THE LEAVES 75 



head. A leaf in the open would receive the greatest 

 amount of light if it were held in a horizontal posi- 

 tion, and as the intensity of the light at noonday is 

 such as to be injurious to many species, they have 

 developed the power of changing the position of the 

 blades at such times. This movement generally 

 consists in tilting, so that the tip points more or less 

 directly upward or downward. In either case the 

 rays of the noonday sun strike the surface at such 

 an acute angle that their effect is not so great. The 

 efficiency of this device can be very readily demon- 

 strated if you should place one tin plate flat upon 

 the surface of the ground, and stand another one 

 upright on its edge, noting the difference in warmth 

 of the two, a half hour later, by the touch. Move- 

 ments of the leaves or leaflets to avoid injury from 

 excessive light may be seen if the bean, locust, pea, 

 or any member of the family should be examined 

 during the warmer part of a summer day. The 

 same plants may also move their leaves to escape 

 dangers of another kind during chilly or cool nights. 



94. Com2yass plants. — If the leaves of the wild 

 lettuce, which is now a common weed in the United 

 States, are examined, it will be found that those on 



