Abstract: This paper attempted to illustrate the mechanisms of callus formation of plant leaf. The theory was proposed that the acid material in the culture medium maintained the plant leaf in the acid environment, and this led to fast degradation of cell wall, breaking away of protoplast from the cell wall. These free protoplasts will aggregation into callus, without fixed organization and functions and being ready to recombine or rebuild for the further differentiation. The acid condition in the culture medium played the most important role in the dedifferentiation of leaves and in the formation of callus, during this course plant cell wall was presumed to serve as signal carrier controlling the cell to finish the normal cell's cycle, not only as the inactive coat of cells.

Key words: formation of callus, mechanism, cell wall, acid degradation