LEAF EXPOSURE TO SUN AND AIR. 101 
plants that they tend to seek the hght. The whole plant 
usually bends toward the quarter from which the strongest 
light comes, and the petioles bend with it. Such movements 
may produce very perceptible changes in the course of a few 
hours. If the position of the plant is shifted after the 
mature portions have taken a permanent bend, the youngest 
FIG. 87.— Shoots of Dwarf Tropzolum, showing bending of young shoots 
toward sunlight. 
The older portions of the shoots have bent to the left, away from the light (as 
climbing plants usually do), and toward a close fence. The younger tips of the 
shoots have bent to the right, the direction from which most light was received. 
portions may be made to bend in the opposite direction, as 
shown in Fig. 87, and a third bending may then be produced, 
giving the longer shoots the form of the capital letter S. 
It is not easy to explain in detail how the tissues of the 
plant act in producing these movements. 
