angiosperms - Angiosperms Flowering Plants Good web site...

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Unformatted text preview: Angiosperms Flowering Plants Good web site Angiosperms Angiosperm means covered seed. The flower is the most important adaptation for the sexual reproduction of plants. Adaptation for attracting pollinators The hallmark of angiosperm reproduction is the Carpel. Produces a fruit which protects seeds and assists in their dispersal Flowers A flower is a system of modified leaves specialized for reproduction. Flowers are only found in angiosperms, not gymnosperms (like pines) or ferns. Fruits are only found in angiosperms, not gymnosperms. Flower Anatomy The modified leaves are present as four whorls positioned at the terminal (sometimes swollen) portion of the branch called the receptacle Always occur in the same order relative to each other. Receptacle Flower Parts The sepal is one member of the first circle of parts. Collectively they are refered to as the calyx. The petal is a member of the second circle of parts. Collectively they are refered to as the corolla. Together, the calyx and corolla are refered to as the perianth. Flower Parts A stamen is one member of the third circle of parts (the male reproductive structure). A stamen consist of a filament and an anther. Collectively the stamens are refered to as the androecium. Flower Parts The carpel is one member of the forth circle of parts. Each carpel is made up of the stigma, style, and ovary. Collectively the carpels make up the gynoecium. The carpel(s) may also be refered to as the pistil(s). Generalized flower structure Calyx composed of sepals lowest position on the receptacle. Usually green and leafy. Corolla composed of petals interior to the sepals. Usually the showy part of the flower (but not always!). The sepals and petals together are called the perianth. Androecium Composed of stamens interior to petals. Each stamen composed of a filament and an anther Gynoecium Composed of carpels Plants may have one or many carpels and the carpels may be separate or fused to each other. Gynoecium Carpel parts Stigma (receptive portion where pollen adheres and germinates), Style (where pollen tubes grow) Ovary (the swollen lower portion). Inside the ovary are the ovules that eventually become seeds. The egg cell is present inside the ovule (don't confuse ovule and egg). Cross section of lily flower showing the sepals, petals, anthers and carpels Cross section of the lily gynoecium showing that it is composed of 3 fused carpels and the presence of ovules. Pollen Formation The Anther is the site of Microsporogenesis Pollen Formation Sporogenous Cells develop within the Anther. Pollen Formation The sporogenous cells become separate and enter meiosis. Pollen Formation The haploid cells form a sphere called a tetrad. Notice that the chromatin is still condensed in the nucleus of each tetrad cell. Pollen Formation The cells of the tetrad separate and prepare for a final division. Pollen Formation The final division produces a small cell which is located in the cytoplasm of a large cell. A thick cell wall develops around the large cell. Pollen Formation The mature pollen grain is the microgametophyte. It has a thick multilayered wall, a large tube cell and a small generative cell. The generative cell floats in the cytoplasm of the tube cell. The generative cell undergoes another division to produce two nonflagellated sperm cells which migrate with the tube cell nucleus to the tip of the pollen tube. Megagametophyte Megasporogenesis starts with the enlargement of one cell within a diploid ovule. Megagametophyte The enlarged cell is called a megasporocyte which means "megaspore forming cell" or megaspore mother cell. Megagametophyte Integuments develop from the ovule epidermis just behind the megasporocyte. These envelop the megagametophyte as it grows. They are incomplete and leave a micropyle (small hole) through which the pollen tube enters. Megagametophyte Eventually, the megasporocyte undergoes the first meiotic division which is a "reduction division" Two haploid nuclei are created. These divide to produce a four nucleate cell. Megagametophyte Two haploid nuclei are created. These divide to produce a four nucleate cell. Megagametophyte The four nuclei represent four megaspores which have NOT formed separating cell walls. Three of these DIE! This leaves 1 functional megaspore. Megagametophyte The functional megaspore divides 4 times to produce a cell with eight nuclei. Megagametophyte Following nuclear migrations & cell formation, the mature megagametophyte (embryo sac) is formed. It is typically composed of an egg apparatus (egg + 2 synergids) at the micropylar end of the megagametophyte. A large, multinucleate central cell occupies most of the volume of the gametophyte. Megagametophyte Three antipodals are found at the chalazal (opposite) end of the megagametophyte. These probably represent an embryo sac which has lost its function over time. Fertilization and Embryogenesis Double Fertilization The pollen tube grows through the micropyle & may enter the senescent synergid. The pollen tube wall degrades and the sperm are released. The egg and the central cell have incomplete cell walls & the sperm has no wall. Fertilization and Embryogenesis One sperm enters the egg while the other enters the central cell. The zygote is formed when a sperm nucleus fuses with the egg nucleus (syngamy). This will develop into the diploid embryo. The endosperm is formed by the fusion of a sperm nucleus with the nuclei of the central cell. It is therefore triploid (3N). The endosperm surrounds the developing embryo and provides a source of nutrition for the embryo during its differentiation & during germination of many plants. Lily Dicot Embryo (Capsella) Mature Embryo in seed SAM = Shoot Apical Meristem RAM = Root Apical Meristem Cotyledons are seed leaves This is a dicot, so there are two of these Monocot Embryo (Zea) ...
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