THE NATURE OF PHOTOSYNTHESIS 89 



c + ci COo 



o + oi O, 



=: P. 



That is, the photosynthetic quotient is determined from the difference 

 in the carbon dioxide and oxygen-content of the air surrounding the plant 

 in the two determinations. In employing this method there are a number 

 of corrections which must be applied. Great care must be exercised to 

 avoid fluctuations in temperature in the two experiments. It is a question 

 whether the rate of respiration is the same in the light and in the dark. 

 Under the experimental conditions of Bonnier and Mangin *'■'' there were 

 a number of disturbing influences which unquestionably affected their 



results. 



They also attempted to separate the two processes of respiration and 

 photosynthesis by comparing the rate of respiration of plant parts that 

 contained chlorophyll with other portions that contained no pigment. It 

 is apparent now, however, that such procedure is not permissible. 



2_Inhibition of Photosynthesis by means of narcotics. Bonnier and 

 Mangin, based upon the old observations of Bernard ** with chloroform, 

 found that photosynthetic activity is much more sensitive to the action 

 of ether than respiration. It was attempted to determine the photo- 

 synthetic quotient by determining the gas interchange in parallel experi- 

 ments with and without ether. The method is not exact, because it is very 

 difficult to establish the exact dosage of the narcotic which completely 

 inhibits photosynthesis and does not affect respiration (see the effect of 

 narcotics on photosynthesis discussed later). It is very doubtful whether 

 the rate of respiration in an anesthetized plant is the same in the light 

 and in the dark.*^ 



3_Suppression of photosynthesis by removal of carbon dioxide. The 

 method depends upon the comparison of the gas interchange of leaves in 

 two similar vessels. One of these contains a solution of barium hydroxide 

 in order' to absorb the carbon dioxide of the atmosphere and that liberated 

 in respiration, the other contains an equal volume of water. From the 

 difference between the oxygen-content and carlion dioxide-content of the 

 two vessels at the end of the experiment the quantities of oxygen evolved 

 and carbon dioxide absorbed during photosynthesis can be calculated. 

 This method is open to serious error on account of the fact that the rate 

 of carbon dioxide emission (respiration) in the two vessels is not the 

 same. Spoehr and McGee *^ have shown that the rate of carbon dioxide 

 emission of leaves is greatly influenced by the partial pressure of carbon 

 dioxide in the surrounding atmosphere. They showed that when the 

 carbon dioxide-content of the air surrounding a leaf is changed from a 



«Bonftier and Mangin, Compt. rend., 96, 1075 (1883), 99. 160 (1.884). Anyi. 

 Set. mt. Bot., (6) 18, 293 (1884). . 



■"Bernard, Legons sur les Phcnomcnes de la Vie, 1. 278. Pans (1878). 

 « Usher and Priestley, Proc. Roy. Soc, B 77, 369 (1906), 78, 318 (1906). 

 ^Spoehr and McGee, Amer. J. Bot., 11, 493 (1924). 



