THE METABOLISM OF PLANTS. 223 



upon the excretion of trimethylamin, N(CH 3 ) 3 , and the leaves 

 of the Stinking Goosefoot (Chenopodium Vulvaria) are con- 

 stantly giving off this gas. More commonly the compound 

 ammonias are ot volatile at ordinary temperatures, and they 

 are then termed alkaloids. The presence of these bodies 

 has been determined in a great number of plants, and it 

 is probable that they are more generally present in plants 

 than is usually supposed. They are not excreted by the 

 plant, but they are especially deposited in those parts which 

 become detached, such as the bark, fruits, and seeds, and it is 

 in this way that they are gradually got rid of. 



With regard to the origin of alkaloids in the plant, there 

 can be little doubt that they are derived more or less directly 

 from proteid. It is an almost necessary assumption that they 

 are built up from ammonia ; we have therefore to enquire into 

 the possibility of the formation of ammonia in the plant. It 

 has been already suggested (p. 1 50) that ammonia is formed in 

 connexion with the processes of destructive metabolism, and 

 it has also been pointed out (p. 210) that, under abnormal 

 conditions, ammonia may be even excreted. The mode of 

 the formation of ammonia in the plant is not difficult to ima- 

 gine. It is well known that the amides are readily decom- 

 posed into organic acids and ammonia. Thus, when asparagin 

 is boiled with dilute acids or alkalies, aspartic acid is formed 

 and ammonia is evolved according to the equation 



C 4 H 8 N 2 O 3 + H 2 O = C 4 H 7 NO 4 + N H 3 . 



It is therefore quite possible that free ammonia may be formed 

 in the plant, and that from this the alkaloids may be built up. 

 It is interesting to note, in connexion with this, that neither 

 urea nor uric acid have ever been found in plants. According 

 to a commonly accepted view it would appear that these 

 bodies may be formed in the animal body from leucin and 

 tyrosin ; these substances apparently undergo decomposition 

 into carbon dioxide and ammonia and the carbon dioxide 

 and ammonia combine in other proportions to form urea and 

 uric acid. There is, in fact, a certain amount of evidence to 

 shew that urea is formed synthetically in the animal body, 



