MINUTE STRUCTURE OF LEAVES Loe 
In the case of an apple tree, where the epidermis of the 
lower surface of the leaf contains about 24,000 stomata to 
the square inch, or the black walnut, with nearly 300,000 
to the square inch, 
the total number 
on a tree is incon- 
ceivably large. 
163. Uses of the 
Parts examined. — 
It will be most con- 
venient to discuss 
the uses of the 
parts of the leaf a 
little later, but it 
will make matters 
simpler to state at 
once that the epi- 
dermis serves as a 
mechanical protec- 
tion. to the parts Fig. 118.—A Stoma of Thyme. (Greatly magnified.) 
beneath and pre- A, section at right angles to surface of leaf; B, sur- 
Sart face view of stoma. cu, cuticle ; g, guard-cells ; 
vents excessive 8, stoma; e, epidermal cells; a, air chamber ; 
evaporation, that c, cells of spongy parenchyma with grains of 
the palisade-cells Saget 
(which it may not be easy to make out very clearly in a 
roughly prepared section) hold large quantities of the green 
coloring matter of the leaf in a position where it can 
receive enough but not too much sunlight, and the cells 
of the spongy parenchyma share the work of the palisade- 
cells, besides evaporating much water. The stomata 
admit air to the interior of the leaf (where the air spaces 
