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Angiosperms: The Life Cycle of a Flower

Angiosperms derive from the Phylum Angiospermae, also called Division Anthophyta, because it encompasses a large group of "flowering plants." The seeds of angiosperms reside within the enclosure of a fruit. Botanists describe the Division Angiospermae as having two main classifications, including the monocotyledones ("monocots") and the dicotyledones ("dicots"). Monocots have specific characteristics that differentiate them from dicots, such as the seeds only have one nutrient storage area, the leaves have parallel veins, the flowers have three parts, the roots have many fibers, and the stems come in bundles scattered throughout the plant. Dicots have two nutrient storage areas, leaves with branching veins, flowers with two, four, or five parts, one main taproot, and several stems arranged in a ring formation.

The Parts of An Angiosperm

Angiosperms consist of four main parts, including the roots, stems, leaves, and flowers. Angiosperm roots generally plant themselves underground in order to absorb water and nutrients. Angiosperm stems come in three different varieties, including stolons (above-ground), rhizomes (underground), and bulbs (fleshy stems that store nutrients). Angiosperm leaves have a simplistic or compounded form, which alternate going up the stem, or have an opposite arrangement on the stem. Whoried leaves originate from the same place on the stem. Angiosperm flowers consist of four whorls of leaves, including sepals (small and green fenced in flowers), petals (flat, widely spread, and brightly colored leaves), stamens (threaded upright leaves), and pistils (fruit bearing material). Pistils consists of three different parts, including the ovary (egg), style (column or the "stalk"), and the stigma (the outer, sticky spot where the pollen sticks as it lands). The angiosperm's fruit has a ripened ovary, which serves to protect seed dispersal. Angiosperms produce simple, aggregate, and multiple fruits. Simple fruits, such as walnut, tomato, orange, apple, and cherry arise from a single ovary in one flower. Aggregate fruits, such as a strawberry, arise from multiple ovaries in a single flower. Multiple fruits, such as pineapple, mulberry, and breadfruit, arise from ovaries that stem from multiple, tightly clustered flowers.

Botanists group angiosperms based on the plants characteristics. Botanists examine the arrangement of flowers, and identify them based off predetermined classifications. For instance, complete flowers have all four layers of the angiosperm. Conversely, incomplete flowers lack one or more layers. Perfect flowers have both sexes, including stamens and pistils. Monoecious plants, such as Easter lilies, peas, dandelions, and roses, have perfect flowers, or both sexes on the same plant. Imperfect flowers do not have pistils or stamens. Dioecious plants, such as hops, persimmons, boxelders, and marijuana, have imperfect flowers on separate male or female plants. Regular flowers are radically symmetrical. Irregular flowers are bilaterally symmetrical. Multiple flowers have clusters of fruits, which serve as a protective measure of seed dispersal.

The Life Cycle of an Angiosperm

The life of an angiosperm alternates between diploid sporophyte and gametophyte generations. The angiosperm's male flowering parts, also called anthers, contains millions of microsporocytes. Microsporocytes divide through a process called meiosis to produce haploid microspores. At the same time, a similar process occurs within the angiosperm's female flowering parts that contains carpels. A single carpel consists of a style, ovary, stigma, and ovule. A single diploid megasporocyte resides in the ovule to produce four haploid megaspores; however, only one survives the production process. Botanists consider angiosperms heterosporous, because they produce both microspores and megaspores. A microspore undergoes mitotic division in order to produce a pollen grain, or a male haploid gametophyte.

The remaining megaspore undergoes a mitotic division in order to produce seven haploid cells. One large haploid cell contains two nuclei, also known as the polar nuclei. One of the cells is the egg. The seven-celled structure comes together to make the female gametophyte. Next, the pollen grain pollinates the female flowering parts by landing on the stigma and then germinates to produce a pollen tube. The pollen tube grows down the style until meeting the female gametophyte. Two sperm from the angiosperm's pollen grain maneuvers through the pollen tube and then enters the female gametophyte. One sperm fertilizes the diploid zygote, while the other fertilizes two polar nuclei creating a triploid cell.

Double fertilization occurs when both the egg and central cell become fertilized at the same time. Double fertilization is the hallmark of an angiosperm's life cycle. Finally, the zygote starts the next cycle of sporophyte generation by developing into the embryo. The embryo sporophyte consists of one or more cotyledons, or nutrient storage pods. In addition, the embryo sporophyte consists of the epicotyl, or the region that lies above the cotyledons that eventually becomes the stem and leaves. Embryo sporophytes also contain the hypocotyl, or the region that lies under the cotyledons, which develops into the root system of the angiosperm. Meanwhile, the triploid cell develops into the endosperm of the seed, whereby it will supply nutrients to the plant. The angiosperm's seed germinates to create a maturing sporophyte, which starts the life cycle all over again. The angisoperm's life cycle contributes to the overall food chain, whereby people and animals can forage the woods looking for succulent fruits to eat.

Follow these links to learn more about angiosperms:

Angiosperm or Gymnosperm?

Anthophyta: Fossil Record

Angiosperms: Major Groups

The Plant List: The Angiosperms (Flowering Plants)

Author : Paul Mousseline