REVIEW OP THE PRINCIPLES OP NUTRITION. 



159 



830. Oaebon (essentially charcoal) enters so largely into the composition of 

 plants that it retains generally the exact form and texture of the wood after the 

 other olemeuta have been expelled by heat. On this element chiefly depends the 

 soUdity and strength. Its proportion is from 40 to 60 per cent. Nitrogen, although 

 equally essential, is less abundant in the tissues, and exists largely only in certain 

 vegetable products, as gluten, albumen, casern, theine. 



831. Oxygen and hydrogen exist in plants combined with other elements, and 

 also combined with each other forming water, especially in all fresh green vege- 

 table matter. The water is expelled by drying, and the following table shows, in 

 a few eases, the proportion for each 100 lbs. 



Peas lose of water 8 lbs. 



Wheat 14 lbs 



Rye and oats 15 lbs. 



Wheat straw 2G lbs. 



Potatoes about 15 lbs. 



Apples and pears 83 lbs. 



Red beet 85 lbs. 



Strawberries and gooseberries. 90 Ita. 



Turnips 93 lbs. 



Watermelons 95 lbs. 



832. Earthy elements. Besides those four universal elements, 

 many other substances, earthy and mineral, are found in quantities 

 greater or less, in different species. Thus forest-trees and most inland 

 plants contain potassa; marine plants, soda, iodine ; the grasses, silex, 

 phosphate of lime ; rhubarb and sorrel, oxalate of lime ; leguminous 

 plants, carbonate of lime ; the Cruciferse, sulphur, etc. 



833. The proportion OP earthy matter is small and may be estimated from 

 the ashes. As drying expels the water, so burning expels all other organic ele- 

 ments, and the inorganic earthy, whatever they be, remain in the form of ash. The 

 following table from Bousingault is instruotivo on this point. 



834. Agricultural chemistry. Since all these elements are found in plants, 

 we infer them to be essential ingredients in the food which they require for liealthy 

 vegetation ; and an inquiry into the sources from which they may be supplied con- 

 stitutes tho_chief object of Agricultural Chemistry. 



835. The food op plants is air, earth, and water. It is evident 

 that plants do not create a particle of matter, and therefore do not 

 originate in themselves any of the elements which compose them. 

 Consequently they must obtain them from sources without. Carbon is 

 derived from the carbonic acid contained in the atmosphere, and from 

 the decaying vegetable matter of the soil. Oxygen is derived from the 



