CONDITIONS ESSENTIAL TO NUTRITION. 139 



Analogous phenomena are presented by the pro- 

 cess of digestion in the human organism. In order 

 that the loss which every part of the body sustains 

 by the processes of respiration and perspiration may 

 be restored to it, the organs of digestion require to 

 be supplied with food, consisting of substances con- 

 taining 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 par- 

 ticularly 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 assim- 

 ilation. Potatoes, which when mixed with hay alone 

 are scarcely capable of supporting the strength of a 

 horse, form with bread and oats a strong and whole- 

 some fodder. 



It will be evident from the preceding considera- 

 tions, that the products generated by a plant may 

 vary exceedingly, according to the substances given 

 it as food. A superabundance of carbon in the state 

 of carbonic acid conveyed through the roots of 

 plants, without being accompanied by nitrogen, can- 

 not be converted either into gluten, albumen, wood, 

 or any other component part of an organ ; but either 

 it will be separated in the form of excrements, such 

 as sugar, starch, oil, wax, resin, mannite,* or gum, 

 or these substances will be deposited in greater or 

 less quantity in the wide cells and vessels. 



contains sugar. Professor Redtenbacher, of Prague, informs me that 

 he has analyzed the crystals, and found them to be perfectly pure 

 sugar. — Ed. 



* Mannite forms the greater part of manna. It is found in the 

 juices of several fruits, in the fermented juice of beet-root, carrots, 

 onions, &c. ; it is also obtained in small quantity when starch is 

 transformed into grape sugar by boiling with dilute sulphuric acid. 

 It crystallizes in prisms, is faintly sweet, soluble in water and hot 

 alcohol. Its aqueous solution cannot be made to undergo the vinous 

 fermentation. Its formula is Ce H? Oe. 



