THE COMPOSITION OF PLANTS. 



27 



where a change in the character of the material takes 

 place, generally from six to ten inches and beneath 

 this is the sub-soil. Deep cultivation increases the 

 depth of the soil, and allows air and moisture to 

 enter, and the roots of plants to penetrate farther in 

 search of food. 



CHAPTER III. 



THE COMPOSITION OF PLANTS. 



50. THE following ten elements are always found 

 in plants, and are believed to be absolutely essential 

 to their growth : 



Non-metallic. Metallic. 



1. Carbon. 7. Potassium. 



2. Hydrogen. 8. Calcium. 



3. Oxygen. 9. Magnesium. 



4. Nitrogen. 10. Iron. 



5. Sulphur. 



6. Phosphorus. 



Four others, sodium, manganese, silicon, and 

 chlorine, are generally found, and in marine, or sea 

 plants, iodine and bromine. Traces of fluorine have 

 also been found, and sometimes minute quantities of 

 Jithium, caesium, and rubidium, and a few other ele- 

 ments. 



51. A great many analyses and experiments have 

 been made by chemists to find out just what ele- 



