[Chap. XXIII PLANT BEHAVIOR RELATED TO OSMOSIS 217 



changes in temperature or light intensity; the opening of buds; the roll- 

 ing and folding of leaves; and pulvinal movements, such as the mo\e- 

 ment of leaves of clover and of "sensitive" plants ( Fig. 76 ) . 



Fig. 76. Sensitive plants (Mimosa pudica). The one on the left was mildly jarred 

 just before the picture was taken. Photo by G. S. Growl. 



If young flowers of tulip are exposed alternately to low and high tem- 

 peratures they may open and close several times in one hour. When the 

 temperature is lowered the rate of growth of the outer side of the 

 sepals and petals is greater than that of the inner side. As the tempera- 

 ture is raised tlie greater rate of growth of the inner side results in 

 opening the flower. White water lily flowers open in light and close in 

 weak light and darkness. A commonly cultivated purple water lily has 

 flowers that close in bright sunlight and are open only in dense shade or 

 at night. 



The opening of buds is the result of growth of the bud scales and 

 young leaves. In many buds there is increased growth on the inner side 

 of the bud scales which results in spreading of the scales. 



The folding and rolling of leaf blades of some species of plants are 

 dependent upon the relative turgidity in rows of special cells in the upper 



