IRRITABILITY (Lat, irritabilitas, from ir ritabilis, irritable, from irritate, to irritate). The state or condition of the 'protoplasm of plants when it responds to a change in external influences or the adjacent protoplasm. The change which initiates the response is called a stimulus. When the nature of the stimulus is unknown, automatism is predicated rather than irritability. Temporarily the protoplasm may lose the power of responding to a stimulus, but when this is permanently lost the protoplasm is dead. Tem porary loss of irritability may be occasioned by prolonged or repeated stimulation, in which ease the loss is said to be due to fatigue; or by un favorable conditions, such as deficient moisture, low or high temperature, lack of oxygen, etc.
The agents affecting plants are extremely numerous and varied. The light changes both in intensity and direction from hour to hour; the temperature fluctuates, the moisture in air and soil is seldom the same for two consecutiv days. The nature of the stimulus does not in any way determine the character of the response, but this is conditioned by the mechanism which the stimu lus sets in action. As there is unlikeness between stimulus and reaction, so there is disproportion between them in the energy involved. Although the stimulus stands to the reaction apparently in the relation of cause to effect, it only initiates a series of changes, in the course of which the energy stored in living protoplasm is set free, and this produces. through the appropriate mechanism, the result observed. The stimulus is only a re leasing cause, and, were the energy not provided by the living protoplasm, no reaction would occur. The only apparent exception to this is seen when a stimulus inhibits the action of an organ whose normal function requires the expen diture of energy. Here the release of energy seems to be prevented. In the absence of exact knowledge as to how this is accomplished. it may lie assumed that it is by counteraction rather than by suppression.
The limitation of irritability by unfavorable external conditions, the phenomena of fatigue, and the disproportion between the stimulus and the reaction it calls forth lead to the formation of an hypothesis regarding the state of the proto plasm when it is irritable. Briefly stated, this hypothesis assumes that a substance \Odell is readily decomposed is produced by the proto plasm or developed as part of it. When a stimu lus acts upon the protoplasm this substance is decomposed, and in its destruction energy is liberated, by means of which the response is made. In those cases of activity which occur without the action of any known stimulus, it may be assumed either that an undiscovered stimulus is acting to initiate the necessary decomposition, or that the protoplasm may decompose itself `at will' and release energy. The energy thus re leased may be applied to contraction; that is, to changing the form of the protoplasmic body, to secretion, to increasing or retarding the rate of growth, etc.
Not all portions of the plants are equally sen sitive to a given stimulus. Indeed, in many cases
the sensitive regions are sharply localized and very lhnited. It has been slim% n, fur example, that sensitiveness of a root to the stimulus of gravity is limited to the terminal millimeter or two. In other parts the whole growing region may be sensitive. When the sensitive region is sharply localized. it may be coincident with the region in which the response is perceived, or it may be separated from it by a considerable dis tance. In the latter case a propagation of the stimulus must occur. There are in plants no defi nite structures corresponding to the nerves of animals, though in roots fibril-like structures in the protoplasm have recently been claimed to be the lines along which the propagation of the stimulus from the perceptive to the active region occurs. Setting in action the mechanism of per ception, the propagation of the stimulus, and the execution of the final response consume time. In some plants the final response follows in a fraction of a second after the stimulus is received, as in the sensitive plant. In most eases, however, the end reaction is separated in time from the initiation of stimulation by seconds, minutes, or even hours. This interval is known as the reac tion time. When it is sufficiently long, a stimu lus may be applied during a portion of the reac tion time and then discontinued. After the usual interval, however, the reaction will still occur, although no stimulus is then operating. Such effects, the results of previously acting stimuli, are known as 'after-effects.' The time during which any stimulus must act in order to call forth a response is the presentation time.
The reaction to a stimulus may consist either in a change in the amount of rate of a function, or in a change in the character of a function. In the former ease the stimulus may either accele rate or retard. The latter form of reaction is much less common than the former. Further, the reaction may he either direct or induced. Induced reactions are such as will not occur in response to the stimulus acting unless some other stimulus first brings about a change; after which reaction to the first stimulus becomes possible. Thus a plant may not respond to stimulation by gravity unless it is first stimulated by light. It is difficult. therefore, to disentangle the effects of different stimuli without extreme care in experi mentation. Every living cell of the plant is ill reality an irritable structure. Since movement is the most easily observed reaction. the mistake has been made of considering motor organs as heing, specially irritable regions. Thus, tendrils, the leaves of the sensitive plant. the peculiar motile leaves of the Venus's fly-trap, the tentacles of the leaves of Drosera, the contractile filaments of many anthers, etc., are looked upon as sensi tive organs par excellence. In reality, however, many of these are far inferior in sensitiveness to structures whose reactions are less evident.