_Effect of transition from darkness to light: Experiment 15._--Here we have to deal first with the immediate effect of sudden transition, and then with the persistent effect of continuous light. In the record given in Fig. 18 the plant had been kept in the dark and the responses taken in the usual manner. It was then subjected to light; the sudden change from darkness to light acted as a stimulus, inducing a transient depression of excitability. In this connection it is interesting to note that Godlewski found that in the phenomenon of growth, transition from darkness to light acted as a stimulus, causing a transient decrease in the normal rate. The effect of continued light on _Mimosa_ is an enhancement of excitability. [Illustration: FIG. 18. Effect of change from darkness to light. The first three records are normal under darkness. Horizontal line below indicates exposure to light. Note preliminary depression followed by enhancement of excitability.] EFFECT OF EXCESSIVE TURGOR. I have often found that the moto-excitability is depressed under excessive turgor. Thus the “over-turgid” leaf of _Biophytum sensitivum_ does not exhibit any mechanical response on rainy days. [Illustration: FIG. 19. Effect of enhanced turgor, artificially induced. First two responses normal. Application of water, at arrow, induces depression of moto-excitability.] _Experiment 16._--The effect of excessive turgor on moto-excitability may be demonstrated in the case of _Mimosa_ by allowing its main pulvinus to absorb water. The result is seen in the above record (Fig. 19), where water was applied on the pulvinus after the second response. It is seen how a depression of moto-excitability results from excessive turgor brought on by absorption of water. In such cases, however, the plant is found to accommodate itself to the abnormal condition and gradually regain its normal excitability in the course of one or two hours. INFLUENCE OF TEMPERATURE. The moto-excitability of the pulvinus of _Mimosa_ is greatly modified under the influence of temperature. For the purpose of this investigation I enclosed the plant in a glass chamber, raising the temperature to the desired degree by means of electric heating. Responses to identical stimuli were then taken at different temperatures. It was found that the effect of heightened temperature, up to an optimum, was to enhance the amplitude of response. Thus with a given specimen it was found that while at 22°C. the amplitude of response was 2.5 mm., it became 22 mm. at 27°C., and 52 mm. at 32°C. The excitability is enhanced under rising, and depressed under falling temperature. The moto-excitability of _Mimosa_ is practically abolished at the minimum temperature of about 19°C. [Illustration: FIG. 20. Effect of moderate cooling during a period shown by horizontal line below. Moderate depression followed by quick restoration.]