274 



PLANT RESPONSE 



period is a matter of several seconds, but varies somewhat 

 with different specimens. I have, again, in some cases 

 observed a very curious phenomenon of two refractory 

 periods. 



We thus find several very interesting parallelisms between 

 the response of BiopJiytum in plants and that of cardiac 

 muscle in animals. We find in both that the minimal 

 response is also the maximal, increasing stimulation pro- 

 ducing no increase of response. In the response-curve itself 



the flattened top is common 

 to both ; both have a pro- 

 longed refractory period ; 

 and we shall see later that 

 in both there is a tendency 

 to the production of mul- 

 tiple rhythmic responses. 



In all these respects the 

 responses of BiopJiytum 

 resemble the cardiac re- 

 sponses, rather than those 

 of skeletal muscle. But 

 they have one peculiarity 

 in which they share the 

 characteristics of the re- 

 sponses of skeletal muscle. 

 In the responses of cardiac 

 muscle, successive effects 

 are not additive, perhaps because that muscle undergoes 

 the maximum contraction possible. In Biopkytiun, however, 

 while any effective stimulus — whether minimal or largely 

 super-maximal — will produce response of the same extent, 

 yet this response, though the greatest possible for a single 

 stimulus, is not the utmost of which the leaflet is capable. 

 Hence, if we superpose successive stimuli, taking care that 

 they do not fall within refractory periods, we shall obtain 

 an extremely interesting response, showing the separate 

 additive effects. I give here two curves, exhibitine these 



Fic;. 113. Additive Eftects seen in Re- 

 sponses o'i Biophytuni to Stimuli which 

 Fall outside the Refractory Period 



The record to the left shows the effect of 

 stimuli, applied at intervals of one, 

 and that to the right of half a minute. 



