12 Macfarlane. — Contributions to the History of 



Repeated experiments were made on the strong green 

 leaves of the plants mentioned above in the laboratory green- 

 house of the University of Pennsylvania during October when 

 the temperature varied from 75 to 83 F., and these always 

 closed after time intervals of sixty to sixty-five seconds, but 

 when extended to eighty-five seconds and then reduced, three 

 to six stimuli could be given, thus : 



1 st stimulus no effect. 



2d " after eighty-five seconds " 



3d " " eighty " slight effect. 



4th " " seventy-five " slight added effect. 



5th " " seventy " leaf closed. 



From tables, like the above, we conclude that in rather 

 weak plants exposed to considerably lower temperatures than 

 the normal — i. e., from 6o° to 75 F., as compared with 8o° to 

 95° F., or 98 F. in North Carolina — sharp memory of the 

 first stimulus is retained for from thirty to forty-five seconds ; 

 but that a decided fall is then experienced, and from fifty-five 

 to sixty seconds the effect of the first stimulus is greatly lost. A 

 summation of one or more stimuli, varying in number with the 

 time-interval between these, is then needed to effect closure. 

 That in strong plants the effect of the first stimulus is sharply 

 retained for from fifty to seventy seconds, the variation being 

 determined by temperature and vegetative strength of the 

 plants. 



But the very important results obtained by Sanderson and 

 Page, supplemented by those now recorded, prove that exactly 

 similar summations of stimuli can be recorded in the contrac- 

 tion of Dioncza leaf as in contraction of muscular tissue, except 

 that the contracting protoplasmic substance in the former is 

 of a greatly less specialized quality, and is to a corresponding 

 degree more convenient and easy for demonstration. The 

 above observers experimented on leaves by applying stimuli 

 at intervals of two minutes, and the following table is given 

 by them to show the result of successive mechanical excita- 

 tion of the hairs at intervals of two minutes, continued until 

 the leaf closed : 



