PLANTS THAT SHOOT OFF THEIR SPORES OR SEEDS.
By numerous devices a large number of the lower plants send off their
ripe spores with considerable force. Some call them sling fruits.
One in particular, _Pilobolus cristallinus_, found about damp
stables, I have observed to shoot black masses of spores to a spot
on a wall six feet above the ground, with enough force to have carried
them not less than twelve feet. When ripe and dry, the spores of most
ferns are shot from the parent plant by a motion forcible enough not
only to burst the _sporangium_, the vessel that contains the spores,
but also to turn it inside out.
[Illustration: FIG. 45.--Spores of _Pilobolus_ before and while
shooting its spores.]
35. Dry pods twist as they split open and throw the seeds.--In December,
while absent from home, I collected for future study some pods of
the Chinese wistaria, and left them on my desk in the library for
the night. The house was heated by a hot-air furnace. In the morning
the pods were in great confusion; most of them had split and curled
up, and the seeds were scattered all about the room. As usual the
little daughter, an only child, was accused of spoiling my specimens,
but she showed her innocence. A little investigation and a few
experiments with some pods not yet opened explained the whole matter
satisfactorily. The stout pods grow and ripen in a highly strained
condition, with a strong tendency to burst spirally, the two
half-pods being ready to coil and spring in opposite directions; when
the valves can no longer hold together, they snap with a sharp noise
and sling the heavy seeds, giving them a good send-off into the world.
As a pair of birds build a nest, hatch eggs, rear their young, and
then send them forth to seek their fortunes, so for months the mother
plant had labored, had produced and matured seeds, which at last it
scattered broadcast. Goethe, Kerner von Marilaun, each independently,
and very likely others, had an experience with ripe pods brought to
a warm room very similar to my own. In many cases the ripe and drying
fruits are "touched off" by wind jostling the branches or by animals
passing among them; in the latter case there is a chance that a portion
of the discharges will be lodged somewhere on the animal and be carried
along with it.
36. A seed case that tears itself from its moorings.--The perennial
phlox in cultivation distributes its seeds in the following manner:
when ripe, the calyx becomes dry and paper-like, and spreads out in
the form of a saucer. The thick-walled dry pistil opens from the top
into three pieces with a snap, spreading open so far against the calyx
that it is torn from the brittle attachment; away go the seeds, mingled
with the fragments of the pistil, no longer of any use.
[Illustration: FIG. 46.--A dry pod of wild bean bursting spirally
to throw the seeds.]
[Illustration: FIG. 47.--Fruit of violet partially dried and split
into three pieces, each piece pinching the seeds so closely that
sooner or later all are thrown out.]
Fruits that sling their seeds are to be found in every neighborhood,
and are first-class objects for the curious person to see and handle.
Very fortunate is the girl or boy who is never fully satisfied with
what he reads and sees pictured, but has a strong desire to learn
how plants are made and how they behave. A considerable number of
seed pods have been illustrated with notes in recent schoolbooks.
Here are some of them: peas and vetches, and some kinds of beans,
violets, balsams, wood sorrel, geranium, castor bean, some of the
mustards and cresses and their cousins, Alfilerilla, richweed,
_Pilea_, witch-hazel, and others. Each of those will well repay study,
especially the fruit and seeds of oxalis. The witch-hazel bears a
hard, woody, nut-like fruit, as large as a hazelnut; when ripe, the
apex gaps open more and more, the sides pressing harder against each
smooth seed, till finally it is shot, sometimes for a distance of
thirty feet. The girl who has shot an apple seed or lemon seed with
pressure of thumb and finger across a small room, can understand the
force needed to shoot a seed but little heavier than that of the apple
two or three times that distance.
[Illustration: FIG. 48.--Dry fruit of witch-hazel shooting seeds.]