THE DETERMINING FACTORS IN PHOTOSYNTHESIS 37 



The remainder of the 70% absorbed by the leaf and not used 

 in photosynthesis, i. e., about 66% of the total solar energy re- 

 ceived, is spent in heating the leaf as well as the surrounding air. 

 This might cause an increase in temperature of 10-15° C. if it 

 were not for the fact that most of this heat is used in the conver- 

 sion of water to water vapor within the leaf. For every gram of 

 water vaporized, 536 calories of heat are employed, and the re- 

 sult is that with an abundant water supply, the leaf may be ac- 

 tually cooler than its surroundings. Green tomato fruits, on the 

 other hand, not having the cooling facilities of leaves, may be 

 nearly 10° C. warmer than red ones (Harvey, 1924). 



Temperature.— According to the researches of Matthaei (1905), 

 temperature is one of the most important of the external con- 

 ditions which influence photosynthesis. While some conifers ac- 

 customed to very cold climates may make some food at — 25° C. 

 and even lower, the minimum for most plants is about — 6° C, 

 the optimum near 37° C, and the maximum about 45° C. Kreusler 

 (1887) had previously reported the optimum at 25° C. but this 

 difference may be explained by the difference in the materials 

 used. For crop plants in the temperate zone it is generally safe 

 to assume an optimum of about 30°, although it is better to know 

 precisely for the individual species. Although Walther (1927) 

 reported that in Vicia photosynthesis did not increase directly 

 with the temperature but that there were maxima at about 10, 

 20, and 30° C. (with drops in between), Honert (1930) found that 

 photosynthesis increased continuously with the temperature from 

 12 to 24° C. in good light. This is more in agreement with the 

 general conception in regard to this matter. 



In studying the effect of temperature upon a reaction it is often 

 useful to know the temperature coefficient. If a reaction goes on 

 twice as fast at 20° C. as at 10° it has a temperature coefficient of 

 2. According to Matthaei and others, the temperature coefficient 

 of photosynthesis between 10° and 30° is 2, i. e., at 20° the process 

 is twice as rapid as at 10° and at 30° twice as rapid as at 20°. 

 Physical reactions generally have a temperature coefficient of 

 less than 2 while chemical reactions commonly have a coefficient 

 of 2 or more. The above results, therefore, indicate a chemical 

 reaction rather than a physical one. Brown and Heise (1917), 

 however, found the temperature coefficient to be 1 instead of 2 

 and hence concluded that they were dealing with a physical 



