‘= 4 
THE LEAF el 
gets warmer. This increase in the rate of evaporation usually 
continues on bright days until some time in the afternoon. As 
the sun goes down the rate decreases, and the decrease con- 
tinues until the low night rate is reached. 
If the rate of evaporation is comparatively low during the 
day, the rate of transpiration may follow that of evaporation 
rather closely. But when the rate of evaporation during the day 
increases to a point where transpiration is excessive, the rate 
of transpiration is restricted by the movements of the guard 
cells and the drying of the chlorenchyma walls. Thus it fre- 
quently happens that while the rate of evaporation continues 
to increase until some time in the afternoon, the highest rate 
of transpiration is reached in the morning. At night the rate 
of transpiration is low, owing to the very low rate of evapora- 
tion and to the closing of the stomata. 
Water content of the leaf. Many leaves contain from 80 to 
90 per cent of water. When, on a dry day, a plant loses more 
water than it absorbs, the water content of the leaves naturally 
diminishes ; thus it very frequently happens that leaves contain 
less water during the middle of the day than during the night. 
In many plants the loss of water goes so far on dry days as to 
cause an actual wilting of the leaf. Not only leaves but other 
parts of plants frequently contain less water during the day 
than at night. It is for this reason that vegetables have a 
crisper and fresher appearance when picked early in the morn- 
ing than when gathered later in the day. 
The relation of moisture to number and size of leaves has 
been treated in a previous paragraph. 
SHAPES OF LEAVES 
While most leaves have petioles, many do not possess them. 
Leaves without petioles are sesstle. The petiole is usually 
attached at the edge of a leaf, but may be attached on the 
lower surface within the margin (Fig. 54). In the latter case 
the leaf is peltate. 
