142 THE BEHAVIOUR OF PLANTS 
class-room so that the pupils may make comparisons; 
although such observations should be made in fields 
and gardens whenever possible. 
Influence of light on the day position of leaves. Light 
has great influence on the position of the leaves during 

Fig. 173. Young sunflower plant ; at left in light, at right 
after being covered two days to shut out light. 
the day, just as it 
has on the position 
of the marrow 
cotyledons which 
we have just 
studied, or on the 
leaves of the sensi- 
tive plant. It acts 
as a stimulus to 
adjust the leaf so 
that the light will 
fall full upon the 
upper surface, or 
nearly so. In 
some plants this 
position becomes 
more or less fixed. 
But in other 
plants, like the 
sunflower, bean, 
oxalis, and many more, the leaves change their position 
night and day. The leaves usually occupy a drooping 
