ECOLOGY OF LEAVES 



109 



extend their surfaces more nearly in a horizontal direction 

 during the cooler hours. 



The so-called "sleep" of plants has long been known, 

 but this subject has been most carefully studied rather 

 recently. The wood sorrel, 

 or oxalis, the common bean, 

 clovers, and the locust tree 

 are some of the most fa- 

 miliar of the plants whose 

 leaves assume decidedly 

 different positions at night 

 from those which they 

 occupy during the day. 

 Sometimes the leaflets rise 

 at night, arid in many 

 instances they droop, as 

 in the red clover (Fig. 76) I're- 75. Opposite Leaves of Deutzia, 

 and the acacia (Fig. 77). a^ arranged on a Vertical Branch. 



One useful purpose, at any rate, that is served by the leaf's 

 taking the nocturnal position is protection from frost. It 



has been proved experi- 

 mentally that when part 

 of the leaves on a plant 

 are prevented from as- 

 suming the folded posi- 

 tion, while others are 

 allowed to do so, and the 

 plant is then exposed 

 during a frosty night, 

 the folded ones may 

 escape while the others are killed. Since many plants in 

 tropical climates fold their leaves at night, it is certain 



Fig. 76. A Leaf of Ked Clover. 



At the left, leaf by day ; at the right, 

 the same leaf asleep at night. 



