24 PLANT PHYSIOLOGY 



that it is necessary to revise somewhat our ideas of the importance 

 of carbon absorption by roots. 



Boussingault (1860-1890) studied all these problems in detail 

 quantitatively, and to him we owe most of our precise information 

 on these points. He carefully measured the carbon dioxide taken 

 in and the oxygen given off and established their equality in vol- 

 ume. One very ingenious and interesting experiment was devised 

 to show that the decomposition of the carbon dioxide into car- 

 bon and oxygen began immediately after the plant was illuminated. 

 Carbon dioxide was mixed with hydrogen and nitrogen and ex- 

 posed with the plant and a piece of phosphorus in an experimental 

 chamber. As soon as light was allowed to fall on the chamber, 

 white fumes of phosphorus pentoxide (P2O5) would appear in- 

 dicating the presence of oxygen. As soon as the apparatus was 

 darkened, no more fumes appeared and those already present 

 would dissolve and disappear in a vessel of water contained in 

 the chamber. 



In this experiment the amount of carbon dioxide was much 

 higher than is normally present in the air and the question arose 

 as to whether the plant could actually remove the small amount 

 of carbon dioxide found in air under ordinary conditions (about 

 0.03%). To settle this point Boussingault placed a plant in a 

 jar through which a current of air was passed, and the air was an- 

 alyzed before and after entering the jar. He found that the plant 

 was able to take out the small quantity of carbon dioxide present, 

 much to the astonishment of the workers in the laboratory, as nar- 

 rated by Timiriazef : 



To what extent the precision of this experiment aroused the admira- 

 tion of his contemporaries (as did most of Boussingault's researches) 

 can best be shown by a story which Boussingault himself told me. "The 

 experiment was carried on with Dumas, each worker making his own 

 weights and records independently in order to secure more reliable results. 

 At first all went nicely and the plants decomposed the carbon dioxide as 

 they were expected to do. Then matters suddenly changed. On a bright, 

 sunny day the plants began to make carbon dioxide instead of decomposing 

 it. That evening we examined the results with much surprise and stared 

 at each other in blank amazement. We remembered the misfortune which 

 had attended Priestley when he tried to repeat his famous experiment. 

 Then one fine morning several days later, Regnault, the famous physician, 

 who had been watching our experiment with much interest, began to 

 laugh at our long faces and admitted that he had been the cause of our 

 trouble. Every day while we were out at lunch he had sneaked into our 



