THE LEAF 165 
182. Distribution of stomata. — While stomata are gen- 
erally more abundant on the under side of leaves, this is not 
always the case. In vertical leaves, like those of the iris, 
which have both sides equally exposed to the sun, they are 
distributed equally on both sides. In plants like the water 
lily, where the under surface lies upon the 
water, they occur only on the upper side. 
Succulent leaves, as a general thing, have 
very few, because they need to conserve 
all their moisture. Submerged leaves 
have none at all; why? 
183. Minute study of a leaf epidermis. 
—Place a bit of the lower epidermis of pig. 220,—A_ small 
a leaf under the microscope, and examine Piece of the under epider- 
3 A : A mis of an oak leaf, highly 
with a high power. It will appear, if a magnified to show the 
monocotyl, to be composed of long, flat, funel ” and minute 
rectangular spaces (Fig. 221) ; if the leaf 
of a dicotyl is used, they will be more or less irregular (Fig. 
220), with the outlines fitting into each other like the tiling 
of a floor or the blocks of a Chinese puzzle. 
These spaces are the cells of the epidermis, 
and the lines are the cell walls. Can you 
find any of the cell contents? The cell 
sap is not often visible; do you see the 
nuclei? Can you give a reason why the 
epidermal cells are so thin and flat? Be- 
tween some of the cells you will see two 
kidney-shaped bodies placed with their 
concave sides together so as to leave a 
lenticular opening between them. This 
is a stoma, and the kidney-shaped bodies 
Fie. 221.— Under (Figs. 218, 219) are guard cells. They 
epidermis of an oat leaf, are given this name because they open 
ES eae or close the mouth of the stoma. If 
you will imagine a toy balloon made in the form of a hol- 
low ring, like the tire of a bicycle, you can easily see, from 
