2i8 FORMATION OF PROTEINS 



(ii) The complex nitrogenous organic compounds of the humus 

 in the soil. 



(iii) The ammonium salts, and 



(iv) Nitrates also present in the soil. 



Among the higher plants only the Leguminosae appear to be 

 able to utilise the free nitrogen of the air (see p. 792), and it has 

 been proved by means of sand- and water-cultures that although 

 green plants are able to make immediate use of ammonium salts 

 and a great variety of organic nitrogenous compounds, such as 

 urea and leucine, they nevertheless thrive best when supplied 

 with nitrogen in the form of nitrates ; this is true even of 

 leguminous plants, which can, under certain conditions, obtain 

 nitrogen from the atmosphere. 



As ammonium salts and the nitrogenous organic compounds 

 of dung, urine and humus when placed in the soil are ultimately 

 changed into nitrates (see p. 785), it is inferred that crops 

 ordinarily obtain the chief portion of the nitrogen which they 

 need from the nitrates of calcium, magnesium, potassium and 

 sodium present in the soil. 



The chemical changes which nitrates undergo after their absorp- 

 tion by plants and in what tissues or organs these changes take 

 place are still practically unknown. 



Plants differ very much in regard to the method of taking up 

 and utilising nitrates ; in some species nitrates can be detected 

 in all parts of the plants, while in others they can only be found 

 in the stem or roots, and in some none are found, in which latter 

 case the decomposition of these compounds appears to take place 

 at the very threshold of entry into the plant, namely, in the root- 

 hairs and delicate fibrils of the root. 



It may safely be concluded that between the simple nitrates 

 absorbed from the soil, and the proteins produced in the plant, 

 there are many intermediate products manufactured. What 

 these products are is not known with certainty, but there is 

 no doubt that asparagine (amido-succinamic acid) and probably 



