THE ASSIMILATION OF NITROGEN IN AUTOTROPHIC PLANTS 145 



lation of the majority of these minerals is as yet out of the question ; we may, 

 therefore, content ourselves by briefly summarizing the more important data 

 available as to the assimilation of sulphur and of phosphorus. These elements 

 claim at least a word, since the former occurs in all proteids and the latter in 

 certain of them. 



The source of the sulphur in proteid is exclusively the sulphates absorbed 

 by the root. The sulphates must certainly be reduced in the process of proteid 

 synthesis, but where, and under what conditions this reduction takes place, we 

 are quite ignorant. The same difficulty which we met with in discussing the 

 assimilation of nitrogen meets us also in an even more pronounced form when 

 we undertake an investigation into the mode of assimilation of sulphur ; for half 

 of any proteid, roughly speaking, consists of carbon, 15-19 per cent, consists 

 of nitrogen, but only 0-4 to about 2 per cent, consists of sulphur. If we write 

 the formula of serumalbumin, as Hofmeister does, as QsoH^goNnsSgOi^o (com- 

 pare CoHNHEiM, 1900), and assume that similar proteids also occur in plants, 

 it is obvious that 75 atoms of carbon must be assimilated for every atom of 

 sulphur. The consumption of sulphates in proteid synthesis must thus 

 obviously be very hmited. Schimper (1890) considered that the assimilation 

 of sulphuric acid also took place in presence of chlorophyll and under the 

 influence of sunlight, but his assumption is by no means well founded, postulat- 

 ing as it does in general the same conditions as were pertinent to the assimila- 

 tion of nitric acid. 



Phosphorus also occurs in the molecules of certain proteid bodies ; 

 it is absorbed only in the form of phosphate, and it would appear 

 that the molecule of phosphoric acid becomes incorporated in the proteid 

 molecule without essential modification, at least without any reduction. 

 According to Postern ak (1900) the assimilation of phosphorus takes place 

 in the leaf by the direct union of phosphoric acid and formaldehyde. 

 The compound so produced, oxymethylphosphoric acid (HjPO^ — CHjO), 

 PosTERNAK claims he has found in the plant, but it is questionable whether 

 it is a first product of assimilation (compare Iwanoff, 1901 b). In addition 

 to the proteids which contain phosphorus, nucleo-albumins and nucleo-proteids, 

 phosphoric acid occurs in lecithins which contain no sulphur ; these latter bodies 

 are very prevalent in plants (Schulze, 1894), and, according to Stoklasa 

 (1893), may also arise in the chlorophylliferous leaf. Sulphur is, moreover, not 

 limited to proteid, it occurs also in other substances of limited distribution, 

 such as oil of mustard (C3H5NCS) in Cruciferae, allyl sulphide (C3H5S), in species 

 of Allium; nitrogen also is not confined to proteids and their anastates, but 

 appears to be used in the construction of the widely distributed alkaloids and 

 certain glucosides as well. As we know little or nothing as to the mode of 

 formation of these bodies it is useless for us to study them in further detail at 

 present. 



Summarizing what we have learned from the last few lectures we may say : 

 that the carbon-dioxide of the air is the only source of carbon available to 

 green plants ; that they convert it into carbohydrates under the influence of 

 sunlight and in the presence of chlorophyll, from which carbohydrates starch 

 is produced as a product specially worthy of note ; that nitric acid is the chief 

 source of nitrogen, and that that element in co-operation with the carbo- 

 hydrates goes to form proteid especially. So far as we know, most cells can 

 carry out synthesis of proteid without requiring sunlight as an essential con- 

 dition of the process. It would also appear probable that a very large part of 

 the proteid is formed in the leaf. In this capacity for assimilating carbon-dioxide 

 and nitric acid the green plant stands in striking contrast to the higher 

 animal, which is unable to construct either carbohydrate or proteid out of such 

 simple compounds. It would be quite a mistake, however, to emphasize this 



JOST L 



