In this article we will discuss about:- 1. Meaning of Stimulus 2. Mode of Reaction of Stimulus.
Meaning of Stimulus:
A stimulus is a change in the intensity or direction of application of energy which produces an appreciable effect upon the living protoplast and brings about a visible reaction or response. The stimulus may be an external agency or it may be due to some internal cause. In the latter case an autonomic or a spontaneous reaction is produced.
The stimuli may be classified as mechanical, chemical or ethereal. The mechanical stimuli depend upon mass movements causing contact, impact, friction, pressure upon the plant, e.g., gravity. The chemical stimuli are those whose action depends upon the chemical composition and molecular structure. The ethereal stimuli are caused by wavelengths, e.g., light, heat and electricity.
Mode of Reaction of Stimulus:
The stimulus brings about stimulation or excitation, causes a change in the electrical condition of the protoplast. It is expressed as a change of turgor, e.g., in the cells of the lower half of the pulvinus of the leaf as in Mimosa.
Again the stimulation may cause greater growth on one side and retardation on the other in an organ, e.g., in tropic movements. It may also bring about more rapid secretion of a fluid from the protoplasm of a gland.
The Internal Sequence:
The visible response may not necessarily occur at a point where the stimulus initially affected the plant. Presumably, some form of internal conduction of excitement occurs. There are three stages which are recognized.
These are briefly discussed below:
The point where the stimulus initially acts is called the region of perception. The reaction also occurs here initially.
Propagation of Reaction or Excitement:
The excitation or the reaction is propagated from protoplast to protoplast through the plasmodesmata. Between the first reaction and the last reaction, a series of reactions occur each of which is a reaction to the preceding stimulus and a stimulus to a succeeding reaction.
The Responsive Region:
The region where the final reaction occurs may be the same or some other point. In the root, for example, the region of perception is the root cap and the region of response is the region of elongation, where the growth rate is unequally affected on the upper and lower side and a curve occurs.
For a specific reaction to occur, a stimulus of given intensity must act for a definite time. This is called the presentation time. There is no response if the time of stimulus falls short of the presentation time. If stimulation is too brief to produce the end reaction its repetition at proper intervals causes the separate effects.
However, they become combined and are sufficient enough to bring forth a response. This is called summation of stimuli. It implies that during the presentation time a specific quantity of stimulus is needed to cause a reaction. In other words, there is no difference when a weak stimulus is prolonged for a long duration and a strong stimulus is applied for a short time.
Thus, the product obtained by multiplying the presentation time with the intensity of stimulus is a constant quantity. This relation is called the law of quantity of stimulus and is applicable to all tropisms and other plant movements.
This law may summed as follows:
Presentation time x Intensity of stimulus = K
The time gap between the initiation of a stimulus and the end reaction is called the reaction time. The reaction time comprises the presentation time +the transmission time +growth time.
Protoplasm in a healthy condition or tone only responds to a stimulus. The tone is influenced by temperature, presence of oxygen, water and the light. Excitability may be diminished for a while by the application of anaesthetics like chloroform or ether, etc. After a while, the effect vanishes away and the tone is restored. However, concentrated dose may be lethal and destroy the tone permanently.
Fatigue and ‘tetanus’:
After an organ or a plant is stimulated once and the response occurs, it regains its original state after a while and is ready to respond as usual. However, when several stimuli follow, before complete recovery, the response is delayed or negated.
This is referred to as fatigue. In many instances, the responses diminish and ultimately cease. In such a state, the organ assumes a fixed position like the un-stimulated one. This is celled tetanus since it resembles the condition of a muscle in tetanus. During the tetanus the reactions cease until rest from excitation permits recovery.