32 A TEXT-BOOK OF BOTANY 
Motile leaves also shift their positions throughout the day 
in reference to light; and at night a very characteristic 
position is assumed, once called a sleeping position, but 
better night position. The contrast between the day and 
night positions of leaves such as those of the sensitive plants, 
and even of the common white clover, is quite striking (Fig. 
28). These night positions, produced by the withdrawal 
of light, may be induced by placing plants in darkness; and 
experiments will show that the power is more common than 
is generally supposed. Just what it means is not clear. 
The suggestion has been made that the night position is a 
protection against danger from the loss of heat, but it may 
have no such meaning. 
(10) Rain is a menace to leaves, for if the water soaks 
in_and fills up the air spaces and stomata, communication 
with the air is cut off; hence leaves shed water with remark- 
able promptness, partly by their positions, partly by their 
structure. Some of the structures 
that prevent the rain from soaking 
in are a smooth epidermis, a cuticle, 
a waxy deposit, felt-like coverings, 
overlapping scales, ete. In the 
rainy tropics it is very common for 
the sunken veins and ribs of the 
leaves to form a sort of drainage 
system for carrying off water, the 
main channel lying along the midrib, 
AT Nera which terminates in a long, spout- 
a) 
Fic. 29.— Diagrammatic sec- like point (Fig. 319). 
tion through a node of horse- 18. Fall of leaves.— Many shrubs 
chestnut, showing the posi- = 
tion of the cutting-off layer #0d trees of temperate regions lose 
(s) and th lar bundl i 208 
oe Arse era undle their leaves at the approach of 
winter, or even earlier, putting out 
new leaves in the following spring. This is called the 
deciduous habit, and it is an adaptation to climate. While 
