444 CONSTRUCTIVE AND DESTRUCTIVE METABOLISM 



deeper insight has been obtained into the complicated series of actions and 

 interactions which are probably at work even in the simplest metabolic 

 activities, will it be possible to follow a molecule of a carbon or other 

 compound in its varying fortunes from its entry into the plant until it is no 

 longer available for metabolism. This task is made all the more difficult by 

 the fact that a substance may be utilized in many different ways, and under 

 certain circumstances may follow a path different from that which it does in 

 other cases. Moreover, the same end may be attained in various ways, as 

 is shown by the fact that a fungus may construct its organic substance 

 from either methane or benzene derivatives (Sect. 66). Similarly amides 

 and carbohydrates may either be formed by synthesis or by the partial 

 or complete decomposition of proteids, and it is sufficiently evident that 

 the substances formed in the latter case may be entirely different to those 

 from which the proteids in question were originally constructed. 



Even the limited knowledge we already possess is sufficient to indicate 

 the very varied character of metabolism in general, although it is certain that 

 a host of metabolic products may yet be discovered. It is indeed extremely 

 probable that in the living protoplast many highly important but unstable 

 proteids or proteid compounds may exist which change into stable forms as 

 soon as death occurs. So little is known as to the actual composition of 

 living protoplasm that no explanation even of the most fundamental pro- 

 cesses which take place in it can be hoped for at present, and hence it is 

 impossible to trace any particular vital activity to its ultimate origin from 

 changes occurring in the living protoplast. The special dependent func- 

 tions which are developed for a particular aim or purpose are more open to 

 experimental study than are the more general functional activities which 

 form an essential attribute of all living substance, and it is probably by 

 means of the former that a knowledge of the working mechanism of the 

 living protoplast will ultimately be obtained. Even when we are fully 

 acquainted with the mode of action of an enzyme, or with the importance 

 of a metabolic product which serves for the maintenance of turgidity or for 

 the formation of the cell-wall, we are unable to say how the substance in 

 question is produced or why the constructive activity of the protoplast 

 should be manifested in this particular manner. Moreover in many partial 

 functions, such as that involved in the production of proteid, the precise 

 mode of synthesis is as yet unknown (Sect. 71). The same is also the 

 case with regard to the assimilation of carbon dioxide, although this takes 

 place in an organ specially adapted for the purpose (Sect. 61). 



Since our knowledge of the essential character of metabolic activity is 

 so incomplete, it is preferable to deal with the visible products and to 

 consider their importance to the plant, for these products afford sure and 

 certain tokens of metabolic activity, although their mode of origin is not 

 always necessarily the same. With regard to their ultimate fate, and their 



