THE LEAF 137 
the oxalis (Fig. 95) are, during the day, spread out to 
the sun, but at night they assume a vertical position. 
By reduction of the exposed surface, loss of heat by 
radiation is diminished, so that this is probably a pro- 
tection against cold. In the sensitive plant (Fig. 95), a 
small shrub of the mimosa family, the leaflets fold 
together, and the whole leaf bends down as on a hinge 
in response to any sudden shock. This may easily be 
a protection against gusts of wind. Leaves must be 
protected against the adhering of water to their 
surfaces, for water would block the stomata. This may 
be prevented by a smooth surface, as in the laurel and 
karaka, or by a felty covering as in the mullein and 
many of the native celmisias. In some tropical plants 
the whole leaf surface resembles a drainage system, the 
blade being channeled and the tip prolonged into a 
kind of gutter, which rapidly conducts away the rain. 
LEAF FORMS. 
The shapes of leaves are as a rule decided by three 
factors. To promote photosynthesis and transpiration 
the blades tend to become as large as possible: on the 
other hand, where environment renders it necessary to 
check transpiration, the tendency is to reduce the leaf 
surface; while, to guard against injury by winds or 
running water, leaves may be narrowed or divided. 
The commonest form of leaf blade is more or less 
egg-shaped as may be seen in the lilac and Coprosma 
robusta (Fig. 96). Such a leaf is said to be ovate 
(L. ovum an egg). Many modifications of this leaf 
occur to suit different conditions and surroundings. 
Where the blunt end is turned outwards as in the 
karaka the leaf is obovate (Fig. 96). When the blade 
is rather heart, than egg-shaped, as in the kawakawa or 
New Zealand pepper (Fig. 96), it is cordate (L. cor 
the heart), and where the heart is reversed, as in the 
obcordate myrtle (Fig. 96) and the leaflets of the 
