98 



LEAF ARRANGEMENT AND MOVEMENTS 



B 



FIG. 107. A leaf of red clover 

 A, leaf by day ; B, the same leaf at night 



part of the leaves on a plant are prevented from assuming the 



folded position, while others are allowed to do so, and the plant is 



then exposed during a frosty 

 night, the folded ones may 

 escape, while the others are 

 killed. Since many plants in 

 tropical climates fold their 

 leaves at night, it is certain 

 that this movement has other 

 purposes than protection 

 from frost, and probably 



there is much yet to be learned about the meaning and impor- 

 tance of leaf movements. 



115. Self -induced movements; sensitive plants. Some leaves, 



notably those of the so-called telegraph plant, 1 have the power 



of maintaining pretty rapid movements without external stimuli. 



The small lateral 



leaflets of this 



plant, through a 



considerable 



range of temper- 



atures above 



72 F. (22C.),in 



light or darkness 



alike, continue to 



move first up, 



then down, so 



that their tips 



make a complete FIG. 108. A leaf of acacia 



circle in from One A, as seen by day; B, the same leaf at night. After 



to three or more Darwin 



minutes. The motion is jerky, like that of the second hand of 

 a watch, and gives one a vivid impression of the plant as a 

 living thing. 



1 Desmodium gyrans. 



