184 



FOUNDATIONS OF BOTANY 



These movements are due to unequal growth induced in 

 the younger portions of the plant by the action of gravi- 

 ^ tation upon it. Other movements (of 



yl ordinary foliage leaves, of the floral leaves 



of many flowers, and of other parts of a 

 few flowers) are produced by changes in 

 the distention or turgescence of some of the 

 cells in the organs which move and have 

 nothing to do with growth. The closing 

 of the leaves of insect-catching plants is 

 briefly described in Sect. 410, and the 

 " sleep " of leaves, due to movements of 

 the pulvini, was described in Sect. 152. 

 A few facts in regard to the opening and 

 closing of flowers will be found in 

 Sect. 440. 



The stimuli which cause movements of 

 leaves or of the irritable parts of flowers 

 are of several kinds. Light is the main 

 cause which induces leaves to open from 

 their night position to that usual in the 

 daytime. In the case of flowers, it is 

 sometimes light and sometimes warmth 

 FIG. 127. stinging which causes them to open. Leaves which 



Hair of Nettle, with . , , ^ , 



Nucleus. (Much catch insects may be made to close by 

 magnified.) The ar- touc hi n g them, but the sensitive-plants, 



rows show the direc- 

 tion of the currents of which there are several kinds found in 



the United States, and a much more sensi- 

 tive one in tropical America, all fold their leaflets, on 

 being touched, into the same position which they assume 

 at night. 



