PHOTOTROPISM 323 



One is the 'up' or A-change, associated with increase 

 of potential energy of the system, and the other is asso- 

 ciated with ' (h)wn ' or D-change, by which there is a 

 run-down or depletion of energy. With moderate stimula- 

 tion the A-and-D effects are more or less comparable to 

 each other. But under strong stimulation the down-change 

 is relatively greater. Hence on cessation of moderate 

 stimulation the increase of potential energy, associated with 

 A-chaiige, finds expression in enhancement of the rate of 

 growth. The depletion of energy under strong stimula- 

 tion is, however, too great to be compensated by the 

 A-change. 



LATENT PERIOD OF PHOTOTROPIC REACTION. 



With reference to the latent period Jost thus summa- 

 rizes the known results :* " The latent period of the helio- 

 tropic stimulus has already been determined. According to 

 Czapek it amounts to 7 minutes in the cotyledons of 

 Avena and in Phycomyces ; 10 minutes in hypocotyls of 

 Sinapis alba and Beta vulgaris, 20 minutes in the hypo- 

 cotyl of Helianthus, and 50 minutes in the epicotyl of 

 Fhaseolus. If one of these organs be unilaterally illumi- 

 nated for the specified time, heliotropic curvature ensues 

 afterwards in the dark, that is to say, we meet with an 

 after-effect in this case as in geotropism. We are quite 

 ignorant, however, as to whether and how the latent 

 period is dependent on the intensity of light." 



With regard to the (juestion of relation of the latent 

 period to the intensity of stimulus I have shown (p. 16(5) 

 that the latent period is shortened under increasing intensity 

 of stimulus. In the case of tropic curvature induced by 

 light, I find that the latent period is reduceu under 



* Jost — Ibid, p. 473. 



