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PHARMACEUTICAL BOTANY 
D. Bifacial. — Leaves with laminae which stand edge on in relation 
to the sun’s rays. The best illustrations are seen among dicotyle- 
dons, such as Eucalyptus, Callistemnon, and other genera of Myrtaceae. 
Both surfaces are similar, having stomata about equal in number. 
The mesophyll is differentiated into a central spongy parenchyme 
containing bundles, and a zone of palisade cells on either side facing 
the surfaces. 
Structure and Development of Stomata.— Stomata are slit-like 
openings in the epidermis of leaves or young green stems surrounded 
by a pair of cells called guard cells whose sides opposite one another 
are concave. They form a communication between the intercellular- 
air-space (respiratory cavity) beneath them and the exterior. The 
slit-like opening taken with the guard cells, constitutes what is known 
as the stomatal apparatus. 
The epidermal cells which abut on the stomatal apparatus are 
called neighboring cells or subsidiary cells. These in many cases, as 
in species of Helleborus, Sambucus, Hyacinthus, Pceonia, Ferns, etc., 
are very similar to the other epidermal cells but in a large number of 
plants they differ in size, arrangement and shape from the other cells 
of the epidermis which do not abut upon the stomatal apparatus. In 
Senna they are two in number one larger than the other and arranged 
parallel to the guard cells of the stoma; in Coca a similar arrangement 
occurs but the cells are more even in size, nevertheless they lack the 
characteristic papillae found on the other epidermal cells; in Pilo- 
carpus they are usually four in number but quite narrow and more or 
less crescent-shaped; in Uva Ursi their number is usually seven to 
eight and their arrangement radial around the stomatal apparatus. 
On all dorsoventral leaves, the stomata arise more abundantly on 
the lower epidermis, less abundantly on the upper. Exceptions to 
this rule are due to the peculiar readaptation of the leaf to its sur- 
roundings. Thus, in the reversed types of leaves (twisted in a half 
circle) the stomata, formerly on the lower surface, have migrated to 
the upper surface which now has become the physiological lower 
surface. 
In Umbrophytic (shade) plants the stomata are either wholly on 
the lower surface or partly so with a number on the upper surface. 
Where the plants are Mesophytic and exposed to dense sunlight and 
