
144 THE BEHAVIOUR OF PLANTS 
was covered one day, and then at noon on the follow- 
ing day the box was lifted off. The leaves of the 
sunflower were in the position shown in Fig. 173, the 
right-hand plant. This shows that if the plant is in 
darkness the leaves droop, and the drooping has nothing 
to do with night time, except that the hight stimulus is 
then removed. When 
the plant is exposed 
to the light, the light 
draws the leaf up into 
the day position. 
The leaves of many 
plants turn so as to 
face the light. From 
some of the foregoing 
studies we learn that 

the leaves of plants 
FIG. 175. ae = 2 eamaad tumed are sensitive to the 
stimulus of light. 
They stand so that the rays of light fall full upon the 
upper surface. In the open, the leaves of many plants 
stand so that the upper surface receives the light directly 
from above, as the light from this direction in cloudy 
days is strongest. The leaves of many other plants 
change their positions through the day if the sun is 
shining, so that their upper surfaces face the sun 
directly, or nearly so, at all times of the day. 
