40 



Science of Plant Life 



approximately at right angles to the Hne along which the 

 greatest amount of light reaches them. Consequently the 

 leaves on most of our common trees, shrubs, and herbs tend to 

 take an approximately horizontal position. The sugar maple 

 and the horse-chestnut are examples of trees whose leaves 

 are displayed in this manner. In the cottonwood and tulip 

 tree the leaves are only sHghtly sensitive to light, and the 

 result is that their leaves assume a great variety of positions. 



If leaves are extremely sensi- 

 tive to light, the blades may 

 turn toward the sun in the 

 early morning and follow the 

 sun throughout the day, always 

 keeping the broad face of the 

 leaf to the light. The leaves 

 of the common mallow move 

 in this way. 



Compass plants. There is 

 another class of plants which 

 are sensitive to light, but 

 which respond to it in a very 

 different manner. These are 

 the so-called compass plants, 

 of which the wild prickly let- 

 tuce is a widely distributed 

 example. In sunny situations 

 the leaves of these plants tend 

 to take positions edgewise to 



Fig. 27. Pricklylettuceplant: 4, viewed the direction of the mOSt in- 



from west ; 5, viewed from south. Drawn ±. „„ r 1,4. a ii ^• 1 1 



from a specimen grown under exposure \^'^^^ ^^g^t. As the SUnhght 



to bright sunlight. is most intense at noon, it is 



