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.]
[Illustration: FIG. 22.]
[Illustration: FIG. 23.]
Gray's First Lessons. Sect. IV. VII, §4. How Plants Grow. Chap. I, 51-62; I, 153.