3. _Transpiration_.--This term is used to denote the evaporation of water
from a plant. The evaporation takes place principally through breathing
pores, which are scattered all over the surface of leaves and young stems.
The _breathing pores_, or _stomata_, of the leaves, are small openings
in the epidermis through which the air can pass into the interior of the
plant. Each of these openings is called a _stoma_. "They are formed by a
transformation of some of the cells of the epidermis; and consist usually
of a pair of cells (called guardian cells), with an opening between
them, which communicates with an air-chamber within, and thence with the
irregular intercellular spaces which permeate the interior of the leaf.
Through the stomata, when open, free interchange may take place between
the external air and that within the leaf, and thus transpiration be
much facilitated. When closed, this interchange will be interrupted or
impeded."[1]

[Footnote 1: Gray's Structural Botany, page 89. For a description of the
mechanism of the stomata, see Physiological Botany, p. 269.]

In these lessons, however, it is not desirable to enter upon subjects
involving the use of the compound microscope. Dr. Goodale says: "Whether
it is best to try to explain to the pupils the structure of these valves,
or stomata, must be left to each teacher. It would seem advisable to
pass by the subject untouched, unless the teacher has become reasonably
familiar with it by practical microscopical study of leaves. For a teacher
to endeavor to explain the complex structure of the leaf, without having
seen it for himself, is open to the same objection which could be urged
against the attempted explanation of complicated machinery by one who has
never seen it, but has heard about it. What is here said with regard to
stomata applies to all the more recondite matters connected with plant
structure."[1]

[Footnote 1: Concerning a few Common Plants, p. 29.]

There are many simple experiments which can be used to illustrate the
subject.

(1) Pass the stem of a cutting through a cork, fitting tightly into the
neck of a bottle of water. Make the cork perfectly air-tight by coating it
with beeswax or paraffine. The level of the liquid in the bottle will be
lowered by the escape of water through the stem and leaves of the cutting
into the atmosphere.

(2) Cut two shoots of any plant, leave one on the table and place the
other in a glass of water.[1] The first