Abstract: Leaf explants of Rosa canina were used to investigate the origins of the rhizoids and protocorm-like bodies formation. Histological studies revealed that remained procambium cells in the vascular bundles were induced by 2,4-D，then divided and enlarged to form long but narrow vascular tissue cells that functioned as pluripotent stem cells with a tendency to form root primordia which further developed to form root meristems. The rhizoids have the same anatomical structure with the roots growing in vitro，including root cap，root meristematic zone，elongation zone and maturation zone，and the rhizoids only has the primary structure not secondary structure. Incubating the rhizoids carrying calli under light/dark conditions on 1/2MS medium containing TDZ，the tips of rhizoids could develop into protocorm-like bodies（PLBs）. Histological and ultrastructure observation revealed that the cells in the tip of rhizoids firstly differentiated into large parenchyma cells，these cells adjacent to the epidermis dedifferentiated to form a great many proembryo calli under the conditions of light and TDZ. And then the embryogenic cells divided and enlarged to form PLBs.

Key words: Rosa canina, leaf explant, rhizoid, protocorm-like bodies, initiation and development, histology, ultrastructure