200 



ELEMENTARY STUDIES IN BOTANY 



But some leaves are so constructed that they can shift their 

 position as the direction of the light changes, or the stem 

 bearing the leaves may shift its position so that a better 

 relation to light is secured (Fig. 162). The leaves of window 

 plants are often seen to adjust themselves so as to face the 

 light, and by turning such a plant around so as to bring its 

 other side towards the light, the leaves will become adjusted 

 gradually to the new condition and face the light again. 



It is evident that the leaves of a plant are in danger of 

 shading one another, and while shading cannot be avoided, 



it can be diminished 

 in various ways. The 

 spacing apart of the 

 leaves by the elonga- 

 tion of the internodes 

 ( 130, p. 224) is the 

 most general method 

 of avoiding extreme 

 shading. The spiral 

 arrangement of leaves 

 on the stem, which 

 prevents two succes- 

 sive leaves from stand- 

 ing in the same plane, 

 is also a very general method of diminishing shading (Fig. 161). 

 In many herbs whose leaves are rather large and close 

 together, the petioles of the lower leaves are usually longer 

 than those above, and thus their blades are thrust beyond 

 the shadow of the upper leaves. The same result is obtained 

 when the lowest leaves of a plant are the largest, and the 

 upper leaves gradually diminish in size. 



Some plants are in such a position that for protection (to be 

 explained later) the leaves are produced in a cluster at the 

 base of the stem. This is called the rosette-habit, and the 

 rosette of leaves frequently lies flat upon the ground or upon 



FIG. 164. Rosette of shepherd's purse. 



