One of the most elegant processes in all of nature is how the egg cell of flowering plants – the angiosperms – is fertilized (angiosperm = enclosed seed). It is a process that sets them apart from all other plant groups. It is a genetic and architectural marvel, and fairly simple to describe – that is, if I can keep the techno-terms to a minimum. I may also have trouble reigning in certain innuendos. After all, this is what the bees are up to in “the birds and the bees.”
The story begins after the bees have done their work of transporting male pollen grains from one flower to the compatible female receptor of another. This receptor is called the stigma, and it is at the outer end of a usually elongate structure called the style. At the other end of the style is the ovary where the eggs are stored. How the two male sperm cells – that’s right, two – within the pollen grain reach an egg in the ovary is the first of the marvels in this process.
There will be more to say in a moment about why there are two male sperm cells in a pollen grain when only one is needed to fertilize the egg cell. But first we need to look at the pollen grain itself. It is made up of three cells: the two sperm cells and a third cell called the tube nucleus. When a pollen grain alights on the female stigma, the tube nucleus cell begins to burrow into the tissue of the stigma, building a very fine tube down through the style to the ovary.
While the tube is under construction, the two sperm cells are positioned near the advancing head of this “tunnel of love.” Once the engineering marvel reaches the ovary, the tube ruptures, allowing the two sperm cells to enter the ovary directly. The sperm cells are not redundant; they have separate tasks. One of them fertilizes the egg cell, and the other sets off on a journey, the conclusion of which is one of nature’s greatest miracles. How great? Well, for one thing, it provides two-thirds of our caloric intake, and we might not have evolved without it.
Elsewhere in the ovary, and separate from the egg cell, are two cells produced by the mother plant, and called the polar nuclei. Just before the boys in the pollen tube arrive, these two female cells fuse together to form a single structure called the fusion nucleus. This fused cell now contains two sets of female chromosomes, and it is the structure the second sperm cell is seeking. When it is found, another mating occurs, and the primary endosperm nucleus is born. It contains three sets of chromosomes: one male and two female.
As its name suggests, this is the structure responsible for creating the endosperm, the food that will feed the seed and a world of animals. It is the flesh of the banana, and it is our daily bread: corn, wheat, and rice. Because the endosperm is born of two parents, it possesses what is known as hybrid vigor. This may have given flowering plants considerable advantage during the early stages of evolution, perhaps by enabling quicker reproduction and greater ability to occupy new places.
And of course it is a trait that has been retained ever since, though often modified. Flowering plants rule the world because their kids get such a great head start, a lunch packed by both parents.
Richard LeBlond is a retired biologist living in North Carolina. His essays and photographs have appeared in many U.S. and international journals, including Montreal Review, Redux, Compose, Concis, Lowestoft Chronicle, Trampset, and Still Point Arts Quarterly. His work has been nominated for “Best American Travel Writing” and “Best of the Net.”