Angiosperm Reproduction and Double Fertilization

Core Idea

In flowering plants (angiosperms), the male gametophyte is the pollen grain and the female gametophyte is the embryo sac inside an ovule. After pollination, a pollen tube grows to the ovule and delivers two sperm. One sperm fertilizes the egg to form a diploid zygote, and the other fuses with the two polar nuclei of the central cell to form a triploid cell that develops into endosperm, the seed's food tissue. This double fertilization is a defining feature of flowering plants; afterward the ovule becomes a seed and the ovary becomes a fruit.

How It's Best Learned

Dissect a lily or tulip flower and find the anthers, stigma, style and ovary, then cut the ovary to see the ovules. Draw the embryo sac with its seven cells and eight nuclei and track the two sperm with different colors. Ending with a bean seed and a corn kernel connects the process to food students eat.

Common Misconceptions

• Pollination and fertilization are the same event. (Pollination is the transfer of pollen to a stigma; fertilization happens later, when sperm delivered by the pollen tube fuse with cells in the ovule.)
• A fruit develops from the ovule. (The ovule becomes the seed; the fruit develops from the ovary wall that surrounds the ovules.)
• Endosperm is part of the embryo. (Endosperm is a separate triploid tissue formed by the second fertilization, and it feeds the embryo.)

Explainer

A flower is a reproductive shoot. Its outer whorls, the sepals and petals, protect the bud and attract pollinators. Inside are the stamens, each an anther on a filament, and in the center one or more carpels, each with a sticky stigma, a stalk called the style and an ovary at the base. The ovary holds one or more ovules.

Both gametophytes form inside the flower. In the anther, diploid cells called microsporocytes divide by meiosis to make haploid microspores. Each microspore divides by mitosis into a pollen grain with two cells: a tube cell and a generative cell. The generative cell will divide to make two sperm. In each ovule, a diploid megasporocyte divides by meiosis to make four haploid megaspores. In most species only one survives. Its nucleus divides three times by mitosis, giving eight nuclei that are partitioned into seven cells. This is the embryo sac, the female gametophyte. Near one end is the egg cell, flanked by two synergids. In the middle is a large central cell with two polar nuclei. At the far end are three antipodal cells.

Pollination moves pollen to a stigma. A compatible grain germinates, and its tube cell grows a pollen tube down through the style to an ovule, entering through a small opening called the micropyle. The two sperm travel down the tube and are released into the embryo sac.

Now comes double fertilization, first described by Sergei Nawaschin in 1898. One sperm fuses with the egg, forming a diploid zygote that will become the embryo. The other sperm fuses with the two polar nuclei in the central cell, forming a triploid nucleus. That cell divides to form the endosperm, a tissue rich in starch, oils and proteins. In corn and wheat the endosperm persists in the mature seed, and it makes up most of a cereal grain. In beans and peas the embryo's seed leaves (cotyledons) absorb the endosperm's food before the seed matures.

After fertilization, the ovule develops into a seed: an embryo, its food supply and a protective seed coat. The ovary wall grows into a fruit, which protects the seeds and often helps disperse them, for example by attracting animals that eat the fruit and drop the seeds elsewhere.

Double fertilization ties the food supply to the embryo. Endosperm forms only in an ovule that has been fertilized, so the plant does not spend resources on seeds that would have no embryo.