STRUCTURE OF LEAF (CROSS SECTION) 285 
adding weights from time to time to take the place of the water lost 
by evaporation. If the plant has large leaves, remove them; after 
the experiment is completed, draw their outline on cross section paper 
and find their combined surface. Compare it with the amount of 
water lost. At the same rate, estimate how much would be given off 
by a plant with a leaf surface ten times as great, or as many times as 
great as you please. Describe the whole experiment in writing, illus¬ 
trating with sketches or photographs, as you please. 
Cover a small fuchsia or balsam overnight with a bell jar. Where 
are the guttation drops found? Why dp they form there? Cover 
young plants of oats, wheat, corn, or grass in the same way. Where 
are the drops formed on them? Make drawings of one or more kinds 
of leaves used in this experiment. Write notes telling what you dis¬ 
covered. 
251. Structure of a Leaf (Cross Section). —While leaves 
vary greatly in number, size, position, and shape, the work 
that they do is very 
similar for each plant. 
A typical leaf is covered 
with a layer of thin cells 
called the epidermis. In 
hot, dry regions the epi¬ 
dermis has a heavy coat¬ 
ing of cutin, a substance 
which prevents evapora¬ 
tion. The epidermis on 
the under side of the leaf 
is thinner and has less cutin than that on the upper side of 
the leaf. It is pierced by many openings called stomata 
(Greek, stoma , a mouth) which allow the entrance of air and 
the exit of water and gas. Between these two layers of epi¬ 
dermis is the mesophyll (mez'o-fil: Greek, mesos, middle; 
phyllos, leaf) of the leaf, divided into two regions, (1) the up¬ 
per or palisade composed of slender, elongated cells placed 
side by side in upright position, (2) spongy layer consisting 
of rounded cells, loosely arranged with many spaces between 
them. 
ispongy layer 
-chlorophyll 
lower epidermis 
Figure 264 . — Cross Section of Bean 
Leaf. 
How many tissues present ? 
