THE METABOLISM OF PLANTS. 22Q 



scales contain much grape-sugar, but neither calcium oxalate and 

 pyrocatechin. But if the internal scales be allowed to become dry, 

 they assume the brown colour of the external scales, the grape-sugar 

 disappears from the cells, and pyrocatechin and crystals of calcium 

 oxalate make their appearance. He has also ascertained that pyro- 

 catechin is commonly present in leaves and shoots, especially in the 

 autumn. 



But if it be admitted that the carbohydrates undergo 

 oxidative decomposition in the plant, it must be true of other 

 substances also. For instance, the alcohols which are doubt- 

 less formed in the plant, would be oxidised to acids. To take 

 glycerin as an example. This alcohol is, as we have seen, 

 set free on the decomposition of glycerides (fats), but free 

 glycerin has never been detected in any plant. It probably 

 undergoes decomposition as soon as it is formed. . It is well 

 known that glycerin is readily oxidised to carbon dioxide 

 and water, and that it is susceptible of more gradual oxida- 

 tion, a number of acids (oxalic, formic, glycolic, etc.) being 

 produced : this is probably its fate in the plant. Again, 

 the amides would yield acids on oxidation. Thus asparagin 

 is readily decomposed into aspartic acid and ammonia, and 

 aspartic acid, when oxidised, yields malic acid. 



Aspartic acid is converted into succinic acid on fermentation. It is 

 probable that the succinic acid which is found among the products of 

 the alcoholic fermentation by Yeast, and of the conversion of alcohol 

 into acetic acid by the Mycoderma aceti, is derived from asparagin 

 formed in the metabolism of the Fungus. 



Again, there can be no doubt that acids are formed by 

 the oxidative decomposition of proteid. It has been found 

 that oxalic acid and amidated acids are formed when proteid 

 is decomposed artificially, that amidated acids, which we have 

 reason to believe are derived from proteids, are present in 

 plants, and that, in the process of fat-formation, fatty acids 

 are produced which doubtless owe their origin to proteids. 



Finally, there can be little doubt that the self-decom- 

 position of protoplasm is attended by a formation of acids. 

 This has already been alluded to in discussing de Saussure's 

 observations on Opuntia (p. 207), and, if we admit that fat is 



