DAILY VARIATION IN PHOTOSYNTHESIS 277 



the precisely opposite effect is produced by strong stimu- 

 lation. 



In photosynthesis the after-effect of stimulus in modifying 

 the tonic condition is found by the Method of the Cyclic 

 Curve, the plant being subjected to light increasing to a 

 maximum and then diminishing to a minimum. Three 

 types of physiological hysteresis are observed under definite 

 conditions ; first, the negative hysteresis, where physiological 

 activity becomes depressed under the previous and long- 

 continued stimulation, the return-curve being to the right ; 

 second, the positive hysteresis, where the feeble activity of 

 a sub-tonic specimen becomes enhanced in consequence of 

 stimulation, the return-curve being to the left ; and third, 

 the zero hysteresis, where the forward- and return-curves 

 coincide, the activity being unchanged (p. 154). 



Daily Variation in Photosynthesis 



The Hydrilla plant growing in the open is subjected 

 to two changing environmental factors — temperature and 

 light. Special apparatus was devised to obtain simultaneous 

 determinations of temperature by the Thermograph, of light 

 by the Electric Photometer, and of photosynthesis by the 

 Photosynthetic Recorder. 



In a typical experiment at the end of January, photo- 

 synthesis was found to be initiated at 7.30 a.m., increasing 

 to a maximum at noon. There was a fall after 1 p.m. ; 

 the decline of activity was rapid from 3 p.m. to 5 p.m., 

 there being a complete arrest at 5.15 p.m. 



The resulting photosynthesis at various periods of the 

 day can be explained as the expression of combined effects 

 of the factors of light and temperature. The activity is 

 continuously increased with increasing intensity of light ; 

 it is also increased under rising temperature up to the 

 optimum ; the fall of activity at about 1 p.m. is explained 

 by the rise of temperature above the optimum. The curves 

 given (fig. 45) show the variations of light and temperature, 

 and of the resulting change of photosynthetic activity. 



