a strong diffuse light coming from all directions, it will be found unresponsive to an increase of the light from one side far in excess of the amount just described. Pfeffer found that, While the spore tubes of ferns were ordinarily affected by an extremely small amount of sodium malate, yet, when placed in a one~hundredth-per-cent solution of this substance, would not respond to an additional amount of the substance until it reached a concentration thirty times as great. The results obtained by other investigators indicate that Weber's law is applicable also to the reactions of plants to light, and perhaps all forms of stimuli.
When a stimulus impinges upon a perceptive zone, the reaction shown by the motor zone does not occur simultaneously with the reception of the stimulus, but after a "latent period" of varying duration.
In Fig. 6 are graphically represented the features of the movement of a tendril which has been stimulated by the contact of a solid body (see Fig. 4). Thirty seconds elapsed after the stimulus was applied before the movement began. The contraction
Fig. 6.—Curve of contraction of Tendril. The distance of the curve from the base represents the amount of displacement of the hip; five centimetres on the base line represents five minutes; 1 to 2. latent period and period of contraction; 2 to 3, period of maintenance of contraction; 3 to 4, period of relaxation.
then went forward with, at first, an accelerating, then a decreasing, rapidity until the maximum of contraction was attained. This position was maintained several minutes, when a relaxation occurred by which the organ was restored to its original position. In comparison with the long familiar reaction of the frog muscle-nerve preparation, it will be seen that in one the latent period is about one hundredth of a. second, in the other thirty seconds; the period of contraction in one is five hundredths of a second, and in the other twelve hundred seconds; the period of maintenance of contraction in one is momentary, and in the other it endures eighteen hundred seconds; the period of relaxation is live hundredths of a second in one, and thirty-three hundred seconds in the other.
The essential features of the mechanism of each of the two movements are so widely different that these are not capable of strict mutual interpretation, but in general the rapidity of con-
