110 ELEMENTS OF BOTANY. 
later, but it will make matters simpler to state at once that 
the epidermis serves as a mechanical protection to the parts 
beneath and prevents excessive evaporation, that the palisade 
cells (which it may not be easy to make out very clearly in a 
roughly prepared section) help to prevent too rapid evapora- 
tion of sap from the leaf when exposed to excessive dryness, 
heat, and direct sunlight, and that they hold large quantities 
of the green coloring-matter of the leaf in a position where it 
Fic. 97. —Epidermis of Leaf of Althea. (Much magnified.) 
I, from upper surface ; II, from lower surface ; h, star-shaped compound hairs ; 
st, stomata; p, upper ends of palisade cells, seen through the epidermis. 
can receive enough, but not too much sunlight. The stomata 
admit air to the interior of the leaf (where the air-spaces 
serve to store and to distribute it) and, above all, they regulate 
the evaporation of water from the plant. 
137. Leaf of ‘‘ India-Rubber Plant.’’ 1— Study with the microscope, 
as the lily leaf was studied, make the same set of sketches, note the 
differences in structure between the two leaves, and try to make out their 
meaning. 
t Ficus elastica. 
