NUTRITIVE METABOLISM 291 



These distinctions are based upon physiological characteristics and 

 purposes, which latter are frequently attained in widely different ways ; 

 thus, assimilatory products may be produced by dissociation as well 

 as by synthesis ; for example, carbohydrates may be formed either by 

 synthesis from water and carbonic acid, or by the self-decomposition of 

 the protoplasm, or by the complete metamorphosis of proteids. Indeed, 

 plastic substances or special constructive materials are very often produced 

 by the decomposition of more complex molecules (Chap. VIII). 



It is of the highest importance to obtain a thorough knowledge of the 

 nature and causes of the chemical changes and processes occurring in 

 the living organism, and we may use the terms synthesis (anabolism) and 

 analysis (katabolism) in the same sense as the chemist does, so long as 

 we are dealing with purely chemical processes, if we remember that dis- 

 similation and assimilation are terms which have a physiological meaning, 

 and hence do not quite correspond to synthesis and analysis, for an 

 assimilatory product may be produced either by analysis or by synthesis, 

 that is, by progressive or retrogressive chemical metamorphosis. 



Constructive and destructive chemical changes are necessarily of 

 constant occurrence, both in constructive metabolism and in the processes 

 by which a supply of energy is obtained. Every chemical decomposition 

 affords a supply of energy to the plant, and whenever any synthesis 

 converts a certain amount of kinetic into potential energy, the former must 

 be derived from processes of decomposition or fermentation, such as lead 

 to the production of carbonic acid, alcohol, &C. 1 Hence, during the 

 progress of development various synthetic products appear in greater or 

 less amount, together with those derived from the unceasing dissimilation. 

 In an adult plant very much less constructive assimilation is necessary, 

 and in cells which merely require a supply of energy, all such synthetic 

 processes may under special nutritive conditions be absent. 



Metabolism is possible only when a supply of appropriate food material 

 is assured, but all plants do not require the same food, and the difference is 

 especially marked with regard to carbon compounds. Thus many fungi 

 are able to grow when sugar is the sole organic substance supplied to them, 

 if the other elements they require are presented in appropriate form as 

 inorganic salts, while in a few cases the sugar may be replaced by formic 

 acid. Certain fungi, however, are unable to form proteids by synthesis when 



already been employed to indicate the assimilation of free nitrogen by certain bacteria, &c. The 

 term carbon dioxide assimilation is moreover the more comprehensive one, and includes the two 

 allied subordinate processes of carbon-assimilation and oxygen-excretion, which processes are 

 probably separated from one another by distinct intervals of time and place, however minute these 

 latter may be.] 



1 See Pfeffer, Studien z. Energetik, 1892. 



U 2 



