IRRITABILITY AND MOVEMENT IN PLANTS. 233 



a strong diffuse light coming from all directions, it will be found 

 unresponsive to an increase of the light from one side far in ex- 

 cess 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-hun- 

 dredth-per-cent solution of this substance, would not respond to 

 an additional amount of the substance until it reached a concen- 

 tration thirty times as great. The results obtained by other in- 

 vestigators indicate that Weber's law is applicable also to the re- 

 actions 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 move- 

 ment of a tendril which has been stimulated by the contact of 

 a solid body (see Fig. 4). Thirty seconds elapsed after the stimu- 

 lus was applied before the movement began. The contraction 



'/ 



2 



y 



Fig. 6. Curve of Contraction of Tendril. The distance of the curve from the base rep- 

 resents the amount of displacement of tbe tip ; 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 decreas- 

 ing, 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 five hun- 

 dredths 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- 



