PLANT FOOD AND GROWTH. 23 



substances are not believed to be absolutely es- 

 sential, but are almost always found in plants and 

 probably serve important purposes. These substances 

 are: 



GASES. 



Oxygen. Nitrogeo. Hydrogen. 



SOLIDS. 



Carbon. Magnesium. 



Sulphur. Iron. 



Phosphorus. 



Potassium. Sodium. 



Calcium or Lime. Silicon. 

 Chlorine. 

 The first five named elements compose the com- 

 bustible parts of plants — that is, these elements pass 

 off into the air as gases when the plant is burned. 

 The five next named are essential ash or mineral ele- 

 ments — as is also sulphur, a part of the sulphur in 

 the plant remaining as ash when the' plant is burned. 

 The three elements last named are those not consid- 

 ered essential to plant growth. In the case of some 

 plants, one or more other chemical elements may be 

 found in very small quantity. 



Oxygen and -hydrogen, chemically united so as 

 to form water, make up the largest part of any living 

 plant. They also, in different combinations, make 

 up perhaps forty per cent of the dry matter. Nearly 

 or quite one-half the dry matter is carbon. Nitrogen 

 sometimes makes as much as four per cent of the dry 

 matter,. but generally not nearly so much. No other 

 element is in nearly so large supply, and the percent- 

 age of some is very small. There may be less than 

 one pound of iron in the hay grown on an acre of 



