INTERMITTENT POINTOLITE ILLUMINATION 95 



rately adjusted by means of rotating sectors interposed in 

 the path of light, the various periods being i, 2, 3, 5, 7, 10, 

 15 and 20 seconds. In order to make the results easily 

 comparable with each other, the activity under continuous 

 light is taken as 100. Under intermission of 1 second, the 

 photosynthesis is found to be 121, i.e. an enhancement of 

 21 per cent. At an intermission of 2 seconds it was 108 ; at 

 an intermission of 3 seconds the activity became lowered 

 to 96. At an intermission of 5 seconds the activity was 

 further reduced to 80 ; at 7 seconds' intermission it was 

 70, and at 10 seconds' there was a maximum reduction of 

 activity to 63. This was the turning-point, for at an 

 intermission of 15 seconds the activity was partially restored 

 to 79, and more fully to 88 at an intermission of 20 seconds. 

 The results are graphically illustrated (fig. 28), where the 

 ordinate represents the activity, and the abscissa the in- 

 creasing periods of intermission in seconds. The typical 

 results given above with Pointolite are very definite and 

 exhibit characteristics similar to those under sky light, 

 the slight difference being ascribable to a different physio- 

 logical condition of the two specimens and to the different 

 intensities of light employed. 



The activity above the normal with a short intermission- 

 period is theoretically of great importance, since it gives 

 some indication of a physico-chemical action underlying 

 photosynthesis. The following is a probable explanation 

 of the phenomenon. Light, as previously stated, gives rise 

 to dual reactions A and D, the resultant being due to their 

 difference. We also found that the D effect often disappears 

 immediately on the cessation of light, while the A effect 

 persists for a short time. This was exhibited on the cessa- 

 tion of light (1) by the transient enhancement of the rate 

 of growth above the normal and (2) by the positive after- 

 effect in electric response (pp. 56, 76). 



The enhancement of photosynthetic activity under short 

 intermission thus appears to be the immediate after-effect 

 of light, the A effect persisting for a short time. With 



