CHAPTER XXXIV 



ON AN ATTEMPT TO DETECT AND MEASURE LATENT 

 STIMULUS, AND ON THE STUDY OF PERIODIC AFTER- 

 EFFECTS 



Positive and negative after-effects — Extreme delicacy of the Method of Balance — 

 Detection of absorbed stimulus by negative after-effect — Constancy of sum of 

 direct and indirect after-effects — Latent component almost vanishing above the 

 optimum — Variation of receptivity— Direct and indirect response of plant in 

 sub-tonic condition — Table showing direct and indirect effects at different 

 temperatures— Is the change induced by stimulus always of an explosive 

 chemical character ? — Relation between stimulus and response in different 

 tonic conditions— After-effect— Factors which determine periodic after-effects : 

 (i) Stimulus of light — (2) Temperature — (3) Chemical stimulus— (4) Tur- 

 gidity— Continuous photographic record of the pulsations of Desmodium — 

 Record of periodic variation of rate of growth — Continuous photographic 

 record of periodic variations of transpiration — Continuous photographic record 

 of the variation of the rate of growth — Annual rings and seasonal periodicity. 



By regarding the plant as a machine, as we did in the course of 

 the earlier chapters, we were enabled to understand the possi- 

 bility of its absorbing, and holding latent, more or less of the 

 incident stimulus (p. 124). The experimental demonstration 

 of this would, however, be difficult, in the case of the ordinary 

 response of motile organs ; for thougn we have seen that ex- 

 ternal stimulus and the absorbed internal energy are opposite 

 in their responsive effects, yet in the ordinary records of 

 mechanical response it is not easy to discriminate that part 

 of the effect which is due to the latter element ; for while 

 it is true that the presence of internal energy would tend to 

 hasten the recovery, it is still impossible to distinguish with 

 certainty a recovery so hastened from one which is natural. 

 The fact that excess of stimulus is transformed into latent 

 energy is demonstrated, however, by the occurrence of 

 multiple response. 



