Formation of Callus Knots on Apple Grafts as Related to the Histology of the Graft Union

Abstract

Histological studies were made of both well and poorly fitted piece-root apple grafts at different times during the first growing season. Callus was produced exclusively by tissues outside of the xylem cylinder of both the scion and root pieces. The callus was inhibited by such factors as crushing, desiccation, and certain fungi. Union was begun through unobstructed contact between callus tissues from both the scion and the stock. The mingling callus tissues from these two sources were so similar that no method was found for their differentiation. In well-matched grafts an arc of cambium was formed in the uniting callus which laid down vascular elements completing the union and forming the second annual ring. The degree of effective continuity in the new vascular sheath about the union influenced not only the size and vigor of the young tree but also, independently, the formation of callus knots. Local obstructions to good vascular union resulted in proliferations of excess callus from which histological sections appeared somewhat like those from crown gall. These proliferations often attained such size that they were called callus knots. They occurred particularly on the scion lips. The size and permanence of the callus knots were determined by the extent and persistence of obstructions to vascular union. Certain phenomena that have been attributed to incompatibility may be interpreted as the result of poor grafting.