_Moving plate method: Experiment 54._--This was carried out with a different specimen of _S. Kysoor_, the record being taken on a moving plate (Fig. 59_b_). The first part of the curve here represents the normal rate of growth. The plant was then subjected to moderate cooling, the subsequent curve with its diminished slope denotes the depression of growth. The question of influence of temperature will be treated in a subsequent Paper of the present series in much greater detail. [Illustration: FIG. 60--Horizontal record shows absence of growth in a dead branch; physical expansion on application of warmth at arrow followed by horizontal record on attainment of steady temperature. (Magnification 2,000 times)] _Precaution against physical disturbance: Experiment 54._--There may be some misgiving about the employment of such high magnification: it may be thought that the accuracy of the record might be vitiated by physical disturbance, such as vibration. In physical experimentation far greater difficulties have, however, been overcome, and the problem of securing freedom from vibration is not at all formidable. The whole apparatus need only be placed on a heavy bracket screwed on the wall to ensure against mechanical disturbance. The extent to which this has been realized will be found from the inspection of the first part of the record in figure 60, taken on a moving plate. A thin dead twig was substituted for the growing plant, and the perfectly horizontal record not only demonstrated the absence of growth movement but also of all disturbance. There is an element of physical change, against which precautions have to be taken in experiments on variation of the rate of growth at different temperatures. In order to determine its character and extent, a record was taken with the dead twig, of the effect of raising the temperature of the plant-chamber through ten degrees. The record (Fig. 60) with a magnification of two thousand times shows that there is an expansion during the rise of temperature, and that the variable period lasted for a minute, after which there was a cessation of physical movement, the record becoming once more horizontal. The obvious precautions to be taken in such a case, is to wait for several minutes for the attainment of steady temperature. The movement caused by physical change abates in a short time whereas the change of rate of growth brought about by physiological reaction is persistent. DETERMINATION OF LATENT PERIOD AND TIME-RELATIONS OF RESPONSE. [Illustration: FIG. 61.--Time-relations of response of growing organ to electric stimulus of increasing intensities applied at the short horizontal lines. Successive dots at intervals of 2 seconds.]