THE STEM AND THE LEAF ce 
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- 
Fic. 60. Branch of Eucalyptus, 
with leaves hanging almost 
vertically downward 
About one fourth natural size. 
After Bonnier and Sablon 
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 pud- 
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 
Fre. 61. A leaf of red clover 
At the left, leaf by day; at the right, the 
same leaf at night. Natural size 
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 
