84 CHAP. XI. PHENOMENON OF PHOTOSYNTHETIC INDUCTION 



effect of increasing stress on a piece of india-rubber string, 

 which has to be continuously stretched before the breaking- 

 point is reached. A certain amount of preliminary work 

 has thus to be performed in stretching the string, so that the 

 final pull brings about rupture. This preliminary work is 

 partially undone if the stretching force acts intermittently 

 instead of continuously. There is thus a greater efficiency 

 with a continuous than with an intermittent force. In 

 photosynthesis under continuous light a steady condition 

 is attained when the successive bubblings occur at intervals 

 of, say, T, which is the time required for the production of a 

 definite volume of gas. But, if the light be discontinued 

 for a time, the preliminary work may be undone and the 

 molecular strain under light preceding dissociation gradually 

 disappear, the more so the longer the duration of stop- 

 page of light. The result of this would be a prolongation 

 of the period preceding the appearance of the first bubble 

 after re-exposure to light, which I designate as the Period 

 of Photosynthetic Induction. From what has already 

 been stated, this induction-period, I, necessary for the 

 evolution of a definite volume of gas, would increase the 

 longer the duration of previous darkness. This increase 

 would not, however, go on indefinitely ; for it could not 

 exceed the period of the complete molecular recovery 

 from the photic stress. The induction-period will thus 

 reach a limiting value beyond which there is no further 

 increase. 



T may be regarded as the induction-period for con- 

 tinuous light ; this will increase to I, I', I", with increasing 

 periods of previous darkness. We shall then have T < I < 



r < i" . . . 



Having explained the theoretical considerations, the 

 following results are given in confirmation. The experi- 

 ment was carried out under uniform light from the sky, 

 the temperature being 20° C. Under continuous light the 

 successive bubbles appeared at intervals of 20 seconds. 

 Light was then interrupted for 20 seconds, and the effect of 



