150 CHAP. XVIIL THE TONIC FACTOR IN PHOTOSYNTHESIS 



Detection of After-effect of Stimulus on Photo- 

 synthesis by the Method of Cyclic Curve 



I have been able to discover parallel effects in photo- 

 synthesis by the Method of Cyclic Curve, which enables 

 us at the same time to follow the history of the induced 

 change. The cycle of operations consists in the applica- 

 tion of stimulus from a minimum to a maximum and 

 back once more to a minimum. After this, the tonic condi- 

 tion of the plant is generally found to be different from 

 that at the beginning : there is residual physiological change, 

 in virtue of which the original activity is enhanced or 

 depressed in a definite manner. I describe this as physio- 

 logical hysteresis, from certain analogous phenomena in 

 physics. 



In the experimental determination of physiological 

 hysteresis in Hydrilla we take a series of observations of 

 photosynthesis under increasing intensity of light, followed 

 by a decrease to the original intensity. The Cyclic Curve 

 plotted from the above data consists of an ascending and 

 a descending curve. These do not usually coincide : under 

 certain definite conditions the descending curve is to the 

 right {negative hysteresis), and under other conditions it is 

 to the left {positive hysteresis). There are occasions, again, 

 when the ascending and the descending curves coincide 

 {zero hysteresis). I give illustrative examples of the three 

 cases, and explain the conditions of their genesis. 



Negative hysteresis. — 'The particular specimen of Hydrilla 

 was moderately sensitive : the photosynthetic activity at 

 500 lux was 41 c.mm. per hour. At 1500 lux the activity 

 increased to 144, and at 3000 lux to 152-5. During the 

 reverse process of diminishing the intensity from 3000 

 to 500 lux, the activity at 1500 was reduced from the first 

 value of 144 to 137 ; at 500 lux it was 22-5 in the place 

 of 41. A depression of activity had thus occurred in 

 consequence of the previous strong and long-continued 

 stimulation (see Table XX., p. 152). The ascending and the 



