ingle
layer, occasionally of two or three layers, of empty cells, mostly of
irregular outline. The sinuous lines which traverse it, and may be
discerned under low powers of the microscope (Fig. 487), are the
boundaries of the epidermal cells.

[Illustration: Fig. 484. Small portion of epidermis of the lower face of
a White-Lily leaf, with stomata.]

[Illustration: Fig. 485. One of these, more magnified, in the closed
state. 486. Another stoma, open.]

[Illustration: Fig. 487. Small portion of epidermis of the Garden
Balsam, highly magnified, showing very sinuous-walled cells, and three
stomata.]

443. =Breathing-pores, or Stomates, Stomata= (singular, a
_Stoma_,--literally, a mouth) are openings through the epidermis into
the air-chambers or intercellular passages, always between and guarded
by a pair of thin-walled guardian cells. Although most abundant in
leaves, especially on their lower face (that which is screened from
direct sunlight), they are found on most other green parts. They
establish a direct communication between the external air and that in
the loose interior of the leaf. Their guardian cells or lips, which are
soft and delicate, like those of the green pulp within, by their greater
or less turgidity open or close the orifice as the moisture or dryness
varies.

444. In the White Lily the stomata are so remarkably large that they may
be seen by a simple microscope of moderate power, and may be discerned
even by a good hand lens. There are about 60,000 of them to the square
inch of the epidermis of the lower face of this Lily-leaf, and only
about 3000 to the same space on the upper face. It is computed that an
average leaf of an Apple-tree has on its lower face about 100,000 of
these mouths.

Sec. 5. PLANT FOOD AND ASSIMILATION.

445. Only plants are capable of originating organizable matter, or the
materials which compose the structure of vegetables and animals. The
essential and peculiar work of plants is to take up portions of earth
and air (water belonging to both) upon which animals cannot live at all,
and to convert them into something organizable; that is, into something
that, under life, may be built up into vegetable and animal structures.
All the food of animals is produced by plants. Animals live upon
vegetables, directly or at second hand, the carnivorous upon the
herbivorous; and vegetables live upon earth and air, immediately or at
second hand.

446. =The Food of plants=, then,