154 LIGHT, VEGETATION AND CHLOROPHYLL 



By the use of anaesthetics, Claude Bernard suspended the 

 assimilating capacity of plants which still continued to breathe, 

 both in the Ught and in the dark. In this way an idea 

 could be formed of the effects of photosynthesis, for 

 experiments comparing plants kept in the dark with con- 

 trols showed that respiration was not modified by the 

 aneasthetic. 



Garreau ingeniously dissociated the two phenomena on a 

 normal branch. Two identical branches were placed in two 

 sealed tubes exposed to the Ught. In one of the tubes he 

 placed baryta, which became turbid, because, Garreau con- 

 cluded, "the plant breathes and produces carbon dioxide, 

 which is fixed immediately by the baryta." In the other, 

 without baryta, the exposure to the fight was the same, but 

 the baryta was not there to compete for the carbon dioxide, 

 which was used entirely and so effectively for assimilation 

 that at the end of the experiment the confined air in this 

 second tube did not make the baryta water turbid. The two 

 phenomena were thus demonstrated, but, for the sake of 

 accuracy, it must be added that turbidity would not have been 

 produced in the first tube if green leaves only had been 

 placed in it, for, in full sunHght, the chloroplasts use the 

 carbon dioxide from respiration even before it has been given 

 off from the leaf; in this experiment, carbon dioxide from the 

 non-green parts passed into the atmosphere where the baryta 

 and the leaves shared it. 



Another method of approaching the problem is to consider 

 the quantities of gas absorbed or given off. Bonnier and 

 Mangin showed that the value of the exchanges due to chloro- 

 phyll could thus be determined independently from the 

 respiratory exchanges which take place at the same time. 

 There is a respiratory quotient, which is the ratio of the 

 carbon dioxide given off to the oxygen taken in, COg/Og, and 

 a photosynthetic quotient, which is the ratio of the oxygen 

 given off to the carbon dioxide absorbed, Og/COg. It may be 

 said, as a first approximation, that carbon dioxide will be 

 more abundant in the final atmosphere if respiration is 



