144 



FOUNDATIONS OF BOTANY 



Flo. 112. —A Leaf of Eed Clover. 



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



leaf asleep at night. 



152. Daily Movements of Leaves. — Many compound 

 leaves have the power of changing the position of their 

 leaflets to accommodate themselves to varying conditions 

 of light and temperature. Some plants have the power 

 of directing the leaves or leaflets edgewise towards the 

 sun during the hottest parts of the day, allowing them to 



extend their surfaces 



more nearly in a hori- 

 zontal 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 familiar of 

 the plants whose leaves assume decidedly different posi- 

 tions at night from those which they occupy during the 

 day. Sometimes the leaflets rise at night, and in many 

 instances they droop, as in the red clover (Fig. 112) and 

 the acacia (Fig. 113). 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 experimentally 

 that when part of the leaves on a plant are prevented from 

 assuming the folded position, 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 that this movement has other pur- 

 poses than protection from frost, and probably there is 



