262 INTERMEDIATES IN REDUCTION OF CO2 CHAP. 10 



pounds which are unsuitable as plant foods in the dark are usually those 

 with L values less than unity, as: formic acid, L = 0.5; oxaHc acid, 

 L = 0.25; malic acid, L = 0.75; succinic acid, L = 0.875; and tartaric 

 acid, L = 0.625. 



Some exceptions from this rule have been reported, citrate being the 

 most notable of them; and we shall consider their significance on page 



266. 



At present, we want only to stress that plants contain enzymatic 

 systems which enable them to convert into carbohydrates in the dark 

 most, if not all, simple organic compounds whose reduction level is so 

 high that their conversion does not require a supply of energy. Thus, 

 even if formaldehyde were definitely proved to be an acceptable food for 

 plants, it would only join the large number of compounds of similar 

 degree of reduction which possess the same property; nobody will claim 

 that all these compounds should be considered as intermediates of photo- 

 synthesis (and few will agree with Lwoff that acetic acid should be picked 

 out as the only such intermediate). 



D. The Problem of Plant Acids* 



The assumption that organic acids play the role of intermediates in 

 photosynthesis, as suggested by Liebig one hundred years ago, is sup- 

 ported indirectly by three kinds of observations. In the first place, some 

 of these acids are present in all green plants (although they are found 

 also in colorless plant organs). In the second place, some plants, at 

 least, can convert these acids in light into carbohydrates. In the third 

 place, they are known to play the part of intermediates in respiration, 

 which in its net result, is a reversal of photosynthesis. 



1. Occurrence of Plant Acids in Leaves 



Table lO.I lists, besides alcohols and aldehydes, a number of organic 

 acids of low molecular weight (e. g., glyoxylic, gly colic, tartronic) as 

 occasionally present in green plants. Table 10.11 shows formic acid 

 and acetic acid among the volatile components of leaves. However, 

 when one speaks of "plant acids" one commonly means not these 

 comparatively rare constituents but the three or four acids which are 

 widely distributed in plants, partly in the free form, and partly as salts. 

 Omitting for the present ascorbic acid, whose role will be discussed in 

 Section E, the three common plant acids are oxalic, malic, and citric. 



Their distribution in the plant world is anything but uniform. Not 

 only are there large variations from species to species and from tissue to 

 tissue, but even from place to place in one and the same tissue. Strong 



* Bibliography, page 277. 



