34 OVER-ALL REACTION OF PHOTOSYNTHESIS CHAP. 3 



photosynthesis — some do not form any visible deposits of reserve ma- 

 terials at all, while others store oils or proteins. Since the latter products 

 are more strongly reduced than the carbohydrates, their formation by 

 photosynthesis would require more hydrogen and lead to the liberation 

 of more oxygen, than does the synthesis of carbohydrates. This would 

 mean an increased value of the photosynthetic quotient. It is therefore 

 important that Barker (1935) found a normal value of the photosynthetic 

 quotient also in oil-storing diatoms (c/. Table 3.1), even in those which 

 have an abnormally low value of the respiratory quotient {Nitzschia 

 palea). This difference illustrates the fact, already stated in the first 

 chapter, that photosynthesis is a universal process, taking the same 

 course in all plants, while the reverse process of oxidation can proceed 

 by many different paths. Whenever compounds other than polymerized 

 carbohydrates (starch, inulin, cellulose) are stored in an organism, the 

 symmetry of photosynthesis and respiration must needs be disturbed. 

 Thus, plants (or animals feeding on carbohydrates) which accumulate 

 fats, may have Qr values above unity while the fats are formed, and 

 below unity while they are consumed. Conversely, plants which store 

 low-molecular organic acids or salts, (e. g., oxalates, tartrates or citrates), 

 often have Qr values below unity during the deposition, and above unity 

 during the consumption of these reserve materials. This is true, for 

 example, of ripening fruits, and of succulents during the nightly accu- 

 mulation of acids (c/. page 264). 



Succulents (e. g., Cacti) often have also abnormally large photosynthetic 

 quotients (Aubert 1892); or, at least, they appear to be large if determined 

 by the usual method of subtracting the gas exchange in the dark from the 

 gas exchange in light. The quotient decreases, however, with prolonged 

 illumination, as shown by Willstatter and Stoll (1918) in experiments 

 with Opuntia. The change is associated with the gradual disappearance 

 of organic acids, which these plants accumulate in darkness. This 

 " deacidification " in light can be interpreted either as a photoxidation, 

 producing free carbon dioxide, or as a photor eduction, converting the 

 acids into carbohydrates. If the first hypothesis is correct, the high 

 photosynthetic quotient is deceptive: the complete oxidation of acids 

 of the type of malic acid produces more carbon dioxide than it consumes 

 oxygen, and thus reduces the net carbon dioxide consumption from the 

 atmosphere more than the net liberation of oxygen. If, however, the 

 second hypothesis is correct, the high photosynthetic quotients are real, 

 and the succulents carry out a "photosynthetic assimilation of organic 

 acids" instead of, or in addition to, the usual photosynthetic assimilation 

 of carbon dioxide. We will return to this problem in chapter 10. 



Abnormal photosynthetic quotients are sometimes shown also by 

 nonsucculent plants at the start of illumination after a period of darkness. 



