32 OVER-ALL REACTION OF PHOTOSYNTHESIS CHAP. 3 



Ingen-Housz knew (or suspected) that plants continue to respire in light, 

 so that their net gas exchange during the day is the l^alance of photo- 

 synthesis and respiration. Tiie calculg-tion of true photosynthesis thus 

 requires the application of a "respiration correction," which cannot be 

 determined without certain arbitrary assumptions. Bonnier and Mangin 

 (188G), who were the first to face this problem, used four methods for its 

 solution. One method (which has since come into common use) was 

 to determine the respiration in darkness, and to assume that its rate 

 remains unchanged in light (c/. Chapter 20). The second method was 

 based on the inhibition of photosynthesis by narcotics (which leaves 

 the respiration almost unchanged, cf. Chapter 12); the third on the pre- 

 vention of photosynthesis by deprivation of carbon dioxide, and the 

 fourth on the comparison of gas exchange in leaves with high and low 

 content of chlorophyll. By all four methods, Bonnier and Mangin ob- 

 tained Qp values considerably above unity (1.1 to 1.3). These results 

 were not confirmed by subseciuent investigators, notably Maquenne and 

 Demoussy (1913) and Willstiitter and StoU (1918), who found that Q? is 

 equal to unity within the limits of experimental error. Maquenne and 

 Demoussy determined the respiration correction by experiments in the 

 dark, while Willstatter and Stoll reduced it to insignificance by working 

 in very strong light and with ample supply of carbon dioxide, so that 

 photosynthesis was twenty or thirty times stronger than respiration. 

 Table 3.1 gives a selection of their results together with those of some 

 recent investigations, in which a different type of plant (lower algae) has 

 been used instead of the higher land plants. 



Table 3.1 shows the remarkable constancy of the photosynthetic 

 quotient — it is independent of light intensity, duration of illumination, 

 temperature, and the concentrations of oxygen and carbon dioxide. 

 Values slightly above 1 seem to predominate, although deviations from 

 unity are hardly beyond the limits of experimental error. Table 3.1 

 shows also that the respiratory quotient: 



(3.2) Qr = ACO2/- aO.> 



is close to unity for most plants (although its deviations from the nor- 

 mal value are more common than are those of the quotient Qp). Very 

 few significant cases of abnormal Qp values have been found. Before 

 discussing them, we may first inquire what the normal value, Qp = 1, 

 means for the chemical mechanism of photosynthesis. It finds a natural 

 explanation in the assumption that the product of photosynthesis is a 

 carbohydrate, i. e.; a compound with the atomic ratio H : O = 2 : 1. We 

 can thus elaborate equation (2.4), by writing: 



light 



(3.3) X CO., + y H2O > Cx(H..O)y + a; O2 



plant 



