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Life of Plants and Animals 207 



often has to do this in cultivating plants partly to 

 keep moisture in the soil and partly to allow free circu- 

 lation of air around the roots of the plants. If air is 

 excluded the plant usually dies. That which enters 

 the stomata passes into air- cavities in the spongy 

 under sides of the leaves; and, from there, into air- 

 tubes found in different parts of the stem. In that 

 way the plant breathes. By the expansion and con- 

 traction of this contained air, as the temperature 

 outside rises or falls, with the changes of seasons or 

 day and night, the sap contained in the stem is sub- 

 jected to pressure, which, if the outer cuticle be re- 

 moved, causes the sap to flow out from the wounded 

 part. Thus the plant bleeds, in one sense, as animals do. 



The air circulating in the leaves and stem of the 

 plant takes up certain waste substances resulting 

 from vital action of the plant, in the form of carbon 

 dioxide, which is exhaled. Otherwise the accumula- 

 tion of the waste would be injurious or even fatal to 

 the plant. This exhalation of carbon dioxide takes 

 place at all times, but is most in evidence at night 

 or in darkness. 



Assimilation. The green substance in the leaf, 

 leaf- green, or chlorophyl, is developed in the living 

 substance in some unknown way through the action 

 of sunlight, somewhat, perhaps, as tan and freckles 

 are developed in our skin when our face is exposed 

 to the sun. If deprived of sunlight it disappears. 

 That can be seen in the white color of clover which 

 has been covered for some time by any opaque body, 

 as a board or a rock. Various patterns can be in- 

 scribed on a leaf by covering it with an opaque pattern, 

 as pieces of cork. 



Chlorophyl is able to utilize the energy of sunlight 

 in doing chemical work. By the aid of the sun's 

 energy salts and other substances remaining in the 

 sap after water has evaporated from the leaf is made 



