98 ELEMENTS OP BOTANY. 



Where the stem on which the leaves are borne stands nei- 

 ther horizontally nor vertically, but at some oblique angle to 

 the earth's surface, the leaf arrangement is more or less 

 irregular, as in Fig. 83, which represents the leafage of a 

 castor-oil plant growing. in an inclined position, because it 

 was shaded on one side. 



123. 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, 

 Tlie so-called " sleep " of 

 plants has long been 

 known, but this subject 

 has been most carefully 

 ^iu.«4, -A Leaf of White Clover. studied rather recently. 



A, leaf by day ; B, the same leaf asleep at ^,^. _ , , . 



njgiit 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 positions at night 

 from those which they occupy during the day. Sometimes 

 tlie leaflets rise at night, and in many instances tlaey droop, 

 as in the white clover, Fig. 84, and the acacia, Fig. 86. One 

 useful purpose, at any rate, that is served by tlie 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. The student may very naturally inquire whether the 

 change to the nocturnal position is brought about by the 

 change from light to darkness or whetlier it depends rather 

 upon the time of day. It will be interesting to try an 

 experiment in regard to this. 



