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When a pollen grain reaches the stigma, a pollen tube extends from the grain, grows down the style, and enters through an opening in the integuments of the ovule. The two sperm cells are deposited in the embryo sac.

What occurs next is called a double fertilization event ( [link] ) and is unique to angiosperms. One sperm and the egg combine, forming a diploid zygote—the future embryo. The other sperm fuses with the diploid nucleus in the center of the embryo sac, forming a triploid cell that will develop into the endosperm: a tissue that serves as a food reserve. The zygote develops into an embryo with a radicle, or small root, and one or two leaf-like organs called cotyledons . Seed food reserves are stored outside the embryo, and the cotyledons serve as conduits to transmit the broken-down food reserves to the developing embryo. The seed consists of a toughened layer of integuments forming the coat, the endosperm with food reserves and, at the center, the well-protected embryo.

Illustration shows three panels. The first has a megagametophyte (embryo sac) with an egg at the bottom and 2 polar nuclei in the middle of the sac. A pollen tube containing a tube nucleus and 2 sperm nuclei is beside the embryo sac. The second panel shows the pollen tube penetrating the embryo sac and releasing the 2 sperm nuclei into the sac. One sperm nucleus fertilizes the 2 polar nuclei, and one sperm fertilizes the egg. The tube nucleus degenerates. The third panel shows the 2 n embryo developed from the fertilized egg, and the 3 n endosperm developed from the fertilization of the 2 polar nuclei. The seed coat has developed from the tissue surrounding the embryo sac.
Double fertilization occurs only in angiosperms. (credit: modification of work by Mariana Ruiz Villareal)

Most flowers carry both stamens and carpels; however, a few species self-pollinate. These are known as “perfect” flowers because they contain both types of sex organs ( [link] . Biochemical and anatomical barriers to self-pollination promote cross-pollination. Self-pollination is a severe form of inbreeding, and can increase the number of genetic defects in offspring.

A plant may have perfect flowers, and thus have both genders in each flower; or, it may have imperfect flowers of both kinds on one plant ( [link] ). In each case, such species are called monoecious plants, meaning “one house.” Some botanists refer to plants with perfect flowers simply as hermaphroditic. Some plants are dioecious, meaning “two houses,” and have male and female flowers (“imperfect flowers”) on different plants. In these species, cross-pollination occurs all the time.

Table with 2 columns. Column on the left has monoecious flowers, column on the right has dioecious flowers. The monoecious flowers are a tulip with male and female structures in one flower, and a begonia plant with male and female flowers on one plant. The dioecious flowers are from 2 separate pussy willow plants, one male and one female.
Monoecious plants have both male and female reproductive structures on the same flower or plant. In dioecious plants, males and females reproductive structures are on separate plants. (credit a: modification of work by Liz West; credit c: modification of work by Scott Zona)

Diversity of angiosperms

Angiosperms are classified in a single division, the Anthophyta    . Modern angiosperms appear to be a monophyletic group, which means that they originate from a single ancestor. Flowering plants are divided into two major groups, according to the structure of the cotyledons, the pollen grains, and other features: monocots , which include grasses and lilies, and eudicots    or dicots , a polyphyletic group. Basal angiosperms are a group of plants that are believed to have branched off before the separation into monocots and eudicots because they exhibit traits from both groups. They are categorized separately in many classification schemes, and correspond to a grouping known as the Magnoliidae. The Magnoliidae group is comprised of magnolia trees, laurels, water lilies, and the pepper family.

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Source:  OpenStax, University of georgia concepts of biology. OpenStax CNX. May 28, 2013 Download for free at https://legacy.cnx.org/content/col11526/1.2
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