288 



PLANTS 



gives back oxygen in its place. In this way the process 

 by which the plant prepares carbohydrates is also the 

 process by which it purifies the air for the use of animals 

 (cf. 58). The making of carbohydrates out of carbon 

 dioxide and water requires chlorophyll and light (usually 



sunlight). The combina- 

 tion generally takes place 

 in leaves ; but other green 

 parts, such as stems, also 

 carry it out. 



311. Modified Leaves. 



- Plants often have spe- 

 cial ways of supporting 

 themselves or of getting 

 food. Thus, plants with 

 slender stems have the 

 climbing habit. To carry 

 out new functions the 

 plant seems to have new 

 organs ; but if we examine 

 the plant carefully, we 

 shall find that the new 

 apparatus is simply one 

 of the original structures (stem, leaves, etc.) , altered to meet 

 the new need. Thus, the morning-glory climbs because its 

 stem is sensitive, and twines around a support, such as a 

 string or a pole. Other plants have tendrils for support. 



The tendrils of the pea are changed parts of its compound leaf 

 (cf. 309). Sometimes the tendrils, instead of twining about an erect, 

 straight object, twist themselves into the rough bark of a tree. But 



FIG. 236. A Spreading Elm. Courtesy of the 

 Field Museum of Natural History. 



