ASSIMILATION OF CARBON 3 



oxygen, a piece of this substance being exposed in the experiment chamber. 

 As soon as light was allowed to enter the apparatus the formation of a white 

 vapor indicated the presence of oxygen, and when the apparatus was darkened 

 the fumes already formed disappeared and no more appeared, showing that 

 the elimination of oxygen had ceased. [The fumes are suspended phosphorus 

 pentoxide (P2O5), which dissolves in water, forming phosphoric acid (H 3 P0 4 ), 

 and thus disappears soon after the apparatus is darkened.] 



Since this experiment was performed in a closed chamber with a high car- 

 bon dioxide content, it was questionable whether the results obtained might 

 justify the conclusion that plants can utilize the small amount of carbon di- 

 oxide in the air under natural conditions (0.028-0.04 percent.). To clear up 

 this point, Boussingault placed a plant in a jar through which a current of air 

 was passed. Analysis of the entering air and of that passing out showed that 

 the plant was able, under favorable conditions of light, to absorb almost all of 

 the carbon dioxide that entered the jar. Regarding this experiment of Boussin- 

 gault, Timiriazev says: 



To what degree the precision of this experiment aroused the wonder of his contemporaries 

 (as did most of Boussingault's researches) can best be shown by an anecdote which I heard 

 from Boussingault himself. "The investigation was undertaken jointly with Dumas, with 

 weighings and records independently made by each worker, in order to secure more reliable 

 results. At first all went well, and the plants decomposed carbon dioxide as they were ex- 

 pected to do. Then things suddenly changed. On a bright, sunny day, the plants began to 

 produce carbon dioxide instead of decomposing it. In the evening we examined the result with 

 astonishment and stared at each other in blank amazement. Involuntarily we remembered 

 the misfortune that had attended Priestley when he attempted to repeat his famous experiment. 

 Several days passed by. Then, one fine morning, Regnault, the famous physicist, who had 

 been watching our experiment with much interest, began to laugh at our long faces and 

 admitted that he had been to blame for our misfortune. Every day, while we were at lunch- 

 eon, he had sneaked over to our apparatus and breathed into it, 'in order,' as he explained, 

 'to be convinced that you were not taking а и for an x, and could really determine such small 

 amounts of carbon dioxide.' " l 



De Saussure and Boussingault showed that the ratio ~ ~ is generally equal 



U2 



to unity. However, it must be remembered that green plant parts also respire 

 while they are assimilating carbon dioxide; that is, they carry on the reverse 

 process, wherein carbon dioxide is eliminated and oxygen is combined. Al- 

 though the process of respiration is much weaker than that of photosynthesis 

 (or " carbon assimilation""), still each must be kept distinct and it must be 



•Timiriazev, K. A., From the field of plant physiology. Public lectures and addresses. [Russian.} 

 Moscow, 1888. P. 245. 



* The term photosynthesis has now come into very general use among English and French 

 physiologists, in place of the more cumbersome expressions previously employed, and there 

 seems to be little room for doubt that it will eventually become universal. The word is of 

 American origin. Barnes (Barnes, C. R., On the food of green plants. Bot. gaz. 18: 40,}- 

 411. 1893) suggested photosyntax, and the other and better form is due to McMillan, and its 

 general introduction to MacDougal. Ewart is partly right in the footnote he appended to his 

 translation of Pfeffer's Plant Physiology (1 : 302. Oxford, 1900), but his objections do not 

 appear valid as against the use of photosynthesis. Of course, this should include all possible 

 forms of chemical synthesis brought about through the action of light, but the formation of 



