142 METABOLISM 



than elsewhere, and, finally, it is possible that when nitrates are present in a 

 chlorophylliferous cell a part of the carbon assimilated may be employed 

 directly in the manufacture of proteid without first going through the carbo- 

 hydrate stage. Still SCHIMPER has not proved that a synthesis of proteid is 

 impossible in the dark in those parts of the plant which have no chlorophyll ; 

 further, it has been often stated recently that nitrogen assimilation may take 

 place in darkness. Thus ZALESKI (1900) has observed a vigorous synthesis of 

 proteid in leaves of Helianthus which had been cultivated in KNOP'S nutritive 

 solution, when a large quantity (4 per cent.) of levulose was added at the 

 same time; when no sugar was added a reduction in quantity of proteid formed 

 was observed. SUZUKI (1898) conducted similar observations on barley, which 

 he found to be able to construct proteid out of nitrates in darkness in the 

 presence of glucose or cane sugar. Certainly researches are not wanting tending 

 to contradict these results, and experimental treatment of the problem on an 

 even wider basis is still desirable ; but we may point to the analogy offered 

 by many Fungi which assuredly form proteids in darkness out of nitrates, 

 a fact which certainly does not militate against the existence of this capacity 

 in Phanerogams. [GODLEWSKI (1903) has observed the formation of some 

 organic nitrogenous compounds to take place in darkness from nitric acid ; 

 increase in proteid-nitrogen takes place only in light, a statement which is 

 confirmed by LAURENT (1904).] 



The absorption of ammonium salts in the undecomposed (nitrified) condition 

 has been clearly proved to take place. Since it nowhere accumulates in the plant 

 in appreciable quantity it follows that it must be rapidly used up. Moreover, 

 its rapid transformation is essential on account of its poisonous properties. 

 Ammonia is used up in the construction of proteid as wefl as in the formation 

 of simpler nitrogenous bodies, of which we shall have to speak later on. The 

 same question arises with regard to the synthesis of proteid from ammonia that 

 we have left unsettled when speaking of nitric acid, viz. the influence of light. 

 LAURENT (1896) held that light was essential [more recently (1904), LAURENT 

 has expressed the opinion that assimilation of ammonia is also possible in the 

 dark] ; HANSTEEN (1899), on the contrary, observed that construction of proteid 

 from ammonia took place also in the dark if the appropriate carbohydrates 

 were present. He found that glucose was of great service in this respect but 

 that cane sugar was useless ; unfortunately he based his conclusions on micro- 

 scopic investigations only. Here also comprehensive studies are urgently 

 needed. 



The problem as to the influence of light on the assimilation of nitric acid 

 or ammonia is of the greatest importance for another and related reason. We 

 have seen how light supplies the energy required in carbon assimilation to form 

 out of carbon-dioxide chemical compounds containing greater supplies of energy. 

 There can be no doubt that an expenditure of energy is also needed to bring 

 about the synthesis of proteid from carbohydrates and nitric acid or ammonia, 

 since, as AD. MAYER (1901, i, 174) has shown, reduction processes are certainly 

 accompaniments of this synthesis. If it could be proved that the synthesis of 

 proteid takes place only in the presence of sunlight, then we might assume 

 that solar energy is the source of energy we are in search of. Since, however, 

 this is not the case, we must look around for another form of energy, and we 

 know of only one other form which we need consider, viz. chemical energy, set 

 free whenever carbohydrates are oxidized. We will return to this subject in 

 speaking of the phenomena of respiration, at present we need consider only one 

 aspect of the process : Synthesis of carbohydrates in the green plant is un- 

 doubtedly a case of photosynthesis ; the sun provides the necessary energy 

 for carrying this out ; the synthesis of proteid, on the other hand, is to be 

 regarded, at least in certain instances, as a case of chemosynthesis. It has 

 already been shown that proteid synthesis is impossible in the absence of 



