THE STEM AND THE LEAF 



77 



FIG. 60. Branch of Eucalyptus, 



with leaves hanging almost 



vertically downward 



About one fourth natural size. 

 After Bonnier and Sablon 



75. Daily movements of leaves. Any one who walks through 

 a clover field at dusk is likely to be struck with the peculiar 

 pale appearance of the leaves, very different from the dark- 

 green color which they have in full 

 sunshine. This paleness is due to 

 the fact that as the daylight fades 

 the leaflets droop, as shown in figure 

 61, so that little except the under 

 surfaces are seen. A large propor- 

 tion of the plants of the Pea family 

 and many other plants have leaves 

 that take a special night position. 

 Some leaves, as those of the bean 

 and the black locust, have three 

 positions one at night, another 

 in ordinary daylight, and a third 

 in intense sunlight. The daylight 

 position is usually almost horizon- 

 tal ; the position for brilliant sunlight is vertical. In the locust 

 the change from vertical to horizontal occurs quickly enough 

 to make it worth while to watch it coming on as the sun 

 moves westward after noon and the leaves are left in the shade. 



In plants of the Pea 

 family the daily leaf move- 

 ments are brought about 

 by means of a sensitive, 

 cushion-like organ, the pul- 

 vinus, situated at the base 

 of the leafstalk. It is 

 easy to see the use of the 

 horizontal and the verti- 

 cal leaf position, but the 

 importance of the night position is not so well understood. 



76. Self -pruning of leaves and twigs. Many trees and shrubs 

 begin to shed some of their leaves even in the spring, very 

 soon after the leaves are well grown. Examples of this are 



FIG. 61. A leaf of red clover 



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

 same leaf at night. Natural size 



