408 THE FOOD OF PLANTS 



it requires from either methane or benzene derivatives, and it is not there- 

 fore surprising that such a plant is able to utilize a great variety of nitrogen- 

 compounds, and thus attain the same result by different means. It is not 

 necessary that amides should always form an intermediate stage in proteid- 

 synthesis, for the process of construction might be a direct one, the 

 assimilated nitrogen-compounds uniting directly with certain molecules 

 of proteids. At the same time, under different conditions, amides might 

 be produced in abundance and used in the construction of proteid, for 

 a living organism is always able to regulate its vital activity according to 

 the conditions under which it exists. Nor does the existence of a benzene- 

 ring in the proteid molecule necessitate that a production of benzole 

 derivates should form one stage in the synthesis of proteid, for the aromatic 

 substances found in plants, such as tannic acid, phloroglucin, appear to 

 be aplastic products. 



The final stages of synthetic metabolism may follow the same paths, 

 although the starting-points differ, and when this is the case the preparatory 

 processes must exhibit various special peculiarities according to the body 

 with which the synthesis of proteid commences, whether it is with ammonia, 

 nitric acid 1 , or nitrites, or whether methane or benzole derivatives are 

 employed as a source of carbon. It is quite uncertain what may be the 

 nature of the primary product in the assimilation of free nitrogen. Nitro- 

 genous metabolism not only involves continual proteid-synthesis, but also 

 unceasing proteid-decomposition, and when the entire supply of kinetic 

 energy is derived from the disintegration of nitrogen-compounds, a large 

 amount of nitrogen may be lost in the form of excreted nitrogen-compounds, 

 or even of free nitrogen in a few cases. 



Even when the chemical composition of products such as carbohydrates, 

 asparagin, &c., is well known, it may be impossible to say how a particular 

 substance is produced by the plant, and this is even more markedly the case with 

 proteids, the chemical constitution of which is still only imperfectly known (Sect. 1 1). 

 According to Loew* the hypothetical aldehyde of aspartic acid is formed, and 

 from this the proteid molecule is derived by condensation, but these are mere 

 speculations. 



A supply of the essential ash constituents must of course always be available, 

 and of these calcium is not necessary for fungi, and hence has no general importance 

 in proteid-synthesis. During the assimilation of nitrates, oxalic acid is pro- 

 duced when necessary to combine with the liberated bases, but calcium oxalate 

 is not always formed during proteid-synthesis, and its appearance does not localize 



1 The ammonia need not necessarily be oxidized into nitric acid, or the latter reduced into 

 ammonia, and, as a matter of fact, higher plants are unable to directly induce such changes. 



3 Loew, Die chem. Kraftquelle im Protopl., 1882, pp. 5,-&c. ; also Ad. Mayer, Agr.-Chem., 

 1895, 4. Aufl., p. 163. 



