248 



LEAVES 



In its home in the tropics this plant is called the shame 

 plant. It seems to shrink in shame. It is a very common 

 weed, and is especially troublesome in the rice. From 

 car windows thousands of these plants may be seen shrink- 

 ing down in response to the vibration caused by the pass- 

 ing train. 



The sensitive plant belongs to that great family (Legumi- 

 nosce) to which also peas and beans belong. Our own 



clovers and the locust trees also 

 belong to this family, and they 

 too show leaf movements. Their 

 compound leaves close up at 

 night, especially in cool weather. 

 The little Oxalis, called sorrel or 

 sour grass, grows often in lawns ; 

 it also has this habit, as you may 

 have noticed at twilight. Its 

 compound leaves resemble those 

 of the white clover. (See Fig- 



ure $7 ) 



_, . . . . . . . . 



Evidently the chief advantage 



of this closure of leaves IS that 



transpiration is checked thereby. 

 But the advantage of closing at night rather than in 

 daytime, or when touched rather than when untouched, is 

 not evident. These may be examples of habits which have 

 no advantages. 



The immediate cause of leaf movements is change of 

 turgor in certain cells of the pulvinus. You will recall 

 that the pulvinus is the enlarged part of the petiole at its 

 base. It is evident that if the cells on one side of the 

 pulvinus suddenly lose their water, while the cells on the 



FIG. 8 7 . - Movement of the leaf 



of wood sorrel (Oxalis). The 

 upper picture shows the leaves 



as they appear in daylight, the 



lower one shows their position 



