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How does an ovule become a seed after fertilization?

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Bumblebee on teasel — plant reproductive system A large bumblebee (*Bombus* species) pollinating a teasel flower head (*Dipsacus* species).

In flowering plants, the transformation of an ovule into a seed is driven by a unique process called double fertilization.12

After pollen reaches the ovule, it releases two generative nuclei (sperm cells). One nucleus fuses with the egg cell to form a diploid zygote, which undergoes mitotic divisions to develop into the embryo. The second generative nucleus fuses with two polar nuclei to create a triploid nucleus, which develops into the endosperm—a nutrient-rich tissue that supports the growing embryo.12

As these internal structures develop, the protective integuments of the ovule harden to form the seed coat, or testa. The resulting seed contains the embryo, storage tissue (endosperm or other reserves), and the seed coat. While the ovule matures into a seed, the surrounding ovary typically develops into a fruit.132