272 PLANT RESPONSE 



than one minute, and the maximum contraction takes place 

 in a relatively short period of about fifteen minutes (fig. 30, 

 /; and c, p. 44). 



Response of Biophytum on the * all or none ' principle. 

 It is well known that in the case of a contractile skeletal 

 muscle, there is a minimal intensity of stimulus which is 

 necessary in order to produce contraction. From this point 

 onwards, as the stimulus is gradually increased, the response 

 increases, till a maximum contraction is arrived at, beyond 

 which still further increase of stimulus produces no increase 

 in effect. In cardiac muscle, however, the range of stimulus 

 between minimal and maximal is practically narrowed to a 

 point, so that the minimally effective stimulation is also at 

 once maximal. It is to be remembered, at the same time, 

 that the differences between cardiac and skeletal response are 

 a question of degree, rather than of kind. 



Curiously, the response of Biophytum is, in this respect, 

 somewhat similar to cardiac response. In an experiment 

 with a particular specimen of Biophytum, the intensity of 

 stimulus was increased by successive increments of the E.M.F. 

 u.sed for charging the condenser. With an E.M.F. of seven 

 volts there was no response. With a charge of nine volts 

 there was always a response, and this was maximal. A 

 charge of eight volts was almost on the threshold of response. 

 That is to say, when I started experimenting, the leaflet was 

 in a somewhat sluggish condition, and an eight-volt charge 

 was ineffective. But after obtaining response to a nine-volt 

 charge, I could obtain response also at eight volts. This was 

 due to the fact that molecular inertness had been removed by 

 the preceding effective shock. Thus we have two determi- 

 nate values of stimulation, giving respectively maximum 

 response and absence of response, the charges, namely, of 

 nine and seven volts. The effective stimulus is, of course, 

 constant for a given individual, but differs with the excita- 

 bility of different specimens. Here, then, we have an instance 

 of the ' all or none ' effect. The leaflet either responds to 

 the utmost, or not at all. 



