well to put the pressed leaves in their note books, as it is difficult to
write in the books without spoiling the specimens. It is better to mount
the specimens on white paper, keeping these sheets in brown paper covers.
The pupils can make illustrations for themselves by sorting leaves
according to the shapes, outlines, etc., and mounting them.


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