Flowering Plant reproduction; Meiosis

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Pollen (male gametophyte) development

Within this group study the illustrations in sequence.

  • Young Lily anther x40. Four pollen sacs and a cross section of the fillament are visible. Diploid (2N chromosomes) microspore mother cells in the middle of the pollen sacs undergo meiosis to make microspores which later become pollen grains. The large green cells lining the pollen sacs provide nourishment for the developing pollen grains.
  • Megaspore tetrads x400. Meiosis produces groups of four haploid (N chromosomes) megaspores which are initially attached to each other as shown here. In some of the illustrated megaspore groups you can't see all four meagaspores because some are out of the plain of section.
  • One celled microspores x400 which will soon divide by mitosis to form two nucleate pollen grains. One of the illustrated spores has just finishing this division. An elaborate pollen wall is forming at this stage of development.
  • Mature pollen grains x400 have two haploid nuclei such as the specimen on the left, and an elaborate wall. Multinucleate pollen grains are young male gametophytes.
  • Mature anther x40. When the pollen is fully formed the pollen sacs break open and pollen is released.
  • Stigma and pollen x100. When pollen is carried by the wind or an animal to the hairy stigma of a flower the pollen germinates and makes a pollen tube. Several purple staining pollen grains are visible with green staining pollen tubes emerging from them.
  • Pollen grain with pollen tube x400. This grain has been germinated in sugar water. When a pollen grain germinates, one of its two nuclei divides by mitosis to produce two sperm nuclei. The generative nucleus and then the two sperm nuclei follow the tip of the pollen tube as it grows toward the ovary and the ovules inside. A germinated pollen grain with pollen tube and three haploid nuclei is the mature male gametophyte in flowering plants.
  • Pollen tubes grow down the hollow center of the style as they head toward the ovary.

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    Female Gametophyte development, Meiosis, Fertilization

    Within this group study the illustrations in the sequence.

  • Lily ovary xs x20 has three chambers that result from the fusion of the three modified leaves (carpels) that formed the ovary. Each of the three chambers has two rows of ovules (unfertilized seeds). Thus, in an ovary cross section like the one illustrated you can see six ovules in various stages of development.
  • Megaspore mother cell x100. This large diploid (2N) cell will undergo meiosis to form four nuclei, each a haploid (N) megaspore. The flaps of tissue on either side of the megaspore mother cell are seed coats. They will grow around (growing toward the upper left in this illustration) the mother cell almost but not quite completely surrounding it.
  • Metaphase I x400. Meiosis of the megaspore mother cell.
  • Two nucleate stage x100. The first two nuclei formed by meiosis. The micropyle is the slit immediately above the old megaspore mother cell with the two nuclei.
  • Metaphase II x400. Meiosis of the megaspore mother cell. You can see two groups of chromosomes, each at metaphase II, and spindle fibers.
  • Four Megaspores x100 within the old megaspore mother cell. The slit-like micropyle is to the right of the megaspores. This tube like opening, where the seed coats failed to fuse as they grow around the megaspore mother cell, is where a pollen tube will penetrate the ovule.
  • Immature female gametophyte x100. In most species of flowering plants three of the four megaspore nuclei die. The remaining megaspore, the one closest to the micropyle, divides several times by mitosis to form the female gametophyte. Illustrated is the 4 nucleate state of this gametophyte
  • 8 nucleate Embryo Sac x40. This is the Lily mature female gametophyte consisting of 7 cells and 8 haploid nuclei. Such a flowering plant female gametophyte is usually called the "embryo sac". The egg and two flanking synergid cells are at the lower right. The large central cell contains two large "polar nuclei", one at the lower left and one at the upper right. The three antipodal cells are at the upper left.
  • Embryo Sac x400. The polar nuclei and egg are labeled. The two "synergid" cells flank the egg. The three "antipodal" cells are above and to the right of the upper polar nucleus.
  • Double Fertilization x400. Lily caught "doing it"! Two sperm nuclei exit the pollen tube. In most species of flowering plants one sperm nucleus (N) fuses with the two polar nuclei (each N) to form the 3N nucleus of the endosperm mother cell. The other sperm nucleus (N) fuses with the nucleus of the egg (N) to form the zygote (2N).

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    Seed and fruit development

    MONOCOT SEEDS AND FRUITS

  • Lily early embryo x40. The zygote, near the micropyle of the seed starts to divide by mitosis to form the first few cells of the (2N, sporophyte) embryo. The endosperm mother cell divides by mitosis to form the (usually 3N) multicellular nutritive tissue known as "endosperm". These are surrounded by the seed coat and the (upper left corner) fruit (ovary) wall.
  • Lily mature seed x20. The endosperm contains large dark staining structures that store reserve food for the embryo. A cross section of the embryo shoot/root axis is in the center.
  • Corn "seed" x12. Actually this is a one seeded fruit of a type known as a "grain". This fruit type is characterized by an ovary wall fused to the seed coat. The embryo's single cotyledon, which gives the monocot group its name, is easily seen. Mature monocot seeds usually have lots of starch filled endosperm.

    DICOT SEEDS AND FRUITS (Study the Capsella illustrations in sequence)

  • Capsella young embryo x100. At this stage the embryo consists of a string of cells (suspensor) that pushes a heart-shaped ball of cells (the proembryo) in toward the center of the seed. Later, all the rest of the embryo's growth will be done by the proembryo. The two lobes of the proembryo will become the two cotyledons. Endosperm is starting to fill the rest of the seed cavity.
  • Capsella young seed within its fruit x40.
  • Capsella embryo with bending cotyledons x100. This intermediate stage in embryo development is an enlargement of the previous illustration.
  • Capsella mature embryos x40. Several mature seeds within their fruit. The embryos have bent into the fetal position filling the entire cavity of the seed. The nutrients of the endosperm have been absorbed by the embryo, mostly into their two cotyledons. Unlike most monocots which have lots of endoserm in their mature seeds, most dicot seeds have little or no endosperm when mature.
  • Bean fruit xs x20. A "green bean" which is actually a fruit with seeds inside. The three layers of the ovary wall can be seen. The seed is obviously a dicot seed, with the two cotyledons filling most of the inside space of the seed.
  • Fragaria (strawberry) fruit x30. You can see a dicot embryo in one of the tiny little achene fruits on the surface of the strawberry. Strawberries are aggregate achenes. The fleshy part that you eat is swollen receptacle (flower stalk).

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    Flower buds

  • Monocot flower bud xs x12. This is a Lily bud with (from the outside toward the center) 3 sepals, 3 petals, 6 stamens each bearing an anther with 4 pollen sacs, and a single central pistil sectioned through the ovary. You can see 6 ovules in the ovary
  • Dicot flower bud xs x20. You can see from the outside toward the center 4 sepals, 4 petals, 6 stamens, and a single central pistil that was formed by the fusion of two carpels. The pistil is sectioned through the ovary and several ovules are visible inside.
  • Another docot flower bud x30. You can see 5 sepals, 5 petals, 5 stamens, and one pistil. The sepals look very leaf like, with venation, mesophyll, etc.
  • Prunus flower bud ls x30. Sepals petals and stamens are all partially fused to each other forming a cup which surrounds the pistil. This type of fusion is called "perigynous". The sepals petals and stamens separate from each other at the level of the style. Since this is a slice through a closed bud you can see cross sections of sepals and petals near the top of the illustration.
  • Prunus flower bud xs x12, sectioned through the style at a level where the sepals petals and stamen are still more or less fused. The outer layer consists of fused sepals. There are numerous stamens and a single pistil. This is the same species of flower as the previous illustration.

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