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.
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.
Fig. 19. Effect of enhanced turgor, artificially induced. First two responses normal. Application of water, at arrow, induces depression of moto-excitability.