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 will soon wilt, while the other will remain fresh.  If the latter shoot be a cutting from some plant that will root in water, such as Ivy, it will not fade at all.  Also, leave one of the plants in the schoolroom unwatered for a day or two, till it begins to wilt.  If the plant be now thoroughly watered, it will recover and the leaves will resume their normal appearance.

[Footnote 1:  Lessons in Elementary Botany, by Daniel Oliver, London.  Macmillan & Co., 1864, pp. 14-15.]