RATE OF PHOTOSYNTHESIS AT DIFFERENT HOURS l6l 



the original value, with 12 spacings. This enhancement was 

 due to the increase in the intensity of light, which was now 

 67, and to the rise of temperature to 22°- 7. The activity 

 continued to be further enhanced, attaining its maximum 

 between noon and 1 p.m. under the combined action of 

 greater intensity of light and higher temperature. There 

 were now 18 spaces in the record, showing an activity over 

 four times that at 7.30 a.m. A decline of activity occurred in 

 the afternoon, as seen in the diminished number of spaces, 

 12, in the record at 4 p.m. Henceforth the fall was extremely 

 rapid : at 4.30 p.m. the number of spacings was 7, about 

 the same as at 8 a.m. Owing to the diminishing intensity 

 of light, the decline in activity was still more pronounced 

 at 5 p.m., the number of spacings being reduced to 2. By 

 5.15 p.m. there was a complete arrest of photosynthesis. 



Determination of the Absolute Rate of Photo- 

 synthesis at Different Hours 



The absolute rate was obtained by applying the various 

 corrections for the successive volumes of oxygen evolved 

 during photosynthesis. The rate of activity (N.T.P.) at 

 various periods is given in Table XXIII. (p. 162), together 

 with the intensity of light and of temperature. 



In fig. 45 are given the three corresponding curves, 

 of which two represent the environmental conditions, and 

 the third the photosynthetic activity : the dotted curve 

 gives the variations of intensity of light ; the curve in 

 thin line gives the variations of temperature ; the curve 

 in thick line gives those of photosynthetic activity. 



I may begin the consideration of the curves by drawing 

 attention to the combined effects of light and temperature 

 in the forenoon and in the early afternoon, which present 

 very interesting features. I have previously explained that 

 though the intensity of sunlight is at its maximum at noon, 

 the light reflected from the northern sky remains practically 

 constant for a considerable length of time. This is seen in 



M 



