_Experiment 160._--The leaf of _Mimosa_ was in this experiment held securely, and two electrical connections made, one with the less excitable upper and the other with the more excitable lower half of the pulvinus. The incident ether-wave induced an electric response in the pulvinus, the more excitable lower half exhibiting galvanometric negativity. On the cessation of stimulus there was a recovery (Fig. 155). It is not at all necessary to employ the sensitive _Mimosa_ for exhibition of electric response; for this is universally exhibited by all plants. The only condition for electric response is that the points of electric contacts should be made with two unequally excitable areas in the plant. This may be secured by artificial means as by causing 'injury' to one point of contact.[28] It is however much better to take advantage of the natural difference of excitability of two different areas in the organ as in the pulvinus of _Mimosa_. This difference of excitability is also found between the inner and outer sides of a hollow tubular organ as in the peduncles of various lilies. I was thus able to secure specimens which were far more sensitive to the action of electric waves than the pulvinus of _Mimosa_. [28] "Comparative Electro-Physiology"--p. 149. EFFECT OF WIRELESS STIMULATION ON GROWTH. There now remains the very interesting question as to whether the effect of long ether waves induce any variation of growth. The results given below show that growing plants not only perceive but respond to the stimulus of electric waves. The effects to be presently described are exhibited by all plants. I shall, however, content myself in describing a typical experiment carried with the seedling of wheat. The specimen was mounted on the Balanced Crescograph, and the growth exactly balanced. This gives a horizontal record; an acceleration of growth above the normal is, in the following records, represented by a down curve, and a retardation by an up-curve. _Effect of feeble stimulus: Experiment 161._--I first studied the effect of feeble stimulus. This was secured by decreasing the energy of sparks of the radiator. The response was an acceleration of rate of growth as seen in figure 156a. The analogy of this with the accelerating effect of sub-minimal intensity of light (p. 224) is very remarkable. [Illustration: FIG. 156.--Record of responses to electric wave by the Balanced Crescograph (_a_) response to feeble stimulus by acceleration of growth, (_b_) response to strong stimulus by retardation, (_c_) responses to medium stimulation--retardation followed by recovery. Down-curve represents acceleration, and up-curve retardation of growth: (Seedling of wheat.)]