228 Plant Physiology 



They must contain carbon, hydrogen, oxygen, and nitro- 

 gen; many contain sulfur, certain forms phosphorus, 

 and others apparently mineral bases, the latter either 

 combined or as ash. Proteins may be regarded as per- 

 fect foods for protoplasm. 



Lack of knowledge respecting the proteins renders 

 impossible a clear picture of their synthesis, although 

 much may be inferred from the decomposition products, 

 which have been extensively studied. Speaking generally, 

 the amides (containing the group NH 2 ) seem to represent 

 products intermediate between certain of the raw mate- 

 rials (organic .acids or carbohydrates and nitrates) and 

 the proteins. Glycin, for a simple example, is an amido- 

 acetic acid [CH 2 (NH 2 ) ' COOH] in which the amide group 

 replaces one H of the acid. It may be assumed that pro- 

 tein is constructed from amido compounds, especially 

 from those derived from carbohydrates, some of which 

 may be further modified by the incorporation of sulfur 

 from sulfates, and others by the introduction of phos- 

 phorus from phosphates. 



121. Soil nitrogen. - - Some data regarding the nitro- 

 gen relations of higher plants have been exhibited in con- 

 nection with the discussion of mineral nutrients. On 

 account of the unusual importance of the nitrogen supply 

 in any permanent system of agriculture, special attention 

 should be given to the interesting transformations of ni- 

 trogenous materials in the soil, and likewise the building 

 up of nitrogenous bodies within the plant require some 

 consideration. 



It is now a matter of common knowledge that nitro- 

 gen exists in the ordinary arable soils in a variety of com- 



