NUTRITION 



65 



The products of photosynthetic activity are, as we have 

 seen, food, water, and oxygen. These, in obedience to the 

 laws of diffusion, pa.ss out of the cells in which they are 

 formed into others which contain less, or into the air. The 

 oxygen set free may be used in part in respiration, but only 

 a small part of the considerable quantity of oxygen liber- 

 ated in active photosynthesis can be respired during that 

 time by the plant producing it. The volume of oxygen 

 given off during the life-time of a plant will equal that of 

 the carbon-dioxide absorbed, for respiration and photo- 

 synthesis balance each other. As the volume of carbon- 

 dioxide given off may be used as a quantitative index of the 

 respiratory activity, so the volume of oxygen may be used 

 as a quantitative index of the photosynthetic activity ; but 

 for reasons corresponding to those discussed under Respira- 

 tion (pp. 38, 39 ) , these indices are not exact. Both gases are 

 at the same time concerned in other processes besides those 

 which we have so far separately examined. The accompany- 

 ing figure shows the relative activity of photosynthesis and 

 respiration as indicated by the volumes of CO^ concerned. 



7.5° 11.3° 15.8° 



37.3° 41.7° iO.l" °0. 



Figure 2. — Photosynthesis and respiration of a leafy branch' of Evbus 

 Suticoaus (after Kreusler*). During photosynthesis the branch was in 

 air containing 0.3% COj, and was illuminated with electric light approxi- 

 mately equal to the diffuse sunlight. The upper curve indicates the amount 

 of CO2 actually consumed in photosynthesis. 



CO2 used in photosynthesis ( per hour in each sq. dm. of 



tion ) 



. .CO2 produced in respiration 



leaf surface. 



* Copied by Pfetfer, Pflanzenphyslologie, Bd. I., p. 321. Eng. transl. I., p. 

 337, from Kreusler'a paper in Landwirthschaftliches Jahrbuch, Bd. 16, 1887. 

 5 



