The
general principle, however, that the leaves are disposed so as to get the
best exposure to air and light is clear. This cannot be shown by the study
of the naked branches merely, because these do not show the beautiful
result of the distribution.[1] Many house plants can be found, which will
afford excellent illustrations (Fig. 21). The Marguerite and Tobacco, both
easily grown in the house, are on the 3/8 plan. The latter shows the eight
ranks most plainly in the rosette of its lower leaves. The distribution is
often brought about by differences in the lengths of the petioles, as in
a Horsechestnut branch (Fig. 22) where the lower, larger leaves stand
out further from the branch than the upper ones; or by a twist in the
petioles, so that the upper faces of the leaves are turned up to the
light, as in Beech (Fig. 23). If it is springtime when the lessons are
given, endless adaptations can be found.
[Footnote 1: Reader in Botany. IX. Leaf-Arrangement.]
[Illustration: FIG. 21. Branch of Geranium, viewed from above.]
[Illustration: FIG. 22.]
[Illustration: FIG. 23.]
_Gray's First Lessons_. Sect. IV. VII, sec. 4. _How Plants Grow_. Chap. I,
51-62; I, 153.
V.
STEMS.
The stem, as the scholars have already learned, is the axis of the plant.
The leaves are produced at certain definite points called nodes, and the
portions of stem between these points are internodes. The internode,
node, and leaf make a single plant-part, and the plant is made up of a
succession of such parts.
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