Abstract: The wild celery (Angelica polymorpha Maxim.) was employed as the experimental material to study the initiation of the embryogemic cells and the ontogeny of the proembryoids in petiole explants examined with light and electron microscopy. Segments of petioles were cultured on the surface of MS medium (MS + 2,4-D 2mg/L + KT 0.25mg/L), and then the DNA synthetic activity and cell divisions were examined to confirm that in which cell departure from quiescent state and initiation of cell division occurred. Results show that the proembryoids originate from the innermost layer of cortex parenchyma cells which exhibit rapid periclinal and anticlinal divisions, then a pericyclic tissue composed of 2--3 layers of smaller but still vacuolate cells form and corrugately cover the vascular bundle. Soon after that, individual cell groups located in the inner region of this tissue undergo regressive change and asynchronously transform to embryogenic clusters, giving rise to nodule-like proembryoids by repeated cell divisions. Meanwhile, the phloem parenchyma cells also proliferate by mitosis. Accompanying with some original sieve tube degeneration, a number of new sieve elements differentiated from the newly formed parenchyma cells. Microscopic observations also reveal that a lot of new tracheary elements differentiated from the xylem parenchyma cells. Evidently, the re-differentiation of the vascular elements in the petiole explants reflects the intensification of nutrient translocation in these segments in close relation to the great demand for nutrients during the ontogeny of proembryoid.

Key words: Wild celery, Bundle sheath cell, Embryogenic cell, Proembryoid