Leaves in Relation to Light 



69 



positions of the blades are not fixed, but are changed according to 

 the intensity and direction of light. A familiar example is the 

 roadside sweet clover. At night the three leaflets of the com- 

 pound leaf droop downward from the petiole; in the medium 

 light of a cloudy day they are held perpendicular to the light ; 

 in the most intense sunhght the blades are raised above the petiole 

 until they are edgewise and point toward the light. Some ob- 

 servation of lima bean seedhngs (Fig. 41), which may readily be 

 grown in the laboratory, will be instructive 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 ac- 

 cording to light intensity, and 

 also when touched or injured 

 in any way (Fig. 39). 



The changes of position in 

 motile leaves is brought about 

 by changes in the water con- 

 tent of the cells on opposite 

 sides of a special organ called 

 the pulvinus (Fig. 40), which 

 is located at the base of the 

 leaves and 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 

 to the woods, the leaves of 



plants growing in the shade Fig. 39- Sensitive plant. The leaves on the 



„ , , J left side are in normal positions ; those on the 



are usually darker and more j-ight side have been touched and the leaflets 



bluish-green than the leaves have folded together wholly or in part, and 



^ • r n ^^^ petioles have folded toward the stem. 



of plants growing m full sun- p is the pulvinus. 



