helping to
buoy those that are heavy. This is a capital scheme, for when the
pods are dry and unfurled, they silently indicate to the slightest
breath of air that they are ready for a flight, and it doesn't take
much to carry them for a long distance. As an active boy delights
to venture again and again over thin ice on a shallow pond in the
pasture, half fearing, yet half hoping, that he may become a hero
by breaking through and escaping, so likewise many of these seeds
and seed-like fruits spread themselves out, as if to tempt the wind
to come along and attack them.

The twin fruits of the parsnip and some of its near relatives are
light and thin and split apart, each holding on lightly to the top
of a slender stem. In this position they are sure to be torn off sooner
or later. Somewhat after the manner of the willow-herb behave the
pods and seeds of willows, poplars, milkweeds, Indian hemp, and
cotton.

[Illustration: FIG. 31.--Dry twin fruits of the parsnip held by
slender stems ready to be blown away. (Much enlarged.)]

24. Why are some seeds so small?--Do you know why so many kinds of
plants produce very small and light seeds? Would it not be better
if they produced fewer and larger seeds, which would then be stronger
and better able to grow under adverse conditions? But a large number
of small seeds cost the plant no more effort than a small number of
large ones, and the lighter and smaller the seeds and the more there
are of them, the better their chances for distribution, especially
for long distances. The minute size of spores of most of the fungi
are given as reasons why so many of them are so widely distributed.

Why is a boy or man of light weight chosen to ride the horse on the
race track? That the animal may have less weight to carry and thereby
use his surplus strength in making better time. The less weight the
parachute of the seed of the willow-herb has to carry, the greater
the chances for success in making a long journey. Of the willow-herb
it takes one hundred seeds to weigh a milligram, including the hairs
attached to them, and it would take thirty thousand to weigh as much
as an ordinary white bean.

25. Seeds with parachutes.--Many years ago large portions of Huron
and Sanilac counties of eastern Michigan were swept by a fire so severe
that the timber was all killed. Fifteen years later the woody growth
consisted mostly of willows, poplars, and birches. The seeds of all
kinds of willows a