[Illustration: FIG. 24.—Series of palmately-veined leaves.]
[Illustration: FIG. 25.—Series of pinnately-veined leaves.]
Let the pupils compare the distribution of the veins in their specimens. They have already distinguished parallel-veined from netted-veined leaves, and learned that this difference is a secondary distinction between monocotyledons and dicotyledons.[1] The veins in netted-veined leaves are arranged in two ways. The veins start from either side of a single midrib (feather-veined or pinnately-veined), or they branch from a number of ribs which all start from the top of the petiole, like the fingers from the palm of the hand (palmately-veined). The compound leaves correspond to these modes of venation; they are either pinnately or palmately compound.
[Footnote 1: See page 34.]
These ribs and veins are the woody framework of the leaf, supporting the soft green pulp. The woody bundles are continuous with those of the stem, and carry the crude sap, brought from the roots, into the cells of every part of the leaf, where it is brought into contact with the external air, and the process of making food (Assimilation 4) is carried on. "Physiologically, leaves are green expansions borne by the stern, outspread in the air and light, in which assimilation and the processes connected with it are carried on."[1]
[Footnote 1: Gray's Structural Botany, p. 85.]
The whole leaf is covered with a delicate skin, or epidermis, continuous with that of the stem.[1]
[Footnote 1: Reader in Botany. XI. Protection of Leaves from the Attacks of Animals.]
2. Descriptions.—As yet the pupils have had no practice in writing technical descriptions. This sort of work may be begun when they come to the study of leaves. In winter a collection of pressed specimens will be useful. Do not attach importance to the memorizing of terms. Let them be looked up as they are needed, and they will become fixed by practice. The pupils may fill out such schedules as the following with any leaves that are at hand.