We have studied leaves as cotyledons, bud-scales, etc., but when we speak
of _leaves_, we do not think of these adapted forms, but of the green
foliage of the plant.
1. _Forms and Structure_.--Provide the pupils with a number of green
leaves, illustrating simple and compound, pinnate and palmate, sessile and
petioled leaves. They must first decide the question, _What are the parts
of a leaf_? All the specimens have a green _blade_ which, in ordinary
speech, we call the leaf. Some have a stalk, or _petiole_, others are
joined directly to the stem. In some of them, as a rose-leaf, for
instance, there are two appendages at the base of the petiole, called
_stipules_. These three parts are all that any leaf has, and a leaf that
has them all is complete.
Let us examine the blade. Those leaves which have the blade in one
piece are called _simple_; those with the blade in separate pieces are
_compound_. We have already answered the question, _What constitutes a
single leaf_?[1] Let the pupils repeat the experiment of cutting off the
top of a seedling Pea, if it is not already clear in their minds, and find
buds in the leaf-axils of other plants.[2]
[Footnote 1: See page 31.]
[Footnote 2: With one class of children, I had much difficulty in making
them understand the difference between simple and compound leaves. I did
not tell them that the way to tell a single leaf was to look for buds in
the axils, but incautiously drew their attention to the stipules at the
base of a rose leaf as a means of knowing that the whole was one. Soon
after, they had a locust leaf to describe; and, immediately, with the
acuteness that children are apt to develop so inconveniently to their
teacher, they triumphantly refuted my statement that it was one leaf, by
pointing to the stiples. There was no getting over the difficulty; and
although I afterwards explained to them about the position of the buds,
and showed them examples, they clung with true childlike tenacity to their
first impression and always insisted that they could not see why each
leaflet was not a separate leaf.]
An excellent way to show the nature of compound leaves is to mount a
series showing every gradation of cutting, from a simple, serrate leaf to
a compound one (Figs. 24 and 25). A teacher, who would prepare in summer
such illustrations as these, would find them of great use in his winter
lessons. The actual objects make an impression that the cuts in the book
cannot give.
[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.