142 CHAP. XVII. EFFECT OF VARIATION OF TEMPERATURE 



Specimen III. 



Activity at temperature 24 -2 C. . . 288-4 



29°-8C. . . 400-4 



400-4 — 288-4 



K = ^ Z ~ — 20 *° 



29-8 — 24-2 



It may be said that a rise of 7 C. practically doubles 

 the activity. 



The average coefficient in the typical instances given 

 above is 20 • 4 : by the following formula the activity A T 



at the higher temperature T is calculable from the activity 

 A^ at the lower temperature t : 



A T = A* + K (t - t) . . . (2) 



According to Van't Hoff, the rate of chemical reaction is 

 increased two or three times for a rise of io° C. According 

 to this the coefficient for a rise of io°, K 10 , varies from 

 2 to 3. The values for K 10 in the above specimens of 

 Hydrilla (II. and III.) are the same, i.e. 2-2, between the 

 temperatures of 18 and 28 C. 



Summary 



In the accurate determination of photosynthesis under 

 variation of temperature, corrections have to be made 

 for loss in respiration. This has been determined by 

 two independent means : (1) by the volumetric method 

 and (2) by the loss of weight by the plant. The two deter- 

 minations give concordant results. At 18 C. the loss due 

 to respiration is about 4 per cent. 



In the determination of the effect of temperature, very 

 careful adjustments are necessary, for any sudden variation 

 produces a shock-effect ; the adjustment is made by the 

 special apparatus which has been fully described. 



The increase of photosynthetic activity is uniform in 

 the middle range. At the optimum temperature the activity 

 is at its maximum, after which there is a very sharp decline. 



