NUTRITIVE METABOLISM 289 



equation, for the result which the energy liberated by a chemical change 

 may produce is entirely dependent upon the inherent nature of the vital 

 mechanism, as well as upon the character, mode of action, and point of 

 application of the impelling force. Similarly, a food substance may be 

 used in various ways, for in different parts of the plant, and even in the 

 same cell, a supply of sugar may be converted into a variety of different 

 carbohydrates, or may be consumed in respiration, or may serve to form 

 the cell-wall; or to produce proteid substance, or may be excreted in the form 

 of nectar in order to attract insects. Again, a change in the internal or 

 external conditions may cause a food substance which has been kept in 

 reserve for a long time to be drawn into the metabolism of the plant, and 

 either to be consumed or used for constructive or other purposes. 



It is possible, in a general sense, to distinguish three classes of metabolic 

 products from one another. These are (i) building material, formative or 

 constructive substances ; (2) plastic or trophic substances ; and (3) aplastic, 

 non-trophic substances. Plastic substances are those which either at once, 

 or after being stored for a time as reserve food, are drawn into metabolism 

 and serve as nutrient material. Aplastic substances are such as take no 

 further part in metabolism, so that they include the substances which 

 form the permanent structural framework of the plant, although we may 

 regard them from a special point of view as building or constructive 

 material l . 



Many substances take no further part in the exclusively vital processes 

 of metabolism, although they have special and definite functions to perform. 

 Thus enzymes may render other substances soluble or prepare them for 

 use, while tannins, ethereal oils, alkaloid poisons, &c., are of greater or 

 less importance as protective or attractive agencies. Substances of this 

 kind, which subserve certain specific ends, are produced only at certain 

 times or only in limited amount, and thus can be readily distinguished 

 from those metabolic products which form the inevitable accompaniment 

 to all vital activity, and which are continually being produced so long as 

 any metabolism is possible (Sect. 77). The carbonic acid of respiration 

 and the alcohol produced by fermentation are metabolic products of this 

 character, and in order that the conditions necessary for continued existence 

 may be maintained, such products must be excreted and removed. Every 

 product which the plant excretes is not necessarily useless, for the enzymes, 

 the ethereal oils, the nectar, &c., are of service to the plant for the most 

 part only when excreted. The same is the case when a plastic substance is 

 excreted from one cell in order that it may be absorbed by another, and 



1 [It is perhaps simpler and less confusing to speak solely of (i) plastic substances which can 

 be used in metabolism, and (2) aplastic substances which, under normal circumstances, can undergo no 

 further metabolic change. Hence an excreted plastic substance is still classified as a plastic product.] 



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