26 



PLANT RESPONSE 



in the case of plant response, by the construction of a spiral 

 spring-recorder, the movement of whose index is approxi- 

 mately proportional to the tension: 

 The recorder is constructed of a fine 

 flattened spiral spring. Springs of 

 this description have the peculiarity 

 that, when they are stretched by 

 tension, the free end of the spiral 

 rotates round the axis of the 

 spring. A slight rotation may be 

 magnified by means of a mirror 

 and reflected spot of light (fig. 20). 

 This arrangement is specially 

 appropriate to the leaf-stalk of 

 Mimosa, where the pull exerted 

 by the excited leaf is consider- 

 able. Fig. 21 gives the isometric 

 response of Mimosa. 



Minimally effective stimulus 

 in Biophytum. — It is well to 

 mention here that at least in the case of Biophytum 

 the minimal intensity of stimulus necessary to cause response 



is very well defined. With 

 a certain specimen for 

 example, when the plant 

 was excited by the dis- 

 charge from a 01 micro- 

 farad condenser, charged 

 to seven volts, there was 

 no response. But when 

 the condenser was charged 

 to nine volts the discharge 

 always produced a large 

 and definite response. 

 Charging of the condenser to nine and seven volts alternately 

 would in the one case produce response, and in the other 

 none. If now, by the action of an external agency, the 



Fig. 20. The Spiral Spring- 

 Recorder 



The leaf is practically prevented 

 from moving. The tension 

 exerted by excited leaf causes 

 rotation of index or mirror. 



Fig. 21. Isometric Response of Mimosa 



