Page:Collected Physical Papers.djvu/371

Rh by rise of temperature is exhibited by the widening of intervals between successive dots. The effect of lowering of temperature in retardation of growth is also exhibited by record (fig. 110b) taken on a moving plate, in which the diminished slope of the curve demonstrates the depressed rate of growth.

The effects of manures, anæsthetics, drugs and poisons, can be similarly determined in the course of a few minutes, and with unprecedented accuracy. A few agents only have hitherto been employed in stimulating growth, whereas there are numerous others of whose actions we have been profoundly ignorant. The crude method hitherto employed in the application of a few chemical agents and of electricity has not been uniformly successful. The cause of the anomaly is found in the discovery of an important factor, namely, the dose of application, which has hitherto not been taken into account. It was thus found that while a particular intensity of electrical current accelerated growth, any excess above a critical point retarded it. The same is true of chemical stimulants; a striking result was obtained with certain poisons which in normal doses killed the plant, but in doses sufficiently minute acted as a highly efficient stimulant in promotion of growth.

The growth of plants is affected by changes of environment which are even below human perception. It now became necessary to devise a new method of still greater sensitiveness which would instantly show by the up- or down-movement of an indicator, the stimulating