76 PLANT RESPONSE 



By wrapping a sensitive film round the recording drum, 

 all these response-records may be obtained photographically. 

 Thus the whole process of stimulation and its record may be 

 rendered automatic. The records given in this and succeeding 

 chapters have been obtained sometimes by photography, and 

 sometimes by employing the simpler process of following the 

 spot of light with a recording pen. 



From a knowledge of the magnification produced by the 

 Optic Lever, and the height of the responses, it is easy to 

 calculate the actual contraction produced by the stimulus. 

 From the length of the specimen experimented on we can 

 also determine the coefficient of responsive contraction — that 

 is to say, the absolute contraction for unit length. When the 

 magnification of the lever, the length of various specimens, 

 and the strength of the stimulus are all kept constant, then 

 the heights of the responses in different cases will be found 

 to afford us a measure of the mechanical excitabilities of 

 the different specimens. 



Demonstration of universality of excitatory longi- 

 tudinal contraction in radial organs. — I shall now describe 

 my experiments on the longitudinal contraction of various 

 radial organs. The first of these was performed on a straight 

 radial internode of Cuscuta, which was attached to the 

 recording Optical Lever in the usual manner. Tetanis- 

 ing shocks were given for twenty-five seconds at a time, 

 from an induction coil, and the successive responses were 

 obtained, at intervals of two minutes (fig. 37). The contractile 

 effect persisted, even after the cessation of stimulation, for 

 a period of five seconds, after which there was recovery, 

 which was completed in a period of ninety seconds. It will 

 be seen from the record that the successive responses were 

 uniform. 



I obtained similar responses from the root of a water- 

 growing plant of the Bindweed family. Successive responses 

 were obtained at intervals of two minutes, the stimulus in 

 each case consisting of tetanising shocks of forty-five seconds. 

 The maximum contraction was attained fifteen seconds after 



