Petiole explants were inoculated on Murashige and Skoog (MS) medium containing 4.1 µM picloram and 4.5 µM Thidiazuron (TDZ) to induce primary nodular callus formation. After 2 months, the primary nodular callus was transferred into liquid or solid MS medium supplemented with 2.0 µM picloram for nodule proliferation. The results indicate that the transfer of primary nodular callus to liquid MS medium is essential for proliferation. Histological analyses revealed that the nodular callus was a mixture of organogenic nodules, instead of what is commonly thought of as callus, which differentiated from meristemoids, originated from the division of responsive epidermic and parenchymal cells. The meristemoids were composed of smaller embryonic cells with large and densely stained nucleus and different tissues of nodule would develop from these cells. Nodules involved in the formation of actively dividing meristematic regions which located on the subepidermal cell layers. Adventitious shoots and somatic embryos were produced from this area.