THE OPTICAL LEVER AS A TOOL IN PHYSIOLOGICAL STUDIES

Recent studies in the responses of the sugar-cane plant to varying percentages of soil moisture required the development of a technique for measuring elongation with considerable precision. A modified form of the optical lever, commonly used in physical laboratories for such measurements as the lineal expansion of metals, has been satisfactorily used in this work. So far as is known by the writer, such a device has never before been reported as a tool in plant physiology. In its modified form the device consists of a small circular mirror mounted normally to a short brass rod which is bifurcated at the end remote from the mirror. This split end is spread to receive the plant member to be under observation. Holes drilled through the two parts of the split end, in a line parallel to the plane of the mirror, permit the pinning of the plant tissue to the end of the lever with a minimum of damage. The weight of the lever is carried upon a metal rod carrying two projections provided with carefully machined depressions in which two steel pins attached to the mirror support are seated when the device is assembled for use. The lever is so counterpoised that its center of gravity is directly over the line of the pins in order that the plant tissue may be protected from stress when attached to the lever arm.