of these stubborn old cones to open their scales and allow the
seeds to escape. What can be the advantage in cones of this nature?
Let us see. A brisk fire passes over the ground at irregular intervals,
usually of from one to ten years; it licks up all dry leaves and sticks,
and kills the pine trees and all else above ground. The soil and the
trunks of trees are blackened, and by lack of reflection the heat
of the sun is rendered more intense; besides, the heat of the fire
acts slowly on the unburned cones as they are left on the dead trees.
By the time the quick hot fire has passed over, the cones have slowly
opened and begun scattering seeds on the vacant and newly burned
ground, at a time when there is the best possible chance for them
to grow. I picked a few unopened cones which, according to my judgment,
were from two to four years old. They were placed under glass in a
dark sheet-iron dish and exposed to the sun. The extra heat caused
the cones to open; many seeds were obtained and sown, and in five
days they began to come up, 95 per cent germinating. From the same
tree I selected at the same time older cones, which I believe to be
from four to six years old at least. From these, 225 seeds were sown,
191 of which germinated--about 85 per cent.

[Illustration: FIG. 43.--Cone of jack-pine closely covering its
seeds, often for several years.]

[Illustration: FIG. 44.--Cone of jack-pine as opened by heat, sowing
seeds.]

CHAPTER VI.
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