1d A TEXTBOOK OF GENERAL BOTANY 
The degree of adjustment of leaves in response to hight varies 
greatly in different plants. Leaves which are very sensitive to 
light follow the course of the sun during the day; leaves which 
are only moderately sensitive usually face the strongest light, 
as is the case with most common trees and shrubs; while leaves 
which are feebly sensitive are not so definitely arranged. | 
Unusual positions of leaves. While the leaves of most plants 
are in general arranged so that they face the source of greatest 
illumination, there are some exceptions. The leaves of many 
plants, particularly of 
those growing in arid re- 
gions, make acute angles 
with the rays of light 
from the source of great- 
est illumination. This 
arrangement has cer- 
tain advantages. Very 
intense light has a tend- 
ency to destroy chlo- 
rophyll. It also has a 
tendency to heat the 
leaves excessively and 
to produce rapid tran- 
spiration. ‘These injurious effects are partly avoided by plants 
that have their leaves arranged in the manner just described. 
Leaf mosaics. ‘I'he petioles of old leaves are usually longer 
than those of younger leaves on the same branch. In many 
plants the bending and twisting of the petioles, or a combina- 
tion of these movements with different lengths of the petioles, 
brings all the blades into approximately the same plane and in 
such a position that they fit in between each other with very 
little overlapping. Such an arrangement of the blades is called 
a leaf mosaic, from the similarity to the fitting in of materials 
in mosaic work. 
Heliotropism. ‘The term heliotropism is often used in place of 

Fig. 111. Arrangement of leaves on hori- 
zontal branch of Cestrum nocturnum; position 
of leaves due to one-sided illumination 
Compare with Fig.110. (x 4) 
the word phototropism. Heliotropism is the orientation of plant- 
