132 THE ART OF CULTURE. 



the substances which do not contain it, these sub- 

 stances will be separated as excrements from the 

 bark, roots, leaves, and branches. The exudations 

 of mannite, gum, and sugar, in strong and healthy 

 plants cannot be ascribed to any other cause *. 



Analogous phenomena are presented by the 

 process of digestion in the human organism. In 

 order that the loss which every part of the body 

 sustains by the processes of respiration and perspira- 

 tion may be restored to it, the 'organs of digestion 

 require to be supplied with food, consisting of sub- 

 stance containing nitrogen, and of others destitute 

 of it, in definite proportions. If the substances 

 which do not contain nitrogen preponderate, either 

 they will be expended in the formation of fat, or 

 they will pass unchanged through the organism. 

 This is particularly observed in those people who live 

 almost exclusively upon potatoes ; their excrements 

 contain a large quantity of unchanged granules of 

 starch, of which no trace can be detected when 

 gluten, or flesh, is taken in proper proportions, 

 because, in this case, the starch has been rendered 

 capable of assimilation. Potatoes which, when 

 mixed with hay alone, are scarcely capable of sup- 

 porting the strength of a horse, form with bread 

 and oats a strong and wholesome fodder. 



* M. Trapp in Giessen possesses a Clerodendron fragram, which 

 grows in the house, and exudes on the surface of its leaves in September 

 large colourless drops of sugar-candy, which form regular crystals upon 

 drying; I am not aware whether the juice of this plant contains sugar. 



