and all beyond, with mean divergence of successive leaves, effect a more
thorough distribution, but with less and less angular distance between the
vertical ranks."

[Footnote 1: Gray's Structural Botany, Chap, iv, p. 126.]

For directions for finding the arrangement of cones, see Gray's Structural
Botany, Chap. IV, Sect. 1.

The subject appears easy when stated in a text-book, but, practically, it
is often exceedingly difficult to determine the arrangement. Stems often
twist so as to alter entirely the apparent disposition of the leaves. 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.]