214 



ELEMENTARY STUDIES IN BOTANY 



the same principle that a sailing vessel may regulate its 

 sail-exposure according to the need. Motile leaves also shift 

 their positions throughout the day in reference to light; 

 and at night a very characteristic position is assumed, once 

 called a " sleeping " position, but better called a night posi- 

 tion. The contrast between the day and night positions of 



many leaves, even clover 

 leaves (Fig. 179), is quite 

 striking. These night posi- 

 tions may be induced ex- 

 perimentally by placing 

 plants in darkness. 



126. The fall of leaves. - 

 Many shrubs and trees of 

 temperate regions lose their 

 leaves every year, usually 

 at the approach of winter, 

 putting out new leaves in 

 the following spring. This 

 is called the deciduous habit, 

 and such trees are called 

 deciduous trees. While in 

 some deciduous trees, as the 

 oaks, there is no special 

 preparation for " shedding" 

 leaves, in most of them a 

 special plate of cells is formed 



near the juncture of the leaf with the stem, known as the " cut- 

 ting-off layer," which gradually separates the leaf from the 

 stem, so that it falls by its own weight or is wrenched off by 

 the wind (Fig. 180). 



In connection with the deciduous habit there often appears 

 the autumnal coloration of leaves, so striking a feature of 

 temperate forests. The colors that appear are shades of 

 yellow and red, either pure or variously intermixed. They 



FIG. 181. A pine twig, showing the 

 needle-leaves, and a cluster of stami- 

 nate cones. Photograph by LAND. 



