THE ALKALOIDS 449 



Under such conditions asparagine and other amides are usually synthesized. 

 The amino groups set free, probably as ammonia, in the oxidation of amino 

 acids are probably used in the construction of asparagine or other amide 

 molecules. If carbohydrate deficiency becomes severe asparagine may also 

 be oxidized, resulting in the liberation of ammonia in the plant cells (Mothes, 

 1926). Under such conditions the concentration of ammonia sometimes 

 becomes great enough to exert a toxic or even lethal effect on the cells since 

 free ammonia is a cell poison. Ammonia toxicity during carbohydrate starva- 

 tion is apparently obviated except in extreme cases by the synthesis of aspara- 

 gine. In general accumulation of ammonia in a plant from any cause seems 

 to favor synthesis of asparagine (Prjanischnikow, 1924). 



Glutamine is believed to play a role in plant metabolism essentially similar 

 to that of asparagine. In some plants the quantity of glutamine exceeds that 

 of asparagine, although the converse is apparently more often true. Etiolated 

 castor bean seedlings, for example, may contain four times as much glutamine 

 as asparagine. There is also some evidence that asparagine and glutamine 

 residues occur in protein molecules. 



4. Urea. — Urea ((NH2)2"CO) occurs abundantly (as much as 11 per 

 cent of the dry weight) in some fungi and is also present in small quantities 

 in certain seed plants (Klein, et nl., 1930). Urea is one of the products 

 formed when the amino acid arginine is hydrolyzed by the enzyme arginase 

 (Chap. XXVII). Urea, like asparagine and glutamine, may also be one of the 

 products of the oxidation of amino acids in tissues with a low carbohydrate 

 content. Under the influence of the enzyme urease urea is hydrolyzed to 

 ammonia and carbon dioxide (Chap. XXVII). Some investigators believe 

 that urea may serve as the starting point in the synthesis of some of the 

 complex cjxlic nitrogen compounds which appear to be present in some pro- 

 teins but there is no actual evidence in favor of this view. The physiological 

 role of urea in the seed plants is probably very similar to that of asparagine 

 and glutamine. 



5. The Alkaloids. — Alkaloids are complex cyclic compounds containing 

 nitrogen that are produced only in certain species of plants. They are espe- 

 cially common in members of the Solanaceae, Papaveraceae, Leguminosae, 

 Ranunculaceae, Rubiaceae, and Apocynaccae. Species of plants which contain 

 one alkaloid are very likely to contain others. More than twenty different 

 alkaloids have been isolated from opium, which is the dried juice of the unripe 

 fruits of certain species of poppies. 



Some of the better known alkaloids are nicotine, from tobacco, quinine 

 from the bark of the cinchona tree, morphine from poppy fruits, strychnine 

 and brucine from the seeds of Strychnos nuxvoniica, and atropine from the 



