Identification of the Excitable Cells in the Petiole of Mimosa pudica by Intracellular Injection of Procion Yellow

Excitable cells in the petiole of Mimosa pudica were located by microelectrode technique and stained with Procion Yellow M×4R which was previously filled in the electrode and injected iontophoretically into the cells. Microscopic observations of sections of the stained petioles revealed that protoxylem parenchyma cells and narrow phloem cells were excitable. The protoxylem localized just inside the metaxylem was composed almost entirely of the parenchyma cells which were 106.3±5.2 μm long (mean±EM, n=15) and 14.2±0.6 μm in diameter (n =33). The excitable phloem cells were 76.4±4.1 μm long (n=7) and 7.0±0.3 pan in diameter (n=37) and were thought to be companion cells or narrow parenchyma cells or both. Amplitudes of action potentials recorded from the petiolar surface had a linear relation to those from the excitable cells in the same petiole. From this fact and the arrangement of excitable cells in the petiole, we conclude that when the transmission of action potential takes place in the petiole all excitable cells in it are activated.