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TYPICAL FLOWER

The typical flower consists of four whorls of modified leaves called sepals, petals, stamens, and carpels, all of which are attached to the receptacle. Such flowers are said to be complete. Plants with flowers that lack any of the first four elements have incomplete flowers.

Usually green because they photosynthesize, the sepals enclose the other parts of the flower inside the bud. Together, the sepals constitute the calyx. After the sepals are the petals, the next structures attached to the receptacle,which is part of the modified stem. Together, all the petals are called the corolla. The calyx and the corolla constitute the perianth.

Next, continuing distally (outward) along the modified stem, are the stamens, each consisting of a long, slender stalk, the filament, which bears pollen on the specialized portion at the tip, known as the anther.

The central part of the flower, and the part farthest along the receptacle, is the carpel, which is composed of modified floral leaves that have become folded over to protect the ovule from otherwise predaceous pollenators. The pistil consists of one or more carpels in the center of the flower. Generally, the pistil has an expanded base, the ovary, in which are one or more sporangia. These sporangia are called the ovules. Each one divides meiotically, creating four haploid megaspores, three of which usually disintegrate. The remaining megaspore divides mitotically to produce the female gametophyte. One of the cells near the micropylar end becomes the egg cell and some polar bodies remain.

Each anther consists of four sporangia. These divide meiotically to produce haploid microspores that mature into pollen grains, each with two nuclei. These pollen grains are the male gametophytes. When the pollen reaches the stigma, the receptive portion of the style extending above the ovary, a pollen tube grows through the stigma and the style into the ovary. When the tip of the pollen tube reaches the ovule, it enters the micropyle and releases both haploid nuclei into the female gametophyte (embryo sac). One of the haploid nuclei, which are known as sperm nuclei although they are not motile, fertilizes the egg, and the zygote develops into the sporophytic embryo. Two polar nuclei from the female gametophyte combine, forming a diploid fusion nucleus. This in turn combines with the second sperm to form a triploid nucleus that divides mitotically and creates triploid tissue known as the endosperm. This tissue surrounds the embryo and acts as the stored food upon which the developing sporophytic seed feeds. Endosperm constitutes the nutritious part of most grains and many seeds that are eaten by much of the world’s human population.

After fertilization, the angiosperm ovary, which surrounds the embryo, develops into the fruit. This protects the embryo from desiccation during the early stages of development. Later, the fruit acts as an agent of dispersal, either by being blown by the wind or falling to the ground and rolling. An animal may also aid in the dispersal process by carrying the fruit in its fur or ingesting the fruit and depositing it in feces.

The angiosperms are composed of two subgroups, the Monocotyledonae and the Dicotyledonae. Monocots, as they are often called, have embryos with one cotyledon, the leaflike structure composing much of the seed with its endosperm, which is composed of nutrients used during germination. Monocots usually have parallel leaf venation. The flower parts are normally in multiples of three. Most forms lack a vascular cambium. The monocots include grasses, lilies, iris, and orchids. The flowers of grasses have greatly reduced petals, stamens, and carpels, and are thought to have evolved from a lily-like ancestor.

In contrast, the dicots have embryos with two cotyledons, their leaf venation is generally netted, and the lower parts are typically in groups of four or five. In addition, those dicots with secondary growth have a vascular cambium. All the flowering plants not included in the monocots are dicots; they are the most numerous and, in many respects, the most successful plants alive.

Дата добавления: 2015-11-16