PLANTS AND ANIMALS. 51 



that they may be readily taken up and used by plants 

 and animals alike, except the- carbon and the nitrogen. 

 The iron, potassium, sodium, sulphur, phosphorus, and 

 the like may be had in solution in the water that perco- 

 lates through the soil, from which it takes up sufficient 

 quantities for either plants or animals. Oxygen can be 

 had from the air and can be used thus as free oxygen ; 

 in addition, it can be had in various common com- 

 pounds, as water itself. Hydrogen also is found in con- 

 nection with oxygen in water (H 2 O) . 



Free nitrogen is abundant in the air, of which it consti- 

 tutes about 80 per cent. ; but most organisms, both plant 

 and animal, are wholly unable to use it in this form. 

 There are some bacteria that seem to have the power of 

 taking up free nitrogen and of building it up into living 

 substance. Nitrogen occurs also in nature, inorganically, 

 in compounds known as nitrites and .nitrates. Plants 

 have the power of using nitrogen from this source. Ani- 

 mals, on the contrary, lack this power of using inor- 

 ganic nitrogen and must get their nitrogen in the form 

 of proteid substances, which are formed by organisms and 

 consist of carbon, hydrogen, oxygen, and nitrogen in com- 

 bination. We are familiar \vith this fact in our own 

 dieting, when we say that we must have eggs or lean meat 

 or beans, in order to get the necessary nitrogen compounds. 



Carbon occurs in the pure state, as in graphite or 

 charcoal. It occurs in the atmosphere in union with 

 oxygen as carbon dioxide (CO 2 ) ; it is found also in various 

 organic compounds, as starch, sugar, fats, and the like, in 

 combination with hydrogen and oxygen ; and in the pro- 

 teids, the most complex of all foods. None of the organ- 

 isms can use the carbon in its pure form. Only green 

 plants can use it in the form of CO 2 . All organisms can 



