Experiment 10.—As a typical example I shall describe the response of a straight tendril of Passiflora. A cut specimen was mounted with its lower end in water. Suitable electric connections were made for sending a feeble induction shock of short duration through the specimen. In this and all other records, unless contrary be stated, up-curve represents contractile movement. On application of stimulus of electric shock, an excitatory movement of contraction occurred which shortly reached its maximum; the apex-time was one minute and forty seconds, and recovery was completed after a further period of five minutes (Fig. 10). Stronger shocks induce greater contraction with prolongation of the period of recovery. The specimen was afterwards killed by application of poisonous solution of potassium cyanide; this brought about a permanent abolition of response. The experiment just described may be taken as typical of response of radial organs.
In a radial organ contraction takes place equally in all directions; it therefore shortens in length, there being no movement in a lateral plane. But if any agency renders one side less excitable than its opposite, diffuse stimulation will then induce greater contraction on the more excitable side which will therefore become concave.
RESPONSE OF AN ANISOTROPIC ORGAN.
Excessive stimulation is found to reduce the excitability of an organ. Under unilateral mechanical stimulation a tendril of Passiflora becomes hooked or coiled, the concave being the excited side. From what has been said, the unexcited convex side will relatively be the more excitable.
Fig. 11. Response of a hooked tendril of Passiflora to electric shock. Successive dots at intervals of 5 seconds.
Experiment 11.—I took a specimen of hooked tendril, and excited it by an electric shock. The response was by the greater contraction of the more excitable convex side, on account of which the curved specimen tended to open out. The record of this response is seen in Fig. 11; the apex-time was nearly two minutes, and the recovery was completed in the further course of 15 minutes.
From the responses of organs rendered anisotropic by the differential action of the environment we pass to others which show certain amount of anatomical and physiological differentiation between their upper and lower sides. I find that many petioles of leaves show movement in response to stimulus. Many pulvini, generally regarded as insensitive, are also found to exhibit responsive movements.