2 PHYSIOLOGY OF NUTRITION 



Since the formation of carbon dioxide in the combustion of carbon is ac- 

 companied by the liberation of heat, energy must be stored in the reverse 

 process, the decomposition of carbon dioxide. From this it is clear why sun- 

 light is so important in this decomposition; the energy of the sunshine ab- 

 sorbed by the plant is partly used in the decomposition of carbon dioxide and in 

 the synthesis of other carbon compounds. The green coloring matter, chlo- 

 rophyll, serves as a screen which absorbs the sun's rays and makes this energy 

 fixation possible. 



§2. Exchange of Gases. — Our first knowledge of the elimination of oxygen 

 by green plants was obtained by Priestley, 1 in 1772. Since animals utilize 

 ''dephlogisticated air" (as Priestley, its discoverer, called oxygen) and thus 

 render the atmosphere unfit for the maintenance of combustion and respiration, 

 he sought a reverse process by which the air might be improved, and he found 

 this process in plants. He placed plants under a bell-jar of air that had been 

 vitiated by animal respiration and was thus unfit for the maintenance of com- 

 bustion and respiration, and found that after some time the air became again 

 capable of supporting these processes. Unfortunately, however, subsequent 

 repetition of this experiment did not always give the same result. Sometimes 

 the plants improved the air, often they did not, and Priestley did not know the 

 reason for these variations. It remained for Ingen-Housz 2 to show that the 

 purifying of the air was effected only by the green parts of plants, and only in 

 sunlight. The importance of this process in the life of the plant was still un- 

 explained; it was regarded as a purposeful arrangement for the improvement of 

 the air for animals. Ingen-Housz had no clear idea as to what gas is taken in by 

 the plant, and even thought that the gas given off by metals under the action 

 of acids might be thus improved by plants. Senebier 3 was later able to show 

 that carbon dioxide alone is absorbed, and that this absorption is a nutritive 

 process. De Saussure 4 then found that the volume of oxygen given out was 

 equal to that of carbon dioxide taken in, that the decomposition of the last- 

 named gas was most rapid when one part of it was present in eleven parts of air, 

 and, finally, that an increase in the weight of the plant occurred as a result of 

 this absorption and decomposition. All these questions were finally taken up 

 by Boussingault, 5 in a series of precise experiments. The equality of the vol- 

 umes of the exchanged gases was established. By an experiment upon the de- 

 composition of carbon dioxide by green plants in a mixture of this gas and hy- 

 drogen or nitrogen, Boussingault was able to show that the decomposition in 

 question began immediately after the illumination of the apparatus, and ceased 

 as soon as it was darkened. Phosphorus was used to show the presence of 



1 Priestley, Joseph, Experiments and observations on different kinds of airs. 324 p. London, 1774. 



2 Ingen-Housz, Jan, Experiments upon vegetables, discovering their great power of purifying common 

 air in the sunshine, and of injuring in the shade and at night. London, 1779. [Ref. in Ger. ed. is ap- 

 parently to Scherer's translation, 3 v., Vienna, 1786, 178S, 1790. This was from author's French ed., 

 1780.] 



3 Senebier, J., Memoires physico-chimiques sur l'influence de la lumiere solaire pour modifier les etres 

 des trois regnes de la nature et sur-tout ceux du regne vegetal. Geneve, 1782. Idem, Physiologie v6g6tale. 

 Geneve, 1800. 



< Saussure, Nicolas Theodore de, Recherches chimiques sur la vegetation. Paris, 1804. 



5 Boussingault, JeanB. J. D., Agronomie, chimie agricole et physiologie. 2nd ed. Paris, 1860-1891. 



