50 



PLANTS AND SUNLIGHT 



80. All the leaves are exposed 

 to light. 



114. The direction or "hang" 

 of the leaf is usually fixed, but 

 there are some leaves which change 

 their direction between daylight 

 and darkness. Thus, leaves of 

 clover (Fig. 82), bean, locust, and 

 many related plants, "sleep" at 

 night; also oxalis. It is not a sleep 

 in the sense in which animals sleep, 

 however ; but its function is not 

 well understood. 



115. Leaves usually expose one 

 particular surface to the light. 



This is because their internal structure is such that light 

 is most efficient when it strikes this surface, as we shall 



learn later on. Some 

 plants, however, expose both 

 surfaces to the light, and 

 their leaves stand vertical. 

 Others endeavor to avoid 

 the intense light of mid- 

 day and to turn in the 

 direction of least light. 

 Leaves standing edgewise 

 are said to exhibit polar- 

 ity. They are "compass 

 plants" if they point north 

 and south. The famous 

 compass plant or silphium 

 of the prairies and the 

 wild lettuce are examples 

 of plants having polar 



81. Phyllotaxy of box-elder, elm, apple. leaves. (Wild 1 6 1 1 U C 6 



[Lactuca Scariola] is a common plant on roadsides; p. 356.) 

 Every leaf has a story to tell of the value of sunlight. 



