THE SOIL AND SOIL FORMATION 69 



and by heat and cold, especially upon freezing and thaw- 

 ing. The soil is literally a slowly seething mass of in- 

 organic and organic activities most vital and interesting. 



Movement of air in the soil. Changes in temperature 

 of the air and soil and barometric air pressure cause more 

 or less movement of air into and out of the soil. Air 

 that is colder than the soil will displace the warmer air, 

 thus causing the circulation of air into the soil ; but air 

 warmer than the soil will not so readily displace the 

 colder air underground. The air upon flowing into the 

 soil carries with it some gaseous products other than 

 nitrogen and oxygen. Air serves as a medium through 

 which the atmospheric gases can slowly diffuse into the 

 soil, and through which free gases forming in the soil 

 can escape into the atmosphere. 



Movement of air in plants. Plants as well as animals 

 are said to breathe. Animals inhale oxygen and exhale 

 carbon dioxide. Plants reversing the process, inhale 

 dioxide and exhale oxygen. On the under surfaces of the 

 leaves and in other places may be seen, by the aid of a 

 microscope, certain small openings called stomata or 

 breathing pores. These breathing pores communicate 

 with the intercellular spaces of the leaves, forming 

 channels through which the air circulates. In the in- 

 terior of the leaves are found cells containing a green 

 substance called chlorophyll. The air enters through 

 these stomata, passes on in between the chlorophyll- 

 bearing cells, and by osmosis gets into the cells contain- 

 ing chlorophyll. Here the carbon dioxide (CO 2 ) of the 

 air is broken up through the action of sunlight into car- 

 bon and oxygen. The carbon combines with the water 

 (H 2 O) that has been taken up by the roots of the plant, 

 and starch is formed. Thus 6 CO 2 , acted upon by sun- 

 light in the chlorophyll bodies, equals 6 C, and 12 O; 

 and 5 H 2 O (5 molecules of water) equals 10 H and 5 O; 

 and when recombined equals C^H 10 O 5 , starch, with 12 O 



