44 



Science of Plant Life 



leaflets of the compound leaf droop downward from the peti- 

 ole ; in the medium light of a cloudy day they are held per- 

 pendicular to the light ; in 

 the most intense sunlight 

 the blades are raised above 

 the petiole until they are 

 edgewise and point toward 

 the light. Some observa- 

 tion of bean seedUngs, which 

 may readily be grown in 

 the laboratory, will be in- 

 structive in this connection. 

 Other examples of motile 

 leaves may be seen in the 

 honey locust, the leaflets of 

 which fold upward at night, 

 and in white clover, oxalis, 

 and the red-bud tree. The 

 leaflets of the sensitive 

 plant vary their positions 

 according to light intensity, 

 and also when touched or 

 injured in any way (Fig. 



31)- 



The change of position in 



motile leaves is brought 

 about by changes in the water content of the cells on oppo- 

 site sides of a special organ called the pulvinus (Fig. 32), 

 which is located at the base of the leaflets. This device may 

 readily be studied in the leaf of the bean. 

 The leaves of shade plants. As may be observed by a trip 



Fig. 31. Sensitiv-e plant. The leaves on the 

 left side are in normal positions ; those on the 

 right side have been touched and the leaflets 

 have folded together wholly or in part and 

 the petioles have folded toward the stem. 

 P is the pulvinus. 



