MINUTE STRUCTURE OF LEAVES 129 
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. 
152. 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 sim- 
pler to state at once 
that the epidermis 
serves as a mechan- 
ical protection to 
the parts beneath 
and prevents exces- 
sive evaporation, 
that the palisade- 
cells (which may 
not be made out 
very clearly in a 
roughly prepared 
section) hold large 
quantities of the 
green coloring mat- 
ter of the leaf in a 
position where it 
can receive enough 
but not too much 
sunlight, and that 
the cells of the 
Fie. 94. A Stoma of Thyme. (Greatly 
magnified.) 
A, section at right angles to surface of leaf; B, 
surface view of stoma. cu, cuticle; g, guard- 
cells; s, stoma; e, epidermal cells; a, air 
chamber; c¢, cells of spongy parenchyma with 
grains of chlorophyll. 
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 serve to 
store and to distribute it), they allow oxygen and carbonic 
