Effects of Excision of Stock and Scion Organs on the Formation of the Graft Union in Coleus: A Histological Study

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Abstract
Effects of removal of leaves, shoots and roots on the formation of the graft union in Coleus stems were investigated. Based on counts of xylem strands bridging the grafts following organ removal, estimates of contributions of xylem-inducing stimuli were made: acropetal flow from stock to scion, 8% of the xylem in grafts of otherwise intact plants; basipetal flow from immature stem tissue, 10%; shoot tips and immature leaves on scion, 10%; scion leaves that are at least half mature, 36%. The remaining 36% of xylem-inducing stimuli may be from synergistic effects of the scion organs. The scion buds and leaves nearest the graft contribute .apprx. 40% of their respective organ-specific xylem-inducing stimuli, but they represent only 30% of their respective organs on the scion. Leaves and shoot tips on the stock have no effect on the graft. Severing the stock below the graft resulted in a reduction of graft-bridging xylem to 22% of that in otherwise intact plants. The ratio of graft-bridging strands of phloem and xylem is not significantly changed by any treatment and averages 0.23:1, compared with 2.2:1 in intact internodes. Independence of control is shown for the following components of the grafting process: callus formation on the cut surfaces, necrotic layer removal, cohesion of the graft partners and bridging of the graft by xylem, phloem and cambium. Grafts of isolated internodes cohere without apparent participation of the other components.