. Principles of modern biology. Biology. 260 - Multicellular Plants. Fig. 14-1. Response of the "sensitive plant," Mimosa pudica: A, before being touched; B, five seconds after being touched (E. H. Runyon). minute, the plant slowly regains its original posture. The receptors, conductors, and effectors of this response of Mimosa are strictly localized. The excitation (p. 190) originates in the epi- dermal cells of the leaf, which possess numer- ous tactile hairs projecting from the surface; and the excitation spreads, mainly along the sieve tubes. The excitation, as indicated by the a
. Principles of modern biology. Biology. 260 - Multicellular Plants. Fig. 14-1. Response of the "sensitive plant," Mimosa pudica: A, before being touched; B, five seconds after being touched (E. H. Runyon). minute, the plant slowly regains its original posture. The receptors, conductors, and effectors of this response of Mimosa are strictly localized. The excitation (p. 190) originates in the epi- dermal cells of the leaf, which possess numer- ous tactile hairs projecting from the surface; and the excitation spreads, mainly along the sieve tubes. The excitation, as indicated by the action potential, follows the course of the vascular bundles through the petiole to the point where the petiole joins the stem. Here lies the pulvinus, a cushion of cells that normally supports the leaf in a horizontal position. When an excitation reaches the pulvinus, the pulvinus cells become more permeable, and losing turgor they relax their support of the leaf. Thus the pulvinus is the effector of the drooping response, and the sieve tubes and epidermal cells, respectively, are the conductors and receptors. Specialized receptor - conductor - effector structures in plants are less clearly defined than in animals, and consequently the re- sponses of plants are less rapid and precise. The drooping of the mimosa leaf is very rapid compared to most other plant re- sponses, but even in this case the propagated excitation—as timed by the spread of the ac- tion potential—is transmitted at a rate of onlv about 5 centimeters per second (compared to 120 meters per second along the nerves of man). Nevertheless the basic nature of ex- citation and conduction appears to be similar in plants and animals. In both cases, the excitation may be blocked by narcotic agents, and in both plants and animals excitation appears to be accompanied by a bioelectric discharge, increased permeability of the ex- cited cells, and a temporary alteration of the local metabolism. Other Turgor Movements. Al
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