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A TEXT-BOOK OF BOTANY 



such a plant grows in the shade the leaves do not assume 

 the profile position. It must not be supposed that there 



is any accuracy in the 

 north or south direction, 

 as the edgewise position 

 is the significant one. In 

 the rosinweed probably 

 the north and south di- 

 rection is the prevailing 

 one; but in the prickly 

 lettuce, a very common 

 weed of waste grounds, 

 and one of the most 

 striking of the compass 

 plants, the edgewise 

 position is frequently as- 

 sumed without any ref- 

 erence to the north or 

 south direction of the 

 apex (Fig. 26). 



(9) Motile leaves have 

 the power of shifting 

 their positions according 

 to their needs, directing 

 their flat surfaces toward the light, or more or less inclining 

 them. Such leaves have been developed most extensively 

 in the great family to which peas and beans belong, the 

 most conspicuous ones being those of the so-called sensitive 

 plants. The name has been given because the leaves respond 

 to various external influences by changing position with 

 remarkable rapidity. A slight touch, or even jarring, will 

 call forth a response from the leaves; and the sudden ap- 

 plication of heat gives striking results (Fig. 27). The most 

 common sensitive plant abounds in dry regions, and may 

 be taken as a type of such plants. The leaves are divided 



FIG. 26. Prickly lettuce, showing the edge- 

 wise or profile leaves from two points of 

 view. 



