PLANT PHYSIOLOGY ii 



and thereby render them fit for root absorption. 

 This corroding action of the roots is due to carbon 

 dioxide, organic acids, and even dilute hydrochloric 

 acid. 



Function of Leaves. — By the agency of the 

 stomata (small openings) the carbon of atmospheric 

 carbon dioxide is absorbed and utilised by the 

 living plant in the preparation of organic substances 

 which become elaborated into various tissues. 

 Assimilation is the name given to the decomposi- 

 tion of the carbon dioxide — the retention of the 

 carbon and the bulk of the oxygen being given 

 back again to the air. 



The air contains about four volumes of carbon 

 dioxide in 10,000, and overlying every square inch 

 of the earth's surface are 15 lbs. weight of air. 

 This comes to 2,160 lbs. per square foot, or rather 

 over 42,000 tons per acre. In 2,160 lbs. of air 

 there are about 21 ounces of carbon dioxide, equal 

 to 5f ounces of carbon, and this amount is equal 

 to 7 tons per acre. This is far in excess of actual 

 total crop requirements, in fact there is constantly 

 floating over the earth a quantity of carbon in the 

 form of carbon dioxide more than five times in 

 excess of any crop's ultimate total requirements. 



This function of assimilation goes on due to the 

 influence of (1) chlorophyll or the green colouring 



