The New Student's Reference Work/Irritability

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Ir′ritabil′ity (in plants). A state or condition of protoplasm in which it undergoes a change in consequence of a change in the neighboring protoplasm or in its surroundings. The change in the surroundings is called a stimulus; the change produced in the protoplasm is called a response or reaction. In plants the reaction follows the stimulus after a perceptible time, seconds (rarely less), minutes or even hours; whereas in most animals the interval (reaction time) is much less. So slow are some plant reactions that the stimulus may cease to act long before any reaction appears. Such late reactions are known as after-effects. The energy of the reaction is often many times that of the stimulus; so, although the sequence is that of cause and effect, the stimulus merely starts the action of protoplasm, which itself is the source of the energy expended in whatever work is done. Since the protoplasm ceases to react after more or less prolonged stimulation, it is supposed that it forms some material which, decomposing under stimulation, becomes exhausted when the stimulus is frequently applied and is only formed again during a period of rest. There are many kinds of stimuli: contact, friction, stress, gravity, light, heat, electricity and chemical substances, including water and air. Only certain parts are sensitive to particular stimuli. The action of the more important stimuli is described under Chemotaxis, Chemotropism, Geotropism, Heliotropism, Hydrotropism, Phototaxis, Rheotropism and Thermotropism. Through the chemical stimuli, probably, one part of the protoplasm influences another. Thus the change in one region, caused by some external stimulus, may act as a stimulus on neighboring protoplasm. These reactions, however, may be unseen, and a visible change may occur only at a distance from the point where the external stimulus acted. Thus, if a leaflet of the sensitive plant be scorched, all the leaflets of that leaf or even all the leaves on the plant may change their position. In such an event the stimulus is said to be transmitted from the region where it is first applied to the region where a visible response occurs. Nothing is satisfactorily known as to the processes in such transmission. Each kind of sensitiveness exists only under conditions favorable to it; prolonged unfavorable conditions produce disease, more or less severe; if conditions unfavorable to general irritability continue too long, death ensues. Indeed Sachs says: “Life is inconceivable without irritability; the dead organism is dead simply because it has lost its irritability.”