PHOTOSYNTHESIS: GENERAL 23 



what gas was removed from the air, and he even thought that 

 air which had been made impure by too much hydrogen resulting 

 from the contact of metals with acids could be improved by plants 

 in the same manner as that vitiated by animals. 



It remained for Senebier (1782) to show that carbon dioxide 

 was the only gas absorbed and that this absorption was related to 

 the nutrition of the plant. He thus explained the results of Bon- 

 net and correlated the work of all the previous investigators. 



De Saussure (1804) then showed that as the carbon dioxide was 

 removed, the same amount of oxygen was returned to the air, or 

 that the C0 2 :0 2 ratio was approximately 1:1. He also showed 

 that, as a result of this absorption and decomposition of carbon di- 

 oxide, the weight of the plant was increased ; and he demonstrated 

 that in an atmosphere deprived of carbon dioxide the green plants 

 would not live. To quote his own words: "The observations of 

 Priestley, Senebier, and Ingen-Housz opened the path which I 

 have followed but they did not reach the goal which I have tried to 

 reach. . . . My researches have enabled me to demonstrate how 

 much greater is the contribution of water and air to the formation 

 of the dry material of plants growing in fertile soil than the mate- 

 rials which they absorb in the water through their roots." 



That De Saussure and his followers minimized the importance 

 of the roots in the absorption of carbon has been shown recently 

 (1927) by Stoklasa who finds that plants absorb comparatively 

 large amounts of carbon in the form of carbonates. This carbon 

 is largely a respiration product of bacteria in the soil. Stoklasa 

 finds that in some cases there are 100 pounds of carbon absorbed 

 by the roots for every 154 pounds of nitrogen and every 106 

 pounds of potash. This seems a considerable amount, but also 

 Livingston and Beall have called attention to the large amount of 

 carbon dioxide which may be taken in by the plant dissolved 

 in water absorbed by the roots: They calculated that with a 

 high rate of water absorption and a high concentration of carbon 

 dioxide in the soil solution, as much as 6-8% of the carbon dioxide 

 decomposed by the leaves of an alfalfa plant might enter through 

 the roots. Confirming these researches is the work of Bergamaschi, 

 who found that plants with their roots in a medium rich in carbon 

 dioxide were much more active in photosynthesis than those in 

 a normal medium. Whether the carbon is absorbed as dissolved 

 carbon dioxide or as carbonates, these researches have shown 



