10 



A TEXT-BOOK OF BOTANY 



light; but if it be turned around so as to bring the other 

 side of the plant to the light, the leaves will become adjusted 

 gradually to the new direction. Many plants have more 

 or less power to direct their leaves, and it would be in- 

 teresting to observe what common plants of any region 

 possess it. ^ 



(2) Problem of shading. It is evident that leaves of the 

 same plant are in danger of shading one another; and while 



it cannot always be pre- 

 vented, there are ways by 

 which the danger is dimin- 



"**** """^SSBI ished. The problem of the 



ttl^^TKPxL plant is to develop as much 



leaf surface as possible and 

 to place it in the most fa- 

 vorable position for work. 

 The spiral arrangement of 

 leaves prevents two suc- 

 cessive leaves standing in 

 the same plane, and results 

 in vertical rows of leaves 

 distributed about the stem. 

 The narrower the leaves, 

 the more numerous may 

 be the vertical rows; and 

 the broader the leaves, the 

 fewer the vertical rows 

 (Fig. 5). In many herbs 

 whose leaves are rather 

 large and close together, 



FIG. 5. A broad-leaved plant, showing the petioles of the lower 

 few vertical rows, and variously directed leaves are USUally longer 

 leaves. 



than those above, and 



thus their blades are thrust beyond the shadow. The 

 same result is obtained when the lowest leaves of a plant 



