THE CHEMISTRY OF PHOTOSYNTHESIS 297 



b. Other Substances From Which Sugar Can Be Produced by the 

 Plant. 



Reference has ah'eady been made to the fact that carl)on monoxide can- 

 not replace carbon dioxide in photosynthesis and is toxic to plants in 

 higher concentrations. On the other hand, this gas has been found in 

 quite high concentrations in the floaters of certain sea weeds, so that its 

 toxicity is probably not as general as was at first concluded. In the 

 earlier forms of the formaldehyde theory, carbon monoxide was assumed 

 to be an intermediate product; for later modifications of the theory the 

 behavior of carbon monoxide is, however, of little significance. 



The results of studies of the nutritive value of carboxylic acids for 

 plants in the light are of little value, because of the fact that most of 

 these acids under the influence of light and mineral nutrients (notably 

 iron) and through the action of micro-organisms are broken down into 

 carbon dioxide and other organic compounds.^* Thus it is not certain 

 whether the plant is not forming carbohydrates from the carbon dioxide 

 present, through the normal photosynthetic process rather than through 

 the utilization of a carboxylic acid. 



Treboux ^^ has made an interesting study of the utilization of various 

 organic acids by algae. He used 40 difi^erent species of algae and the 

 gain in dry weight of the plants was determined after they had been 

 in solutions of the ammonium and potassium salts of diflterent organic 

 acids in the dark for one to two months. About one-half of the plants 

 used, developed with the organic acids as the only source of carbon. It 

 was found that the organic acids with a long chain of carbon atoms were 

 not as good nutrients as the relatively simple acetic acid. The optimal 

 concentration of the potassium salts was about 0.25 per cent. The acids 

 which could be used in this manner were acetic, lactic, citric and butyric 

 acids; some of these also form starch. The amino acids glycocoll, alanine, 

 leucin, asparagin and aspartic acid are apparently less favorable sources of 

 carbon, while formic, oxalic, propionic, valeric, succinic, malic and tartaric 

 acids are doubtful sources. Bokorny ^^ has prepared a list of the acids 

 that can be used by the plant, though some of those included are of ques- 

 tionable value. Different species of plants apparently vary greatly in 

 their ability to utilize different organic compounds and there exists still 

 a great lack of agreement on many of these questions. 



Other substances which have been reported as capable of being used 

 by plants for starch formation are methyl alcohol, glycol, phenol, glycerol 

 and urea.°' In considering intermediate products of photosynthesis these 

 substances are not of very great interest beyond demonstrating the fact 



"Stutzer, Ber. cliem. Gcs., 9, 1395 (1876); Landiv. Vcrsuchs., 21, 104 (1878). 

 Schmoger, Ber. chem. Gcs., 12, 753 (1879). Spoehr, Biochcm. Zcit.. 57, 95 (1913). 



"'Treboux, Ber. bot. Ges., 23, 432 (1905). 



''Bokorny, Biol. Centralbl., 17, 5 (1897). Loew and Bokorny, Jour. Prakt. 

 Chem., 144, 272 (1887). 



"Lippmann, "Chemie der Zuckerarten," II, 1763 (1904). 



