Pollen Grains (Magnified), and Stigma
The pollen grain is also a little sac containing protoplasm. Thus we have these two little sacs of living substance, each growing in a similar manner, one to the inside of an ovary, the other to the inside of an anther. Naturally, it is the living substance in these little sacs that is important. It is the living substance of the ovule that unites with the living substance of the pollen grain to become a seed; or, to say the same thing another way, it is the living substance of the pollen grain that unites with that of the ovule to become a seed; or yet again, it is the union of these two living substances that enables the seed to develop.
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.
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 can be turned upon the wonderful mystery—one of the great mysteries of the universe—of how this tiny atom can influence the whole future plant. There is ample opportunity here to elevate his mind and spirit to a high plane, and, by talking of the wonders of inheritance, to give many a hint for future reflection. Without this law of inheritance the world would be chaos. Imagine the seed of a rose sometimes developing into an oak tree, the egg of a bird into a bee or a trout. Imagine eggs developing haphazard into anything. There would be no use in living. Nothing could be depended upon. But there is no danger that any such thing will happen: the law of inheritance is unyielding. From a rose seed must come a rose bush,—and this is good. But on the other hand, from the seed of a weak, poor plant will grow another weak, poor plant. Whatever the parent is, good or bad, that must the offspring be. But sometimes the offspring inherits only the best in the parents, and so is better than they.