y fruits
are said to have wings, with the general understanding that they are
by this means better fitted to be sustained in air. We shall find
that all or nearly all flattened seeds and dry fruits, also winged
seeds and fruits, are one-sided, unbalanced, and more or less
twisted; consequently, in falling to the ground they whirl about,
and are thus kept much longer in the air than they would be if shaped
more like a winged arrow. Even the wings on the fruit of some of the
ashes are twisted, though many of them are flat. Experiments with
these things are sure to interest inquisitive children, or even older
persons, when once started right; they are likely to prove as
interesting as flying kites, skating, fishing, or coasting on the
hillside. Try experiments with seeds of catalpa, trumpet-creeper,
wild yam, pine, spruce, arbor vitae, and fruits of maple, box elder,
birch, hop tree, blue beech, ailanthus, ash, tulip tree,--in fact,
anything of this nature you can find, whether the name is familiar
or not. No two of them will behave in all respects alike.

[Illustration: FIG. 42. Winged seed of pine. Want of symmetry causes
it to whirl about while falling.]

34. Plants which preserve a portion of their seeds for an
emergency.--Many a great general or business man has learned by
experience and observation that it is usually unwise to exhaust all
resources in one effort. If possible, he always plans to have
something in reserve for an emergency--a loophole for escape from
difficulty. We have seen in many instances that plants are endowed
with the same trait. This is well illustrated by the way in which
the jack-pine, _Pinus_ [_Banksiana_] _divaricata_, holds in reserve
a portion of its seeds, to be used in case the parent trees are killed
by fire. In 1888 I made a study of this tree as it lives on the sandy
plains of Michigan. The tree is often killed by fire, and never sprouts
from the stump, as do oaks, willows, cherries, and most other trees.
The jack-pine grows readily and rapidly from seed dropped on the sand,
and begins to bear cones and seeds in abundance while it is yet only
a few years old, perhaps as young as five years in some instances.
The cones open slowly to liberate their seeds, some of them only after
months or even years, and in some cases they never open at all. I
have seen cones containing good seeds that had been nearly grown over
by the tree. Dry weather, the dryer and hotter the better, causes
many