HELIOTROPIC MOVEMENTS 101 



This adjustment is most perfect in the compass plant of the 

 prairies of the Mississippi basin. Its leaves stand nearly upright, 

 many with their edges just about north and south (Fig. 110), so 

 that the rays of the midsummer sun will, during every bright 

 day, strike the leaf surfaces nearly at right angles during a 

 considerable portion of the forenoon and afternoon, while at 

 midday only the edge of each leaf is exposed to the sun. 



FIG. 111. Nearly vertical leaves of the olive 



118. Heliotropic movements. The whole plant above ground 

 usually bends toward the quarter from which most light comes. 



Any set of flowering plants growing close to a wall, or of 

 house plants in a window, generally offers many illustrations of 

 this principle. Movements caused by light are called heliotropic 

 movements (from two words meaning turning toward light). 



119. Positive and negative heliotropic movements ; how 

 produced. Plants may bend either toward or away from the 

 strongest light. In the former case they are said to show posi- 

 tive Tieliotropism, in the latter negative heliotropism. In both 

 cases the movement is produced by unequal growth, brought 

 about by the stimulus of unequal lighting of different sides of 

 the stem. A plant if placed on a revolving table before a window 

 and slowly turned during the hours of daylight grows upright, 

 like a plant out of doors. This is because it is not left with a 

 one-sided illumination long enough to produce any bending. 



