138 METABOLISM 



for drawing upon a large extent of soil for the absorption of a substance 

 occurring but sparingly in it. 



The continued growth of plants on natural soil indicates at least that they 

 are always able to obtain the necessary nitrogen from it. It is otherwise 

 with cultivated plants. Just as in the case of the materials of the ash, agri- 

 culture prospers according to the amount of nitrogen removed. If 50 kg. of 

 nitrogen be extracted from a hectare at each harvest, and in large part 

 removed permanently from the field, and at the same time not quite 3 kg, of 

 nitrogen is added annually, the soil must become rapidly impoverished, and 

 that condition can be remedied only by manuring. Part of the nitrogen 

 removed may be replaced by the excrement of cattle, and this explains the 

 favourable results obtained by manuring with excrement, customary even in 

 the most primitive forms of agriculture. Excrement is insufficient of itself to 

 replace the loss in nitrogen suffered by the land, for part of the nitrogen is 

 sold off the land directly with the harvest or indirectly with the cattle ; the 

 remainder, which is contained in dung, is entirely transformed into ammonia, 

 and as such becomes further diminished by evaporation or is washed out after 

 undergoing nitrification. The formation of free nitrogen in dunghills may be 

 reckoned as a further source of nitrogenous loss. Thus, in all logical schemes of 

 agriculture, artificial manuring with nitrogen is essential. Since nitrate of 

 potash is too expensive, by far the most valuable manure is Chili-saltpetre 

 (nitrate of soda), which occurs in immense beds in Peru, traceable in its origin 

 to vital activity. This substance came into use in England seventy-five years 

 ago and is still employed in very large quantities. In addition to Chili-saltpetre 

 may be mentioned sulphate of ammonia, a by-product in the manufacture of 

 coal gas and almost as valuable for the purpose as nitrate of soda. [Probably 

 calcium cyanamide (CaCN^) is also of great service as a nitrogenous manure.] 

 Finally, those plants which bring about the fixation of the free nitrogen of the 

 air are of the greatest importance in agriculture. These have been several 

 times referred to but we shall speak of them in greater detail later. 



Having now become acquainted with the compounds of nitrogen which 

 may be made use of by the plant, and having noted that these substances are 

 absorbed especially by the root, we may turn to the question as to where and 

 how they are assimilated. As we remarked, however, at the beginning of the 

 lecture, our knowledge of the assimilation of nitrogen is very defective. The 

 final products of the assimilation of nitrogen at all events are proteids. These 

 bodies are rightly considered as forming a series of chemical compounds of 

 special importance, and hence deserve a few words at this stage in our work. 

 Unfortunately, from one point of view, the advances in the chemistry of proteids, 

 which have taken place during recent years, owing to the efforts of physio- 

 logical chemists, have dealt rather with animal than vegetable proteids (compare 

 the comprehensive expositions of Hammarsten, 1895, Cohnheim, 1900, Kossel, 

 1901, Hofmeister, 1902). Thus we do not even yet know whether the very 

 important animal proteids occur in the plant kingdom also, while we know little 

 or nothing as to the peculiarities of vegetable proteids. Consequently the 

 following notes, which we extract from Cohnheim' s works, must of necessity 

 be very fragmentary. [Compare Czapek, Biochemie, vol. II.] 



Proteids cannot be so easily represented by formulae as carbohydrates or 

 fats. Five elements for the most part enter into their composition, hydrogen, 

 nitrogen, oxygen, carbon, and sulphur, to which we may add also phosphorus. 

 The relative amount of these elements in the different proteids varies greatly 

 and little is to be deduced from the statements made as to the percentage com- 

 position of each. Generally speaking, however, proteids possess certain physical 

 characters, give certain chemical reactions, and especially give rise to similar 

 decomposition products, so that we may conclude that they form a natural series 

 of compounds and not merely a heterogenous collection of organic bodies which 

 cannot be catalogued under other and more fully studied groups. 



