THE NATURE OF PHOTOSYNTHESIS 93 



previous table are given a number of the possible reduction products 

 together with the photosynthetic quotient which corresponds to their 

 formation calculated from the carbon dioxide absorbed and oxygen split off. 

 When the photosynthetic quotient is exactly 1 it is evident that the 

 carbon dioxide has been reduced to carbon, CO2 ^ C + O2, or in the 

 hydrated form carbonic acid has been reduced to a carbohydrate. Since 

 the best evidence indicates that the photosynthetic quotient is 1, this would 

 signify that carbonic acid is reduced to the formaldehyde stage. How- 

 ever, the evidence of the value of the photosynthetic quotient does not 

 throw any light on the question as to the exact compound which is formed. 

 It may be formaldehyde, glycolaldehyde, or any other carbohydrate of 

 the general formula CnH^-uOn, so that the results of the gaseous exchange 

 during photosynthesis can contribute little toward settling this much de- 

 bated question. The evidence of the photosynthetic quotient does con- 

 tribute to determining whether the first products formed in photosynthesis 

 are compounds other than those possessing an empirical formula CnHonOn. 

 The old theory of Liebig ^^ that organic acids are the precursors of carbo- 

 hydrates in photosynthesis still finds adherents who have modified and 

 elaborated the original theory. These views will be discussed in another 

 chapter. Suffice it to point out here, that it is now clearly established 

 that the hydroxy-acids which play so important a role in the Liebig theory 

 are products of a modified respiratory activity and that a photosynthetic 

 quotient of 1 excludes them from being the first direct products of carbon 

 dioxide reduction. Similarly the theory that fats are the first products 

 of photosynthesis finds no support in the values of the photosynthetic 



CO2 absorbed 



quotient, for these substances would demand a quotient, -— -, ;; — 



^ O2 emitted 



in the neighborhood of 0.7. 



One other point regarding the photosynthetic quotient deserves con- 

 sideration. It has been known for a long time that the fleshy plants or 

 succulents exhibit respiratory and photosynthetic quotients which differ 

 greatly from the values obtained with thin leaf species. These plants 

 ordinarily show a very low photosynthetic ratio. 



The structural arrangements of the succulents are such that the plant 

 loses relatively little water through transpiration. The ratio of the sur- 

 face to the volume is low and there are relatively few stomata. This 

 results in inhibiting the gaseous exchange. A striking feature of the 

 metabolism of these plants is the accumulation of organic acids at night or 

 in the dark and the decomposition of these acids during periods of illumi- 

 nation. This phenomenon has been investigated very thoroughly recently 

 by Richards '^* who also discusses the older literature. The accumulation 



■"Liebig, Die Chcviic in Hirer Anzvendtmg auf Agrikultur iind Physiologic, I, 52 

 (1862). 



*• Richards, Carnegie Inst. Washington, Pub. No. 209 (1915). Hempel, Compt. 

 rend, du Laboratoire de Carlsberg, 13, 1-129 (1917). 



