HYSTERESIS 



153 



The rise of the tonic level as an after-effect of stimulus 

 was also shown in a different and very interesting manner. 

 Stimulus of the intensity o-io S did not, as stated before, 

 produce any photosynthesis, that particular stimulus being 

 then ineffective ; but after the cycle of operations (which 

 raised the tonic level from P to P'), the former ineffective 

 stimulus became effective, the rate of photosynthesis under 

 o-io S being now 89 c.mm. per hour. The heightened 

 tonicity is thus manifested in the induced lowering of the 

 intensity of the minimally effective stimulus. 



Zero hysteresis. — In this experiment a more sensitive 

 spring-specimen was used ; care was taken to avoid ex- 

 cessive stimulation by prolonged application of light, which 

 might have otherwise caused fatigue. The observations 

 were taken for the comparatively short period required for 

 application of 400, 600, 800 and 1000 lux, both during the 

 ascent and descent ; the photosynthesis during ascent and 

 descent was found to be practically the same (Table XXII.). 

 The two curves coincide, and the hysteresis is zero. The 

 physiological condition (P) of the plant at the end of the 

 operations was the same as at the beginning. 



Table XXII. — Cyclic Photosynthetic Curve exhibiting 



Zero Hysteresis 



Intensity of light in lux 



400 



600 



800 



IOOO 



Activity in c.mm. O per hour 



During ascent 



il7-I 



182-5 

 I 237-3 



V 243-1 



During descent 



118-3 



183-4 



A 237*7 



I 243-1 



This is the ideal method of experimentation for obtain- 

 ing a perfect photosynthetic curve and for the most correct 

 determination of the physiological constant. The entire 

 observation should be completed in as short a time as 



