Angiosperms perform double fertilization

In most angiosperm species, the mature pollen grain consists of two cells: a large tube cell enclosing a much smaller generative cell. When a compatible pollen grain lands on the stigma of a plant of the same species, it germinates, and the pollen tube grows through the style tissue to the embryo sac. During this process, the generative cell undergoes one mitotic division and cytokinesis to produce two haploid sperm cells (Figure 37.5, steps 1 and 2).

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Figure 37.5 Double Fertilization Two sperm are involved in two cell fusion events, hence the term “double fertilization.” One sperm is involved in the formation of the diploid zygote, and the other results in the formation of the triploid endosperm cell that divides to form endosperm. Double fertilization is a characteristic feature of angiosperm reproduction.

Animation 37.1 Double Fertilization

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Two fertilization events now occur. One of the two synergids degenerates when the pollen tube arrives and the two sperm cells are released into its remains (Figure 37.5, step 3). Each sperm cell then fuses with a different cell of the embryo sac (Figure 37.5, steps 4 and 5). One sperm cell fuses with the egg cell, and the two nuclei fuse, producing the diploid zygote. The other sperm cell fuses with the central cell, and its nucleus fuses with the two polar nuclei, forming a triploid (3n) cell. Immediately after fertilization the triploid nucleus undergoes rapid mitotic divisions to form a specialized nutritive tissue, the endosperm. In most species the endosperm nucleus initially divides without cytokinesis, forming a large, multinucleate cell, and cell walls form later between the nuclei. After the endosperm begins developing, the zygote undergoes mitotic division to form the new sporophyte embryo. The developing embryo uses the endosperm tissue as a source of nutrients, energy, and carbon-based anabolic building blocks. In some cases the endosperm persists until germination and is used as a source of nutrients by the developing seedling; this is prevalent in grains such as corn, wheat, and rice. The source of nutrients is important because the seedling often begins its development underground and cannot perform photosynthesis right away.

The remaining cells of the male and female gametophytes—the antipodal cells, the remaining synergid, and the pollen tube nucleus—degenerate as the embryo begins to develop.

Double fertilization, a hallmark of angiosperms, is so named because it involves two cell fusion events:

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The fusion of a sperm cell with the central cell to form the triploid endosperm nucleus is one of the defining characteristics of angiosperms.