vule to become a seed; or yet again,
it is the union of these two living substances that enables the seed to
develop.

[Illustration: THE POLLEN TUBE PASSING THROUGH THE STYLE TO THE OVARY]

To understand how the pollen substance finds its way to the ovule
substance let us examine the pollen grain a little more carefully.
Pollen grains are of many shapes, though usually they are globe-shaped,
or football-shaped. Tiny as they are, the outer skin is often marked
with grooves and ridges in a very ornamental manner. They have two
skins, an outer hard one, a softer inner one. The outer skin is not
equally thick and hard all over. It has little glazed spots sometimes,
like little glazed windows. Now, when the pistil is ripe the stigma is
_sticky_. When the pollen grain falls upon this sticky stigma its inside
wall swells up, just as the bean does when we soak it. But the outside
wall cannot swell, consequently the inner wall finally breaks through at
one of the weak spots in the outer wall. Then the inner wall absorbing
moisture and nutriment from the stigma actually grows, becoming a tube,
which finds its way down through the style. The living substance of the
pollen grain runs into the tip of this tube, and so is carried with it
down through the style. The tube is nourished by the juices of the style
as it goes along, and finally it gets to the ovary and the ovule. Every
ovule has a tiny opening, or micropyle as it is called, and it is now
easy to guess what that is for. The pollen tube pushes straight toward
the micropyle, enters into the ovule through the micropyle, and then the
living substance it has carried all this distance in its tip breaks
through its delicate wall and mingles with the living substance of the
ovule. When this has happened, the ovule begins to grow and to develop
into a seed.

[Illustration: ENTRANCE OF POLLEN-TUBE THROUGH THE MICROPYLE TO THE
OVULE]

We see that the whole pollen grain could not possibly force its way
down to the ovule. It cannot move of itself, for one thing, and if it
could it is too large to pass between the tissues of the style. So it
simply sends down the long tube, which grows fast, pushing along through
the style, whose tissues are rather loose, and carrying with it the only
valuable part of the pollen grain, its living protoplasm. No ovule can
possibly grow into a grain without this tiny bit of pollen.

In explaining this union of the two protoplasms, the child's mind ca