartig, Diseases of Trees, Engl. ed. 1894, pp. 225 - 269.