STUDIES IN PLANT RESPIRATION AND 

 PHOTOSYNTHESIS. 



Bt H. a. Spoehr and J. M. McGee. 



I. THE CARBOHYDRATE-AMINO-ACID RELATION IN 

 THE RESPIRATION OF LEAVES. 

 INTRODUCTORY DISCUSSION. 



Already at the beginning of the nineteenth century it had been 

 realized that the materials which support the respiratory activity 

 in plants are carbohydrates and fats. The observations of Lavoisier 

 and of De Saussure that the respiration coefficient, CO2/O2, in 

 mature higher plants is usually very close to unity were even at that 

 time accepted as evidence of the preponderance of carbohydrates 

 as the fuel material. Since then a continually increasing number 

 of chemical compounds has been discovered in plants. When 

 derived from the higher, autotrophic plants, these multifarious 

 products must be considered as coming primarily from the carbohy- 

 drates through the various intricate channels of plant metabolism. 

 So while the substances which are used as sources of energy by the 

 lower plants and bacteria are of great variety, the higher, mature 

 chlorophyllous plants are in general consumers of carbohydrates. 



It would, however, be drawing a very incomplete picture of plant 

 respiration if this were confined to the carbohydrates and fats and 

 the more elusive and complex nitrogenous components were to be 

 omitted. But unfortunately our knowledge of the function of 

 nitrogen compounds in the metaboUsm, particularly of the higher 

 plants, is most fragmentary. It is our belief that a rational con- 

 ception of the function of nitrogen in the economy of the higher 

 plants can be gained only through an analytic study of definite 

 groups of nitrogen compounds. This belief has its foundation first 

 of all in the fact that nitrogen is capable of forming such a variety 

 of compounds of basically different physiological properties, and 

 furthermore because our chemical knowledge of the nitrogen com- 

 pounds derived from protoplasmic proteins is fairly well organized 

 and related and so can serve as a most valuable experimental guide. 



Even a cursory study of the carbohydrate economy of the higher 

 plants must emphasize the old conclusion that physiology is essen- 

 tially a study of dynamics. Thus far organic chemistry has dealt 

 largely with the properties and reactions of carbon compounds. 

 The recent tendency to enter into various phases of energetics and the 

 fundamental causes of chemical reactivity will make chemistry an 

 even more valuable adjunct to physiology than heretofore. 



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