PLANT FOOD AND PLANT GROWTH 29 



is fourteen times, and the other sixteen times, heavier than hy- 

 drogen. 



At the ordinary temperature, hydrogen and oxygen gases can 

 be mixed together and remain a mixture; but, if heated or ignited, 

 the bonds which hold the atoms in molecules (H 2 and O 2 ) are 

 broken, and the mixture explodes with terrific force and loud re- 

 port. The only product of the explosion, or at least of the reac- 

 tion, is water, H 2 O; and, if either gas was present in the mixture 

 in excess of these proportions, the excess remains unchanged. 

 The corn kernel contains 6.4 per cent of hydrogen, or about 97^ 

 per cent of the three elements, oxygen, carbon, and hydrogen. 



Life. The fixation of carbon is the most important process in the 

 growth of plants. By the term fixation is meant the changing of a 

 gas or soluble substance to a solid or insoluble form by means that 

 involve chemical reaction. (The fixation of atmospheric nitrogen, 

 the fixation of soluble phosphorus in soils, and the fixation of po- 

 tassium and other bases will be explained in the following pages.) 



The process known as the fixation of carbon is the more impor- 

 tant, because it involves, not only the fixation of carbon itself, 

 but likewise the fixation of both oxygen and hydrogen. Its im- 

 portance is better appreciated by recalling that these three ele- 

 ments compose about 95 per cent of the entire weight of most 

 agricultural plants or crops. Both the carbon and oxygen utilized 

 in plant growth are derived from the carbon dioxid contained in 

 the air. It is truly remarkable that 90 per cent (90 pounds in 100) 

 of our common crops must be secured from .04 per cent (4 parts in 

 10,000) of the air. 



The fixation of carbon, oxygen, and hydrogen takes place in the 

 green parts of plants. The carbon dioxid enters through the breath- 

 ing pores 1 on the under side of the leaf; and the water, composed 



1 A breathing pore consists of two guard cells with a slit between them, passing 

 through the outer coat of the leaf. These slits or openings are greatly influenced 

 by the moisture and temperature of the air. In the absence of light they remain 

 closed. When the breathing pores are open, the outside air has free access to the 

 intercellular spaces and passages within the leaf. 



The number of breathing pores varies with different plants. About 17,000 per 

 square inch have been found on oat leaves, 102,000 on corn leaves, and 216,000 

 per square inch on the leaves of red clover. They are found chiefly on the under side 

 of leaves and on green stems, but sometimes in small numbers on the upper leaf 

 surface and even on underground stems. 



