312 PHOTOSYNTHESIS 



NHo — CHo — COOH + MHCO3 ^ NH — CH^ — COOM + HoO 



I 

 COOH 



The resulting products are carbamino acids and the reaction is usually 

 arried out in alkaline solution so that the secondary salts are formed. 

 Willstatter and Stoll suggested that the absorption of carbon dioxide by 

 Jie leaf material was also based upon this type of reaction. 



On the basis of preliminary experiments Spoehr and McGee came to 

 conclusions which supported those of Willstatter and Stoll, that the leaf 

 absorbs carbon dioxide from the atmosphere by a mechanism similar 

 to that by which the blood serves in freeing the tissues of this gas. 

 Willstatter and Stoll found that 100 grams of dry leaf material absorb 

 from an atmosphere containing about 3.8 per cent carbon dioxide 160 mg. 

 of this gas. In the experiments of Spoehr and McGee ^*® the absorption 

 of carbon dioxide was determined directly; that is, the leaf material was 

 first freed from carbon dioxide by passing an air-stream, free of carbon 

 dioxide, over the leaf material for 24 hours and then the amount of the 

 gas which the leaf material took up was determined directly by exposing 

 it to air containing a known quantity of carbon dioxide and analyzing the 

 air after several hours. Thus, they obtained an absorption by 100 grams 

 leaf material with 1.3 per cent carbon dioxide, at 25°, of 495 mg. or 

 272 cc. of carbon dioxide. 



On further examination of the phenomenon of the absorption of car- 

 bon dioxide by dried leaf material by Spoehr and Newton it was found 

 that the theory which ascribes the absorption to amino acids or proteins 

 is untenable. The leaves were dried at about 45° in a stream of dry 

 air. This material was powdered and extracted with water saturated 

 with ether. The water extract from sunflower and nettle leaves, on the 

 addition of alcohol, formed a precipitate which showed decided absorp- 

 tion of carbon dioxide, about 25 mg. per gram of the sunflower material. 

 A number of other species of leaves was tested for absorption and the 

 alcoholic precipitates from these absorbed either only slightly or not at 

 all. The absorption of the dried leaf material itself could be estimated 

 only in the case of the sunflower and nettle ; in the case of the other 

 plants studied the evolution of carbon dioxide due to post mortal respira- 

 tion overbalanced any absorption. 



That the amino acids and proteins in the alcoholic precipitates men- 

 tioned were not responsible for the absorption of carbon dioxide was 

 indicated by the following facts : the amount of total nitrogen and of 

 amino nitrogen in the precipitates bore no relation to the amount of 

 carbon dioxide absorbed ; the amount of nitrogen was also too small to 

 account for the amount of carbon dioxide absorbed on the basis of the 

 carbamino reaction. The alcoholic precipitates were dializable ; the mate- 

 rial which passed through the memlDrane, on reprecipitation with alcohol, 



'*" Spoehr and McGee, Science, 59, 513 (1924). 



