GRAFT UNION FORMATION IN DOUGLAS-FIR

Greenhouse-grown Douglas-fir (Pseudotsuga menziesii Mirb.] Franco) graft unions were examined between 2 and 84 days after grafting. Room temperature was maintained at 60-70 F throughout the growing season. In most respects grafts of Douglas-fir followed development patterns previously reported for spruce and pine grafts, but specific differences were noted in contributing cell types, time of formation, and mode of healing. The time interval from first occurrence to occurrence in 80% of the grafts is as follows: contact layers, 2 days; callus bridges, 10-14 days; periderm, 10-17 days; cambia, 17-23 days. Callus bridges were generally of secondary phloem or cortex origin. Callus lignification began along cut edges of the union at 14 days and was completed across the entire length of the union by 17 days. Lignified tracheids were continuous across union zones at 35 days. When proper grafting techniques were used, all tissue systems necessary for a successful union were present 35 days after grafting. Poor grafting techniques at times retarded cambium formation for 3 months or more.