LEAVES 



213 



inclining them. Such leaves have been developed most 

 extensively in the pea family, and the most conspicuous illus- 



FIG. 179. The day (A) and night (B) positions of the leaves of a clover-like plant. 

 After STRASBURGER. 



trations are the " sensitive plants " of the drier regions. The 

 name has been given because the leaves respond to various 

 stimuli by changing position with 

 remarkable rapidity. A slight touch, 

 or even jarring, will call forth a 

 response from the leaves, and the 

 sudden application of heat gives 

 striking results. The leaves of the 

 best known sensitive plant (Fig. 

 178) are divided into very numer- 

 ous small leaflets, which stand in 

 pairs along the leaf branches. 

 When a drought begins, some pairs 

 of leaflets come together face to 

 face, slightly reducing the surface 

 exposure. If the drought continues, 

 more leaflets come together, then 

 still others, until finally all the 

 leaflets may be in contact in pairs, and the leaves them- 

 selves may droop against the stem. In this way the ex- 

 posed surface may be regulated according to the need, on 

 15 



FIG. 180. Diagrammatic sec- 

 tion through a node of horse 

 chestnut, showing the posi- 

 tion of the cutting-off layer 

 (s) and the vascular bundle 

 (b) not cut through. 



