INTERNAL ENERGY AND EXTERNAL STIMULUS 433 



thus easy to obtain exact records of the effect of tension on 

 growth. The normal record is first taken, with the very- 

 slight tension exerted by the recording lever itself. The 

 short arm of the lever, 5 cm. in length, is, it should be 

 remembered, attached to the growing organ. A rider, half a 

 gramme in weight, can be placed on the longer arm of the 

 lever, at distances of '5, 1, l'5, 2, 25, or 3 cm. from the ful- 

 crum. The effective tension may thus be gradually in- 

 creased, and the corresponding effects on growth recorded. 

 It may be stated here that, generally speaking, any sudden 

 change of external conditions, such as sudden cooling, 

 sudden warming, or sudden variation of tension, acts on the 

 organ as an external stimulus ; and I shall presently show 

 that an external stimulus always induces a contraction or 

 retardation of growth. When the organ is subjected to 

 sudden increase of tension, the preliminary effect of con- 

 traction occurs therefore, as we should expect. But after 

 this temporary disturbance has disappeared we are able to 

 observe the permanent effect of increase of tension on growth. 

 For these experiments I took different specimens of Crinum 

 Lily, and the results obtained show that increase of tension 

 enhances the rate of growth. This increase, however, appears 

 to reach a limit at a certain optimum point, beyond which 

 increase of tension would seem to retard growth. The follow- 

 ing table exhibits the results of two experiments on different 

 specimens of Crinum. 



Table showing Effect of Tension on Rate of Growth 



F F 



