BEHAVIOUR OF PLANTS TOWARD LIGHT 145 
Turning of the sunflower plant toward the sun. During 
the period of growth of the sunflower plant the leaves, 
as well as the grow- 
ing part of the stem, 
are very sensitive to 
light. On sunny 
days the leaves on 
the growing end of 
the stem are drawn 
somewhat together 
so that they form a 
rosette. They also 
turn so that the ro- 
sette faces the sun FIG. 176. Same sunflower plant photographed 
just at sundown. 

when it is rising. 
The growing part of 
the stem also turns 
toward the sun; this 
aids in bringing the 
upper surfaces of the 
leaves to face the sun. 
All through the day, 
if the sun continues 
to shine, the rosette 
of leaves follows it, 
and at sundown the 

Fig. 177. Same plant a little older when the head 
rosette faces squar ely does not turn, but the stem and leaves do. 

