THE ASSIMILATION OF NITROGEN IN AUTOTROPHIC PLANTS 143 



carbohydrate synthesis, and that in the long run every case of proteid formation 

 is dependent on sunhght, though indirectly. 



Since the nitrogenous decomposition products which appear as a result 

 of the breaking down of proteids, not only in the plant but apart from it, 

 as a result of boiling in acids, are always those mentioned at p. 139, it may 

 be assumed that the synthesis of these bodies, which we term amides, precedes 

 the synthesis of proteids [perhaps with the formation of polypeptides as 

 intermediate products]. In fact, these substances, more especially asparagin, 

 are known to be of widespread occurrence in plants, although it has not as 

 yet been shown whether they are, primarily, intermediate stages in the 

 formation of proteid from ammonia or nitric acid, or secondary products re- 

 sulting from the decomposition of already formed proteid. Although Frank 

 and Otto (1890) found that leaves generally contain more asparagin in the 

 evening after illumination than in the morning, we must not conclude on that 

 account that synthesis of asparagin has taken place ; it might just as easily 

 arise from an increase in the proteid contents of the leaf and a concomitantly 

 increased decomposition of proteid. It may be possible, however, to determine 

 whether the amides found are produced there by the breaking down of complex 

 molecules or by the synthesis out of simpler bodies. Perhaps an investigation 

 of leaves whose proteid is prevented from escaping from them by their re- 

 moval from the stem, may serve, in comparison with normal leaves, as a 

 starting-point. A more comprehensive view of the process would be obtained 

 if we could establish a vigorous assimilation of carbon and of nitrogen 

 in leaves while preventing a concomitant production of proteid. Sulphur is 

 present in proteids under all conditions, and although this element is required 

 only in small quantity it might be still possible perhaps to obtain a more vigor- 

 ous anabolism of amides in the leaf-blade by withdrawal of sulphates and 

 finally to induce a subsequent combination of these into proteids by adding 

 sulphur afterwards. Experiments in this direction are still much needed. 



Under present circumstances it is of interest to know that the power of 

 the plant to construct proteid out of the nitrogenous organic substances named 

 above, and others also, has been repeatedly proved. Older experiments {for 

 literature see Pfeffer, Phys. I, 397) have shown how substances like urea, 

 glycocol, asparagin, leucin, tyrosin, guanin, creatin, hippuric acid, uric acid, &c., 

 may be supplied to plants in water-cultures in place of ammonia and nitric 

 acid, whilst more recently Lutz (1899) has shown that, in addition to acetamide, 

 methylamylamin, ethylamylamin, &c., may be employed. The plant can 

 recoup itself so far as nitrogen is concerned from such substances, although they 

 are not all equally good for the purpose. If a marked increase in dry weight 

 takes place, proteid must have been synthesized from such nitrogenous bodies. 

 It is a well-estabhshed fact, however, that the transition to proteid is never 

 direct, but that it is usually preceded by decomposition processes. This is well 

 known to be the case with hippuric acid, which breaks up into benzoic acid 

 and glycocol, the latter only undergoing further transformation. Further, all 

 these bodies are easily changed into ammonia through the action of micro- 

 organisms. Although it has been often expressly stated that the formation of 

 ammonia could not be demonstrated in certain experiments, it by no means 

 follows that it did not occur. It may well be that the ammonia is at once 

 absorbed by the plant as soon as it appears. The systematic exclusion of 

 micro-organisms has not been considered worth while in the majority of the 

 researches, and in those which have been carried out with antiseptic pre- 

 cautions (Lutz, 1899), other sources of error are not absent (compare Schulze, 

 •1902). Notwithstanding, we cannot doubt that a transformation of amides 

 into proteids takes place in the plant. In Lectures XIII and XIV we shall 

 learn that the plant produces such substances in the course of metabolism and 

 how it reforms proteid from them. In this case, as in the older water-culture 



