MINUTE STRUCTURE OF LEAVIS 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 
: - A, section at right angles to surface of leaf; B, 
can receive enough surface view of stoma. cu, cuticle; g, guard- 
but not too much cells; s, stoma; e, epidermal cells; a, air 
sunlight, and that a ago ci cle 
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 serve to 
store and to distribute it), they allow oxygen and carbonic 
Fic. 94. A Stoma of Thyme. (Greatly 
magnified.) 
