The criterion employed for test of perception is the movement induced in response to stimulus. The responsive _mechanical_ movement is rendered possible only by the contractility of the organ, and mechanical and anatomical facilities offered by it for unhampered movement. The petiole of _Mimosa_ when locally stimulated does not itself exhibit any movement. The fortunate circumstance of the presence of a motile pulvinus in the neighbourhood enables us to recognise the perceptive power of the petiole, since it transmits an impulse which causes the fall of the leaf. There is no motile pulvinus in ordinary leaves, and stimulation of the petiole gives rise to no direct or transmitted motile reaction; from this we are apt to draw the inference that the petiole of ordinary leaves are devoid of perception. This conclusion is, however, erroneous, since under stimulus the petiole exhibits the electric response characteristic of excitation. Moreover my electric investigations have shown that every living tissue not only perceives but also responds to stimulation.[21] Hence considerable doubt may be entertained as regards the supposed absence of perception in the hypocotyl of _Setaria_. [21] "Response in the Living and Non-Living"--p. 17. I shall in the present paper describe my investigations on the mechanical response of _Setaria_ under direct and indirect stimulation which will be given in the following order:-- (1) The response to unilateral stimulation of the tip of the seedling. (2) The response of growing hypocotyl to direct stimulation. (3) Summated effects of direct and indirect stimulation. EXPERIMENTAL ARRANGEMENTS. _The Recorder._--The pull exerted by the tropic curvature of the seedling is very feeble; it was therefore necessary to construct a very light and nearly balanced recording lever. A long glass fibre is supported by lateral pivots on jewel bearings. The seedling is attached to the short arm of the lever by means of a cocoon thread. The recording plate oscillates to and fro once in a minute; the successive dots give therefore the time relations of the responsive movement. The positive curvature towards light is recorded as an up-curve, the negative curvature being represented by a down-curve. [Illustration: FIG. 135.--Arrangement for local application of light to the tip and the growing region. O, O', apertures on a metallic screen. Light is focussed by a lens on the tip, and on the growing region at o, o'. Figure to the right shows front view of the shutter resting on a pivot and worked by string, T.]