THE; PLANT AND THE: ATMOSPHERE; 39 



Carbon dioxide is soluble in water, and is brought to the earth 

 with rain. It exerts a solvent action on constituents of soils and 

 rocks, especially on carbonate of lime. The amount of carbon 

 dioxide dissolved by water varies according to temperature and 

 pressure. Carbon dioxide is usually found in springs, wells, and 

 river water, as well as in dew or rain. 



Circulation of Carbon. Carbon circulates from the air to 

 plants, from plants to animals, and from plants and animals back 

 to the air. The oxidation of carbon in decay of organic matter 

 is due to bacteria. The diagram on page 38 illustrates the cir- 

 culation of carbon. 



Effect of Light on Plants. The energy stored by plants comes 

 for the most part from the sunlight. In the presence of light, 

 green leaves absorb carbon dioxide and give off oxygen. This 

 can be demonstrated by a simple experiment. Some fresh green 

 leaves are placed in a funnel filled with water containing carbon 

 dioxide, inverted in a vessel of water, and placed in the sunlight. 

 Bubbles of gas begin to rise, which may be collected in a small 

 test-tube attached to the stem of the funnel, and tested with a 

 spark on a splinter. This bursts into flame, and so proves the 

 gas to be oxygen. 



In darkness, plants take up oxygen and give off carbon dioxide, 

 though the amount is small in comparison with the reverse action 

 in the light. For example, Corenwinder 1 ascertained that three 

 pea plants exhaled 24 cc. carbon dioxide during an entire night, 

 while they absorbed 86 cc. carbon dioxide during an hour of 

 direct sunshine. 



The amount of light most favorable to the growth and develop- 

 ment of the plant, depends on the kind of plant. Some plants 

 grow better in the sunshine, while others thrive only in the 

 shade. In the diffused light of cloudy days, or the softened light 

 of a forest, plants may exhale either carbon dioxide or oxygen, 

 according to the kind of plant, the intensity of the light, and the 

 stage of development of the plant. 



Chlorophyll, the green coloring matter of the leaf, seems to 

 1 Jahresber Agr. Chem., 1858, p. 106. 



