2 FEEDS AND FEEDING 



every portion of the structure, being especially abundant in the leaves 

 and growing 'parts. A surprising amount of water is needed by plants 

 during growth. For every pound of dry matter which they manufacture, 

 from '200 to 500 Ibs. of water are drawn from the soil in humid climates, 

 and as high as 1,800 Ibs. in arid districts. 



Next to water, the great food material of plants is carbon dioxid, or car- 

 bonic acid gas (composed of carbon and oxygen). This is obtained from 

 the air, ten thousand parts of which contain 3 to 4 parts by volume of 

 carbon dioxid. About 28 tons of this gas rest over each acre of the 

 earth's surface. The air, carrying this carbon dioxid, is taken into the 

 leaves thru the innumerable minute openings on their under surfaces, 



r Corbon ct/oxid 



(Carbon ont/oxyptn) 



fl?/nera/ /naffer 

 Su/phur Ca/c/um 

 Phosphorus ftJag/jew'um 

 Potass/urn fron 



WHERE PLANTS SECURE THEIR Fo6p 



Plants obtain carbon dioxid from the air, and water, mineral matter, and 

 nitrates from the soil. Legumes are able to use indirectly the nitrogen of the 

 air. Plants give off water and free oxygen gas to the air thru their leaves. 



which lead inward among the cells of which the leaves are composed. 

 The carbon dioxid is then absorbed by the cells and is used in building 

 plant compounds, as described later. In producing a 12-ton crop of 

 green corn over 4 tons of carbon dioxid are required, to obtain which the 

 plants must take in about 10,000 tons of air. Yet the supply of carbon 

 dioxid is never exhausted, for it is being continuously returned to the 

 air thru the breathing out of carbon dioxid by animals and the decay of 

 plant and animal matter. 



Nitrogen abounds in the living, growing parts of plants, Altho about 

 three-fourths of the air is nitrogen gas, plants in general cannot use the 

 free nitrogen of the air, but obtain their supply from nitrogen-containing 



