THE NATURE OF PHOTOSYNTHESIS 87 



is high during rapid photosynthesis. Conversely when the rate of photo- 

 synthesis is low, oxygen formation is low, the escaping gas stream is 

 slow, and the nitrogen and carbon dioxide which have been swept out 

 are replaced in the intercellular spaces from the surrounding water. 

 Under these conditions the gas which escapes from the plant contains 

 a higher percentage of nitrogen and carbon dioxide than when the photo- 

 synthetic rate is high. 



Kniep's results also show that when the rate of the escaping gas 

 stream is high the absolute amount of nitrogen evolved in unit time is 

 higher than during a low rate of gas emission. This is in spite of the 

 fact that the percentage of nitrogen is lower during a high rate than 

 during a low one. This would indicate that during high rate of gas 

 emission the partial pressure of nitrogen in the intercellular spaces is 

 sufficiently reduced to cause a relatively rapid escape of this gas from 

 the surrounding water into the intercellular spaces. 



In all probability oxygen is the only gas which is emitted by the 

 plant during photosynthesis. Pollacci ^^ reported the presence of small 

 quantities of hydrogen and some hydrocarbon in the emitted gas, but 

 this single observation has never been substantiated. 



Carbon monoxide ^^ has been found in plants as a product of respira- 

 tory activity but has not been found in the gas emitted during photo- 

 synthesis. 



Most of the analyses of gases emitted during photosynthesis are based 

 upon relatively small samples of gas ; it might prove interesting if large 

 quantities of the gas were analyzed for traces of other gases. 



40 



d. The Photosynthetic Quotient. 



In the study of the respiration of both animals and plants the deter- 

 mination of the ratio of the volume of oxygen absorbed to that of carbon 

 dioxide expired has been a most valuable means of determining the 

 nature of the oxidation processes. Similarly in photosynthesis the ratio 

 of the volume of carbon dioxide absorbed to that of oxygen emitted 

 has been extensively studied in attempting to establish the nature of 

 the reduction process. The much quoted early work of de Saussure, 

 Boussingault and some others, while it established that this ratio was 

 close to unity, is nevertheless, in the light of modern methods, not suffi- 

 ciently accurate for our present needs. There are a number of factors 

 in the determination of the ratio which exert a profound influence on 

 the values obtained, but which are extremely difficult to regulate. Prob- 

 ably the most significant of these is respiration. The oxidative processes 

 comprising respiration, in which carbon dioxide is formed, it is generally 

 assumed, are going on simultaneously with photosynthesis during the 



^Pollacci, Atti deir R. hist. Bot. Pavia. 7, 97 (1902). 

 "-Langdon, G. C, Science, 49, 573 (1919). 

 ■^Boussingault, Agronomic, etc., 3, 271 (1864). 



