LEAVES AND THEIR FUNCTIONS 



127 



3un. In the case of the dandelion a rosette or whorled cluster of 

 leaves is found. In the horse-chestnut, where the leaves come 

 out opposite each other, the older leaves have longer petioles than 

 the young ones. In the mullein the entire plant forms a cone. 

 The old leaves near the bottom have long petioles, and the little 

 ones near the apex come out close to the main stalk. In every 

 case each leaf receives a large amount of light. Other modifica- 



Oxalis plant in the light. 



Oxalis plant after two hours in the 

 dark. 



tions of these forms may easily be found on any field trip. See 

 how many different examples of leaf mosaics you can bring in to 

 record in your notebook. 



Effect of Absence of Light upon the Leaves of the Oxalis.^ — Place a potted 

 oxalis in a dark closet or box for two or three hours, having previously 

 exposed it to direct sunlight for a short time. What is the general posi- 

 tion of the leaves after removal from the darkness? Compare with the 

 position of the same leaves in the light. 



Sleep Movements. — The examination of a number of young 

 seedlings of buckwheat or sunflower shows that after a few hours 

 in darkness the leaves droop. The leaves of the bean and oxalis 



* See Hunter and Valentine, Manual, page 246. 



