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.
Fig. 20. Effect of moderate cooling during a period shown by horizontal line below. Moderate depression followed by quick restoration.
Effect of lowering of temperature: Experiment 17.—A simple way of exhibiting the effect of lowering of temperature is by artificial cooling of the pulvinus. This cannot very well be done by application of a stream of cooled water, because, as we have seen, absorption of water by the pulvinus is attended by a loss of excitability: diluted glycerine has, however, no such drawback. This fluid at ordinary temperature was first applied on the pulvinus, and after an interval of half an hour records were taken in the usual manner. Cooled glycerine was then applied and the record taken once more; the results are seen in Figs. 20 and 21. In the former, the first response was normal at the temperature of the room, which was 32°C.; the next two exhibit depression of excitability under moderate cooling; the duration of application of moderately cooled glycerine is there indicated by the horizontal line below. On the cessation of application, the normal temperature was quickly restored, with the restoration of normal excitability.
In the next record (Fig. 21) is shown the effect of a more intense cold. It will be noticed that the first effect was a depression, and subsequently, a complete abolition of excitability. Thick dots in the record represent applications of stimulus which proved ineffective. It will also be noticed that even on the cessation of cooling, and the return of the tissue to normal temperature the induced abolition of excitability persisted as an after-effect for a considerable time. I have likewise found that the after-effect of cold in abolishing the conduction of excitation is also very persistent. These experiments show that owing to physiological inertia, the variations of excitability in the plant often lag considerably behind the external changes which induce them.
