Page:Researches on Irritability of Plants.djvu/29

6 concomitant curve. For this there must be an axis, supported on frictionless jewelled bearings, and carrying two arms of a horizontal lever and a thin vertical wire with a bent tip, to serve as the Writer. The different parts, as far as possible, should be made of aluminium, to secure the utmost lightness. A point of the petiole of the responding leaf would be attached by a silk thread to one arm of the lever, the other having on it a small weight, to act as counterpoise. On the fall of the leaf, under excitation, it would pull down with it the attached arm of the lever. The vertical writer would then also move, say, to the left. If the finely pointed bent end of the writer were to press lightly against the smoked surface of a glass plate, which was allowed to fall, at a uniform rate, by means of clockwork, a curve would then be traced which would not only record the responsive movement and recovery but also give their time-relations (fig. 1). To obtain the latter, it would be necessary to know the rate of movement of the plate on Which successive vertical lines might be traced by a time-marker at intervals of, say, one minute.

In order to find out the absolute movement of the leaf we must know the degree of magnification or reduction that has been effected by the recording arrangement. This will depend upon the relative lengths of the writer and the lever, and the distance of the point of attachment on the leaf from the pulvinus. When the lengths of the lever-arm and the writer are equal, then the writer will describe a movement which is equal to that of the point of leaf-attachment. By shortening the arm of the lever to half the length of the writer, we should obtain the magnification of two. This