RELATIONS OF CARBONIC ACID TO ASSIMILATION. 319 



841. The amount of carbonic acid most favorable to assimilation. 



The results of the most exhaustive study of the amount of car- 

 bonic acid most favorable to assimilation have been given by 

 their recorder as follows : 



(1) Increase in the amount of carbonic acid in the air, up to 

 a certain limit (the optimum), favors the evolution of oxygen 

 by plants ; beyond this it is more or less injurious. 



(2) The optimum of carbonic acid is different for different 

 plants : for Glyceria spectabilis on clear days it is between 8 

 and 10 per cent ; for Typha latifolia, between 5 and 7 per cent ; 

 for Oleander, somewhat less. 



(3) Any given increase in the amount of carbonic acid below 

 the optimum favors the evolution of oxj'gen far more than a 

 similar increase above the optimum hinders it. 



(4) The stronger the intensity of the light the more the evolu- 

 tion of oxygen is favored by increase in the amount of carbonic 

 acid up to the optimum ; and when this limit is passed the 

 evolution is checked so much the less. 



(5) From (4) it follows that the influence of the intensity of 

 the light on the evolution of oxygen is greater in proportion to 

 the amount of carbonic acid in the air. 



842. Ratio of the oxygen evolved by plants to the carbonic acid 

 decomposed. The volume of oxygen evolved by plants during 

 assimilation proper is very nearly that of the carbonic acid 

 decomposed. 1 



Numerous experiments by Boussingault exhibit this relation 

 in a very striking manner. In forty-one experiments the volume 

 of carbonic acid was to that of the oxygen set free as 100 : 98.7. 



1 Saussure (Recherches chimiques sur la vegetation, 1804, pp. 40, 59) is 

 regarded as the first to indicate this. He arrived at this conclusion by experi- 

 menting upon a number of plants under different conditions. His first recorded 

 experiment consisted in surrounding seven plants of Vinca (Periwinkle) with 

 an atmosphere containing a known quantity of carbonic acid gas. The plants 

 were exposed to sunlight from five to eleven o'clock in the morning for six 

 days, after which the air in the bell-jar was examined. 



Air in the jar Air in the jar 



before the experiment. after the experiment. 



Nitrogen 4199 cubic cent. . . 4338 cubic cent. 



Oxygen . . . . 1116 " 1408 



Carbonic acid . . . 431 " " 



Total volume . . . 5746 " . . 5746 " 



Saussure's conclusion is that plants, in decomposing carbonic acid, assimi- 

 late a part of the oxygen gas therein contained, and, further, that the amount 

 of carbon retained by the plant bears a definite relation to the amount of C0 2 

 taken up by it, 



