40 SCIENTIFIC AGRICULTURE. 



state, as it is perfectly insoluble. In the gaseous 

 form, as carbon dioxide, it is absorbed in large quan- 

 tities by the leaves, and it also enters through the 

 roots. By the action of sunlight this gas is decom- 

 posed in the green leaf, the carbon retained, and the 

 oxygen restored to the air. This change cannot take 

 place without the influence of sunlight, and hence 

 plants grow much more rapidly in the daytime than at 

 night. It is believed, by those who have studied the 

 subject closely, that all the carbon contained in farm 

 crops is derived from the atmosphere. 



85. Hydrogen and oxygen enter in the form 

 of water through the roots, and carry with them the 

 various soluble matters of the soil that are needed 

 for growth. Hydrogen also enters in the form of 

 ammonia, which is a compound of nitrogen and hy- 

 drogen, and as hydrogen compounds formed by the 

 decomposition of organic matters in the soil. In the 

 same manner oxygen, in combination with carbon 

 and with hydrogen and other elements, is absorbed 

 in large quantities. 



86. Nitrogen is also taken in as food by both 

 leaves and roots, but always in combination. This 

 element forms nearly four-fifths of the atmosphere, 

 and yet, as free nitrogen, it does not contribute in 

 the least to vegetable growth. The whole of the 

 nitrogen is obtained from compounds of ammonia 

 and from nitrates. 



87. Plants contain very little nitrogen, generally 

 from one half to three per cent. This, however, is 



