LEAVES 
27 
clear; and doubtless they may serve other purposes also, 
or sometimes may even be of no use whatever to the 
plant. It has been 
suggested that in re- 
gions of intense light a 
covering of hairs is an 
effective sun - screen. 
The explanation is 
that being dead struc- 
tures, containing air, 
they reflect the light, 
thus diminishing the 
amount that reaches 
the working cells. As 
is well known, hairs 
are by no means re- 
stricted to leaves, but 
occur on all parts of 
plants. 
(5) Small leaves are 
characteristic of dry  - 
Frc. 23.—Scale from the leaf of Shepherdia; such 
scales overlap and form a complete covering. 
regions, in this way each leaf exposing a small surface to 
the drying air and intense light. 
Fic. 24.—Section through a leaf of Be- 
gonia, showing epidermal layers (e), 
colorless water storage tissue (w), 
and the central cells containing 
chloroplasts (c). 
That this reduction in 
size holds a direct relation to 
the dry conditions is evident 
from the fact that the same 
plant often produces small 
leaves in a dry region and 
larger ones in moist condi- 
tions. In the case of the 
cactus, a large group in the 
dry regions of the Southwest, 
the leaves have become so 
much reduced that they are 
no longer used in photosyn- 
