In vitro studies on the anatomy and morphology of bud regeneration in melon cotyledons

The anatomy and morphology of bud regeneration were investigated in melon ( Cucumis melo L.) cv. Galia, which regenerates in vitro only by direct organ ogenesis from the cotyledon explant. Explants were cut from the cotyledon p roximal to the apex from 3-d-old in vitro seedlings. After 3 d on Murashige and Skoog medium with N-6-benzyladenine, cell division can be observed in the epidermal layer on the adaxial side in the center of the explant, near the most proximal (wounded) cut edge. Over the next week, the area of the m eristem increases laterally. Additional cell layers are added to the merist ematic area by cell division in the epidermis. In places the epidermis rema ins active in cell division. Alongside those active areas there are zones w here the epidermis has become inactive, although the subepidermal layers co ntinue to divide. In transverse section, the explant now has small protuber ances on the adaxial surface. After 10 d on cytokinin-containing medium, th e first signs of development are visible on the adaxial surface adjacent to the proximal cut edge. The protuberances observed after 10 d are neither p rimordia nor buds, although some meristematic bulges are observed. The firs t regenerated shoot buds are observed histologically after 15 d, by which t ime the surface has many protuberances and some small leaves. The first sho ot is found by histology after 22 d. By this time the surface is covered wi th protrusions and leaves, mostly without accompanying buds. The leaves may be produced from the protrusions initially visible after 10 d.