In plant physiology, the word ‘stimulus’ is often used in a very indefinite manner. This is probably due to the different meanings which have been attached to the word. An agent is said to _stimulate_ growth, when it induces an acceleration. But the normal effect of stimulus is to cause a retardation of growth. It is probably on account of lack of precision in the use of the term that we often find it stated, that a stimulus sometimes accelerates, and at other times, retards growth. In order to avoid any ambiguity, it is very desirable that the term stimulus should always be used in the sense as definite as in animal physiology. An induction shock, a condenser discharge, the make or break of a constant current, a sudden variation of temperature, and a mechanical shock bring about an excitatory contraction in a muscle. These various forms of stimuli cause, as we have seen, a similar excitatory contraction of the motile pulvinus of _Mimosa pudica_. We shall enquire whether the diverse forms of stimuli enumerated above, exert similar or different reactions on the growing organ. EFFECT OF ELECTRIC STIMULUS OF VARYING INTENSITY AND DURATION. The form of stimulus which is extensively used in physiological investigations, is the electric stimulus of induction shock which is easily graduated by the use of the well known sliding induction coil, in which the approach of the secondary to the primary coil, indicated by the higher reading of the scale, gives rise to increasing intensity of stimulus. The retarding effect of electrical stimulus on growth has already been demonstrated in record taken on a moving plate (Fig. 61). I shall adopt for unit stimulus, that intensity of electric shock which induces a barely perceptible sensation in a human being. It is very interesting to find, as stated before, that growth is often affected by an electric stimulus, which is below the range of human perception. [Illustration: FIG. 72.--Effects of electrical stimulus of increasing intensities: of 0.25 unit, 1 unit, and 8 units. Short dashes represent the moments of application of stimulus.] _Effect of Intensity: Experiment 74._--I shall now describe a typical experiment on the effect of intensity of stimulus in retarding the rate of growth. The normal rate of growth of the bud of _Crinum_ was 0.35 µ per second. On the application of electric shock of unit intensity for 5 seconds, the rate became reduced to 0.22 µ per second. When the stimulus was increased to 2 units, the retarded rate of growth was 0.07 µ per second. When the intensity was raised to 4 units, there was a complete arrest of growth. In figure 72 is given records of a different experiment which show the effects of increasing intensity of stimulus in retardation of growth. [Illustration: FIG. 73.--Effect of continuous electric stimulation of increasing intensity. The last record exhibits the actual shortening of the growing organ under strong stimulus.]