BOOTS AND THEIB FUNCTIONS. 45 



thrive best in heavy, compact clay or loamy soils j others 

 flourish in loose sands and peat-bogs ; while still other 

 kinds grow only in water, or wholly exposed to the air, 

 as seen in some of the epiphytes. 



The food of plants consisbs principally of a few simple 

 elements, viz., oxygen, hydrogen, carbon, and nitrogen. 

 These are indispensable nutrients, out of which all com- 

 bustible parts of the plant are formed by the chemical 

 and vital processes of nutrition. It is true that other 

 substances are usually found in plants, such as potassium, 

 calcium, magnesia, iron, phosphorus, silicon, sodium, and 

 various other elements, but just what position they hold 

 in vegetable economy has not been fully determined. 

 There are also elements which may be essential to some 

 kinds (like iodine in marine plants) that are of no value 

 to others. 



Oxygen is a very important element of plants, for every 

 nine pounds of water contains eight of oxygen, and it is 

 always present in organic compounds. Plants take up 

 oxygen, chiefly in its combination with hydrogen, in the 

 form of water, and we all know how important moisture 

 is to vegetation in general, as it is the vehicle which con- 

 veys to plants the great bulk of their food. Oxygen com- 

 bines with various other elements to form the solid rocks 

 of the globe, as well as the bodies of animals. 



Hydrogen is an invisible element of plants and the 

 lightest of all known substances. It is not found free in 

 nature, but combined with oxygen in water, and it is in 

 this state of combination that it is taken up and utilized 

 by plants. As water is composed of eight parts (by 

 weight) of oxygen and one of hydrogen, the latter may 

 be considered as always present where there is moisture, 

 and without this compound all plants soon perish. Hy- 

 drogen is always present in all organic compounds, but 

 ]t is not supposed to enter into the composition of the 



