THE FOOD OF PLANTS 2$ 



stances. Thus, the air is a mixture of gases, among 

 which are oxygen, nitrogen, and carbon dioxid. The 

 first two of these are simple substances, but the last is 

 itself a combination of oxygen and carbon. Water is a 

 combination of two simple gases, oxygen and hydrogen. 

 The bodies of living things are exceedingly complex com- 

 binations and mixtures. 



Elements. Simple substances, that is, substances which 

 cannot possibly be divided so as to give parts of different 

 kinds, are called elements. Iron, gold, silver, and copper 

 are simple substances. About seventy elements make 

 up the world, and everything on it, as well as the sun, 

 moon, and stars. 



Elements Essential to Plants. Of the seventy elements, 

 there are ten, without all of which no green plant can live. 

 These are carbon, nitrogen, hydrogen, oxygen, sulphur, 

 phosphorus, iron, calcium, magnesium, and potassium. 

 Fungi can live without two of the ten needed by green 

 plants calcium and iron. Besides the ten elements men- 

 tioned, animals must also have chlorine and sodium. 



The Organic Elements. Carbon, nitrogen, hydrogen, and 

 oxygen make up a very large part of all living things, 

 and are therefore often called the organic elements. Be- 

 side these four, sulphur and phosphorus are part of the 

 living substance itself, of all living things. The uses of 

 the individual elements are not well known. 



Calcium, Iron, and Potassium. Without calcium, plants 

 cannot move food from the leaves to the stem and roots; 

 so these parts may starve and die, though the leaves may 



