beech, and occasionally a maple.

22. Buttonwood balls.--Nature seems to have no end of devices for
sowing seeds to advantage. Here is one which always interests me.
The fruit of the buttonwood, or sycamore, which grows along streams,
is in the form of balls an inch and a half in diameter. These balls
grow on the tops of the highest branches, and hold on into winter
or longer. The stems are about two inches long, and soon after drying,
through the action of the winds, they become very flexible, each
resembling a cluster of tough strings. The slightest breeze moves
them, and they bob around against each other and the small branches
in an odd sort of way. After so much threshing that they can hold
no longer, the little nuts become loosened and begin to drop off a
few at a time. Certain birds eat a few and loosen others, which escape.
The illustration shows some of these nuts, each supplied with a ring
of bristles about the base, which acts as a parachute to permit the
wind the easier to carry them for some distance before falling, or
to drift them on the surface of the snow or ice.

[Illustration: FIG. 29.--(a) Lax stem, supporting (b) fruit of the
buttonwood, or sycamore; (c) a single fruit separated, ready for the
wind or water.]

23. Seeds that tempt the wind by spreading their sails.--On low lands
in the cool, temperate climate of Europe, Asia, and North America,
is a common plant here known as great willow-herb, a kind of fireweed
(_Epilobium angustifolium_). There are several kinds of fireweeds.
This one grows from three to five feet high, and bears pretty pink
flowers. In mellow soil the slender rootstocks spread extensively,
and each year new sprouts spring up all around, six to eight feet
distant. Below each flower ripens a long, slender pod, which splits
open from the top into four parts, that slowly curve away from a
central column. The apex of each seed is provided with a cluster of
white silky hairs nearly half an inch long.

[Illustration: FIG. 30.--Fruit of willow-herb exposing seeds for
distribution by the wind.]

The tips of the hairs stick slightly to the inside of the recurved
valves, some hairs to one valve, and often others to the adjacent
valve, thus spreading them apart with the seed suspended between.
Four rows of the seeds are thus held out at one time. Often not over
half, or even a tenth part, of the seeds are well developed, yet the
silky hairs are present and float away in clusters, thus