448 CONSTRUCTIVE AND DESTRUCTIVE METABOLISM 



unknown, a person who controlled only the entry and exit could form no 

 idea as to the complicated processes by means of which the different 

 products were formed in the factory. This is precisely our position with 

 regard to the living plant, in which reside powers and properties of which 

 the most complicated machines constructed by the hand of man can give 

 but the faintest conception. 



No machine or factory has ever been constructed which can auto- 

 matically regulate its entire mechanism, maintain itself, and when necessary 

 be capable of regenerating particular parts. The latter might, however, 

 be possible in certain machines granted the necessary supply of energy and 

 of building material. It is impossible, however, to conceive of a machine 

 which could reproduce its kind by sexual or asexual means. In spite how- 

 ever of the special peculiarities of the living organism the comparison with 

 a physical mechanism may suffice to indicate the impossibility of predicting 

 from the nature of the visible products what are the invisible processes 

 which led to their formation. The special powers and properties of the 

 protoplast, along with its regulatory mechanism and its tendency to activity, 

 primarily determine whether a particular food-substance shall enter into 

 metabolism. If from any cause no tendency to the production of proteids 

 exists at a given moment, no formation of the latter is possible, even though 

 asparagin, sugar, and all the necessary building materials are present in the 

 most appropriate form and position within the protoplast, and in direct 

 contact with it and with one another. Moreover even in the most minute 

 cell the different substances it contains may not be in actual contact, and 

 hence may be incapable of reacting upon one another. 



The comparative study of a number of plants is of the utmost importance 

 in rendering possible a clear comprehension of the nature of metabolism, and 

 since the protoplasm always retains the same general composition, the final 

 products of constructive metabolism must in all cases be similar, or must be 

 closely related substances. The specific differences in the metabolism of 

 different plants are due to the fact that the same end is not always attained 

 by the same means, so that special substances may be formed as unavoidable 

 by-products, or as products which have definite functions to perform. The 

 most marked deviations from normal metabolism as illustrated by the higher 

 plants are exhibited among the lower organisms as the natural consequence 

 of their adaption to every variety of external conditions, and certain of the 

 lower plants have actually the power of liberating kinetic energy without 

 the aid of free oxygen. Intermediate forms, however, exist which connect 

 aerobic organisms with these anaerobic forms, and certain bacteria are 

 known in which, as in the higher plants, carbon dioxide is the sole 

 excrete product of respiration. All substances \\hich are continually 

 produced by katabolism must either accumulate or be excreted unless 

 they arc reassimilated as fast as they are formed. Accarding to the 



