200 LECTURE X. 



be that the supply of, say, phosphorus, for example, is unusually small. 

 The plants would then only be capable of utilizing the nitrates in propor- 

 tion to the amount of phosphorus present. It is perhaps possible that 

 the conditions of nourishment are different in different zones. 



In any case, the free nitrogen in organic nature plays an exceedingly 

 important part in the nitrogen cycle. The amounts of nitrogen produced 

 artificially, whether by the combustion of organic substances or by the 

 explosion of gunpowder, although large, to be sure, have little effect upon 

 the equilibrium of the nitrogen cycle. Such amounts are in time recom- 

 bined and again take part in the natural cycle. 



Albumin contains, besides nitrogen, also carbon, hydrogen, and sulphur. 

 We have already pointed out the central position that the carbohydrates 

 in the plant organism play in the assimilation of carbon dioxide, and called 

 attention to the fact that it evidently forms the starting point of the syn- 

 thesis of albumin. In another place we shall go into this matter more in 

 detail. Here we shall merely suggest that certain relations are known 

 to exist between the simple carbohydrates and individual amino acids, so 

 that we can easily understand the formation of the latter from the former. 

 Thus carbon and hydrogen for the synthesis of albumin originate in the 

 air and water. In this form the animal organism gives back these elements 

 to the vegetable kingdom again. 



The plants obtain their sulphur from the soil, in which this element is 

 present as sulphates of the alkalies and alkaline earths. Plants utilize 

 sulphur almost exclusively for the formation of albumin, and it also reaches 

 the animal organism in this form. In the animal, this sulphur is largely 

 converted into sulphuric acid, and given back to the general cycle in the 

 form of its alkali salts. 



It is difficult to estimate the value of albuminous substances to the vege- 

 table kingdom, from the experiments at hand. Our knowledge of the 

 metabolism of albumin in plants is remarkably slight. We are certain 

 that it by no means plays the same part in the plant that it does in the 

 animal organism. We should like above all to know whether the plants 

 consume albumin at all, i.e., oxidize it. Oxidation processes, as we have 

 seen in considering the assimilation of carbonic acid, play a subordinate 

 role in plants to processes of reduction. They undoubtedly take place to 

 some extent. We do know that the albumin in the animal organism is 

 almost entirely decomposed, partly into urea, or partly into uric acid. Such 

 substances have been looked for in vain in the vegetable kingdom. 1 It is 

 interesting, however, to learn that many plants produce substances closely 

 allied to the uric acid group, namely, methylated-xanthine derivatives, 

 such as theo-bromine and caffeine, both of which are important con- 



1 A discovery of urea has been reported among the varieties of Ly coper dacece. Cf. 

 Max Bamberger and Anton Landsiedl: Monatsh. 24, 218 (1903). 



