152 FOUNDATIONS OF BOTANY 
somewhat kidney-shaped and become more or less curved 
as they are fuller or less full of water (see Sect. 170). 
162. Calculation of Number of Stomata per Unit of Area. 
—JIn order to get a fairly exact idea of the number of 
stomata on a unit of leaf-surface, the most convenient 
Fie. 117. — Epidermis of Leaf of Althza. 
(Much magnified.) 
A, from upper surface; B, from lower surface. 
h, star-shaped compound hairs; st, stomata; p, 
upper ends of palisade-cells, seen through the 
epidermis ; e, cells of epidermis. 
plan is to make 
use of a_photo- 
micrograph. The 
bromide  enlarge- 
ment No. 12 of 
the Tower series 
represents about 
a twenty-five- 
hundredth of a 
square inch of the 
lower epidermis of 
the cyclamen leaf, 
magnified until it 
is about fifteen 
inches square. 
Count the number 
of stomata on the 
entire photograph, 
then calculate the 
number of stomata 
on a square inch 
of the surface of 
this leaf. If a cyclamen plant has twelve leaves, each 
with an average area of six square inches, calculate the 
number of stomata of the lower epidermis of all the leaves 
taken together. 
