X. THE CHEMICAL CONSTITUENTS OF PLANTS 213 



been detected in certain vegetables, e.g., in plane-tree leaves, in the 

 sprouts of many young plants and in cereals. It crystallises in 

 prisms, soluble in hot water and in alcohol, and has a neutral 

 reaction. It is a di-ureide of glyoxylic acid and has the constitution 



NH CH NH 



/ I \ 

 CO CO NH, 



NH CO 



The simpler ammo-compounds, the " amides " of food analyses, are 

 generally regarded as not being capable of forming flesh in an animal, 

 but only to function in fat formation or heat production. Bemem- 

 bering that proteids, by hydrolysis, are converted into amino-acids in 

 the process of digestion and that these are subsequently built up into 

 proteids in the animals, it would appear probable that the " amides" 

 of a food, if present in sufficient variety, might take some part in the 

 building up of nitrogenous tissue. Indeed, recent experiments tend 

 to show that this is the case under certain conditions, or at least that 

 " amides " can, to a certain extent, replace a portion of the albumin- 

 oids in a ration. 



They cannot, nevertheless, be regarded as possessing the same 

 value as true albuminoids and, in the analysis of products rich in 

 " amides," distinction has to be made between the nitrogen existing in 

 the two classes of compounds. The total nitrogen in the foodstuff 

 is determined by the Kjeldahl process (vide Chap. V, p. 85). Another 

 portion of the food is then digested with water containing, in suspen- 

 sion, recently precipitated copper hydroxide. This renders the true 

 albuminoids insoluble but permits the "amides" to dissolve. The 

 precipitate is filtered off, washed, and a nitrogen determination by 

 Kjeldahl's process made in the residue. This is assumed to be the 

 nitrogen existing as real albuminoids. 



(iii) The Alkaloids. These substances are nitrogenous bases, 

 possessed usually of powerful therapeutic properties. They are very 

 numerous and in constitution are generally to be regarded as derived 

 from ammonia, NH 3 , by the replacement of a part of or all the hydro- 

 gen by complex groups. With few exceptions, e.g., nicotine, coniine 

 and sparteine, they contain oxygen. They exist in the plant probably 

 as salts of organic acids. They are only slightly soluble in water, 

 more so in alcohol. They are found in various parts of plants and may 

 be extracted by digesting the finely divided material with dilute sulphuric 

 acid, nearly neutralising the solution with alkali, boiling down, and 

 adding alcohol, when most of the gums, mucilage, etc., are precipitated 

 and the alkaloids remain in solution. 



Some of the more important alkaloids are 



Coniine, C 8 H 17 N = propyl piperidine, C 5 H 10 (C a H 7 )N. 

 Nicotine, C 10 H 14 N., = /5-pyridyl-a-N-methylpyrrolidine, 

 C 5 H 4 N.C 4 H 7 N(CH 3 ) 



