Chapter XII — 187 — Dionaea and Aldrovanda 



of the same kind were necessary before any movement ensued," but 

 failed to indicate a general rule. 



Following in the trail left by Burdon-S Anderson and Munk, 

 the method of electrical stimulation has been used by Brown and 

 Sharp (1910) to study time and intensity relations. They found 

 first of all that at 15 deg. C. two stimuli were always required, and 

 must be applied within an interval of from 1.5 to 20 seconds. But 

 at the higher temperature of 35 deg. C. frequently only one stimulus 

 was required, while at 40 deg. C. only one stimulus was required 

 in 50% of the instances. In order to elucidate this behavior, Brown 

 and Sharp tried the effect of electrical shock in various intensities 

 and found that the number of shocks required varied inversely with 

 their intensity. The authors then proceeded to determine the number 

 of stimuli (by bending the sensitive hairs) required when applied at 

 various intervals, viz., 20 seconds, i, 2, and 3 minutes, and found 

 that for these intervals, 2.0, 3.8, 6.2 and 8.7 stimuH (averages of 

 several tests) were required. It thus appears that "the number of 

 stimuU necessary for complete response varies almost directly with 

 the length of the intervals. It would seem, therefore, that the re- 

 sponse follows on a definite amount of accumulated efiect, "possibly 

 the accumulation of some chemical substance as the result of ex- 

 citation." It should be added that the physiological condition of 

 individual leaves has a modifying effect — at a given time and place 

 all leaves are not equally sensitive. 



In 1873, stimulated by Darwin's studies, J. Burdon-S Anderson 

 published the first observations on the electric current in leaves as 

 indicating physiological disturbance, using those of Dionaea. Having 

 demonstrated that there is a normal current from base to apex of 

 the trap while there is one in the reverse sense in the petiole, related 

 quantitatively so that if the petiole were cut off at different lengths, 

 the current in the trap was increased, he then studied the effect of 

 stimulating the sensitive hairs on the current. Whenever a fly was 

 allowed to walk into the trap it disturbed the hairs and at once there 

 was observed a deflection of a galvanometer. If the stimulus were 

 repeated, the galvanometer indicator came to rest in a different position 

 (more to the left) each time. Disturbance of the hairs with a camel's 

 hair brush had the same effect. Thus the fact that movements of 

 the sensitive hairs constitute a stimulus was demonstrated by noting 

 consequent electrical disturbance. 



Localization of perception. — It had been generally accepted since 

 OuDEMANs' time, in spite of Meyen's evidence to the contrary, that 

 StimuH leading to closure could be received only by the sensitive 

 hairs. Oudemans could not repeat Meyen's (1839) result, namely, 

 causing closure by scraping the midnerve. Darwin, however, found 

 both the area within the triangle formed by the sensitive hairs and 

 the surface along the midrib to be sensitive, so that when scratched 

 or pricked with a needle, closure followed. Macfarlane found that 

 this occurred on pinching the blade of the trap with steel forceps, 

 but that two stimuli were required. Brown and Sharp confirmed 

 Macfarlane's observation, only quaUfying the numerical expression 

 since they found that one only, or two or more pinches might be re- 



