Dormant buds, or preventitious buds, are such as receive no sufficient supply of water and food materials to enable them to open with the other buds in ordinary years, for in most trees only the upper buds on the branches develop into new shoots. The lower buds do not die, however, but merely keep pace with the growth in thickness of the parent branch, and may be elongated sufficiently each year to raise the minute tips level with the bark, their proper cambium only remaining alive but not thickening the bud.
When, by the breaking of the branch above the insertion of the dormant bud - or by pruning, defoliation by insects, etc. - the transpiration current and supplies of food materials are in any way deflected to the minute cambium and growing points of the dormant buds, they are stimulated to normal growth, and may grow out as epicormic shoots or "shoots from the old wood." In many cases such epicormic shoots are stimulated to grow out by suddenly exposing an old tree to more favourable conditions of root-action and assimilatory activity, owing to the felling of competing trees which previously hemmed it in from light and air, and restricted the spread and action of its roots in the soil. This is often seen in old Elms, Limes, etc.
It is by such means as the above that substitution branches are obtained when a leader is broken or cut away.