GENERAL 



447 



in which rubidium can replace it, a corresponding change must occur in the 

 plasma whenever such substitution is enforced on the plant. It is also 

 probable that in respiratory and fermentative processes a certain amount 

 of substitution is possible, for in nitre-bacteria nitrous or nitric acid forms 

 the final excrete product of physiological combustion instead of carbon 

 dioxide as in ordinary plants, and hence the processes of respiration can 

 hardly be identical in the two cases. 



In those fungi which can be nourished with a variety of different 

 substances the final processes of metabolism may be precisely similar, 

 and only the preparatory processes be different (Sect. 66), as is indicated 

 by the fact that in the same fungus similar plastic substances are produced, 

 whether it is fed with sugar, or with tartaric or quinolic acid. According to 

 the food-material, however, various substances may be produced, probably 

 as by-products of the preparatory processes, as is certainly the case when 

 quinine is produced from arbutin, or benzoic acid from hippuric acid, for 

 both of these substances undergo no further change. Similarly fungi when 

 fed solely with peptone may produce large quantities of ammonia. 



The whole course of metabolism can be completed in a single cell or in 

 a unicellular organism, and mutual interchanges together with the prevention 

 of direct contact between particular products play an important part in 

 modifying its character. Hardly anything definite is known as regards even 

 the most essential vital activities, and thus it has yet to be determined 

 whether the proteid decomposition by which energy i's liberated occurs mainly 

 between comparatively stable structural elements (micellae or pangens), 

 or whether these also undergo perpetual decomposition and reconstruction 

 so long as life remains. A living protoplast may be compared to an 

 extremely complex chemical manufactory in which the character and 

 mechanical possibilities of the different departments, as well as the nature 

 of the materials supplied, the requirements of the factory, and the demands 

 made upon it, determine what products shall be produced and in what 

 relative amount. The different operations may be more or less complicated, 

 and may involve a series of processes taking place at different times in 

 different departments, so that from the same raw material a variety of 

 products may be formed, while the same products may be constructed from 

 various raw materials, either by synthetic or analytic means. 



A factory can continue to exist only by satisfying all its varying 

 requirements and the demands made upon it from without, so that in 

 addition to the stream of primary and accessory products which pass 

 away outwardly, the manufacture of new substances must be undertaken 

 when necessary, and that of others temporarily or permanently discontinued. 

 The amount of expansion possible is always limited, and the different 

 construction and powers of different factories cause each to have its own 

 specific productive power. So long as the internal mechanism remained 



