GENERAL 



449 



cultural conditions many fungi and bacteria may either excrete carbon 

 dioxide only, or in addition a variety of other metabolic products. Continual 

 removal of the products renders possible a greater production than can occur 

 when they are retained, and this is why a given organism may ferment so 

 relatively large a quantity of fermentable material. 



Higher plants produce quite as great a variety of substances as lower 

 ones do, and scarcely a substance is formed by the latter which cannot also 

 be produced by one or other of the former, although marked differences may 

 exist between closely allied plants with regard to their metabolic products. 

 This is especially the case with bacteria. It must be remembered, however, 

 that the physiological importance of a substance is by no means wholly 

 dependent upon its chemical constitution. There can be no doubt that 

 a fundamental agreement exists between the metabolism of the higher and 

 of the lower plants, and the same holds good for plants and animals. Even as 

 regards the higher animals marked resemblances with plants are exhibited 1 , 

 and it may with certainty be expected that the little known lower animals 

 will be found to agree more closely with certain plants than with the higher 

 animals as regards their metabolic products. The supposed fundamental 

 difference between vegetable and animal metabolism was due to the failure 

 to distinguish between the processes by which food is obtained, and those 

 by which plastic products are produced and energy is liberated. Moreover 

 the photosynthetic assimilation of carbon dioxide is possible only to certain 

 plants, as is also the power of synthesizing proteids, while plants which 

 have not this latter property must obtain fresh proteids from without in 

 order to replace the continual loss by destructive metabolism. 



Only the substances actually present in plants can be detected by analysis, 

 but these suffice to indicate what are the products formed by metabolism from 

 the assimilated food-material. This is especially obvious when the disappearance 

 of one substance is accompanied by the appearance of others derived from it. 

 Qualitative tests may suffice to render such changes evident, although sometimes 

 quantitative determinations are necessary, but hardly any idea can be obtained 

 by such means as to the progress or causes of such changes, or as to the regions 

 where they occur. Nor can all the substances present in the plant be detected 

 by analysis, for many of them may at once decompose when the plant is killed 

 and subjected to chemical analysis. 



Macrochemical analysis affords however a basis for further research, and 

 by means of microchemical methods the source and origin of the substances 

 discovered by macrochemistry may be more precisely determined. Many con- 

 clusions may indeed be arrived at as to the importance of the metamorphoses which 

 particular substances may undergo, from their distribution in cells or in tissues, 



1 Cf. E. Schulze, In wie \veit stimmen Pflanzenkorper u. Thierkorper in ihrer Zusammensetzung 

 iiberein ? 1894 (Sep.-abdr. a. Vierteljahrsschr. d. Naturf.-Ges. z. Ziirich, Bd. xxxix). 



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