THE CONSTITUENTS OF PLANTS. 9 



of study, be divided into two classes: first, those which 

 are lost or driven away by burning, or are capable of being 

 burned ; and second, those which are contained in the ash 

 or residue after burning. The first are called "combus- 

 tible" or burnable constituents; the others are called the 

 " incombustible " or ash constituents. 



The part that is removed by burning contains the con- 

 stituents that have been derived largely from the air, 

 while the remainder contains the mineral substances which 

 have been derived largely from the soil. The burning 

 of wood illustrates the separation of these two classes 

 of constituents and the proportions of each. 



Each one of the constituents contained in a plant is 

 a distinct chemical element; and the number and kind 

 which have been found to be absolutely essential to the 

 growth of plants are carbon, oxygen, nitrogen, hydrogen, 

 potassium, magnesium, calcium, iron, phosphorus, ana 

 sulphur. These are called plant-food elements, and all 

 healthy plants contain them; if, by any chance, any 

 one of them could not be obtained, the plant could not 

 make normal growth, and in this sense no one of them 

 is of more importance than another. 



Besides these, however, the chemical elements, silicon, 

 chlorine, sodium, manganese, and sometimes others, do 

 exist in the plant, though plants can be grown without 

 them. Thus, at most, but fourteen only of the seventy 

 known chemical elements are necessary for the growth 

 of a plant, and form its food. The large number of 

 different species and varieties of plants now existing is, 

 however, capable of being produced from them, the 

 different properties and forms being entirely due to a 



