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(also excitability), the property of intracellular formations, cells, tissues, and organs to react to various external and internal environmental factors by means of structural and functional changes.
Plants can respond to a variety of agents. They are particularly sensitive to such vital factors as light, temperature, gravity, moisture, aeration, concentration and composition of salts, and acidity and alkalinity of the soil solution. Responses to environmental changes determine the position of plant organs in the air and in the soil.
All living plant cells exhibit irritability, but most sensitive are the shoot apices and root tips from which excitation is transmitted to the growth zones of the shoots and roots, where corresponding changes in the direction of their growth occur. Very high sensitivity to contact is characteristic of the stems, petioles, and tendrils of twining and climbing plants and of the stamens and pistils of certain other plants. The flowers and leaves of many plants react sharply to changes in light or temperature. Highly sensitive plants, including such insectivorous plants as mimosa, Venus’s-flytrap, and sundew, react very quickly to stimulation, as do the hyphae of carnivorous fungi. Stimuli can alter the movement of the cytoplasm, the nucleus, chromosomes, chloroplasts, mitochondria, and other structures of plant cells, as well as the movements of lower plants, zoospores, and spermatozoa not attached to a substrate.
The phenomena of irritability in plants and animals have much in common, although their manifestations in plants differ sharply from the usual forms of animal motor and nervous activity. Excitation also develops in plants in response to stimulation; in other words, there is temporary intensification of the vital activity of cells, tissues, and organs. The degree of excitation is generally proportional to the intensity of stimulation (product of the force and the duration of the stimulation). The excited portion of tissue or organ acquires a negative charge in relation to the unexcited portions owing to a change in the ion permeability of the cell membranes at the site of stimulation. If the stimulation is weak, excitation will be local; if it is fairly strong, excitation spreads to the adjacent cells chiefly in the form of biological currents (action potential) with the participation of plant hormones. For example, the action potential in multicellular algae (Nitella), in especially sensitive plants (mimosa, Venus’s-flytrap), and in the conducting tissues of ordinary plants is similar to the action potential in animal tissues. The rate at which excitation is conducted in plants depends on the species and state of the plant, the type of tissues, and the properties of the stimulant. Geotropic and phototropic excitations spread the slowest (about 1 cm/hr), and excitation caused by the movement of organic matter in phloem is transmitted more quickly (several tenths of a cm/hr). The conduction of excitation by water flow in xylem attains a rate of 5–10 m/hr; action currents spread through the companion cells surrounding the sieve cells of the vascular bundles at a rate of 50–100 m/hr. Very strong stimuli inhibit the vital processes of plants. The greater the physiological activity of a stimulus, the quicker the transition from stimulating to inhibiting doses and concentrations.
Every plant cell contains the entire genetic program for the growth and development of the particular plant. However, each cell is highly sensitive to certain external and internal stimuli. The particular stimuli to which a cell is sensitive is determined by its function and specialization. Hereditary requirements and changing environmental conditions demand complex and coordinated activity of all cells, tissues, and organs at each stage of plant development. This coordination is achieved by a regulatory system that includes plasmatic, hormonal, vascular, and bioelectric relations and that unites billions of cells into an integral organism.
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I. I. GUNAR