98 



LEAF ARRANGEMENT AND MOVEMENTS 



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 



FIG. 107. A leaf of red clover purp oses than protection 



, leaf by day ; B, the same leaf at night f rorn 



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 

 cons iderable 

 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 ill 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. 



