flower

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Longitudinal cross section of a flower at the time of fertilization, showing a pollen grain and a pollen tube.

flower, name for the specialized part of a plant containing the reproductive organs, applied to angiosperms only. A flower may be thought of as a modified, short, compact branch bearing lateral appendages. Like twigs, flowers develop from buds, and the basic floral parts (sepal, petal, stamen, and carpel) are in actual fact greatly modified leaves. A typical flower is a concentric arrangement of these parts attached at their base to the receptacle, the tip of the stem. Outermost is a whorl of leaflike green sepals (the calyx) encircling a whorl of usually showy, colored petals (the corolla). Within the corolla the stamens, bearing anther sacs full of pollen, surround the central carpels (ovary). Inside the ovary at the base of the pistil are the ovules, containing the female sex cells; after fertilization of the egg, the ovule becomes the seed and the ovary becomes the fruit. The ovary and stamens are termed essential flower parts, the petals and sepals accessory parts. The number and arrangement of the floral organs vary considerably among the many families and orders of plants and are used in the classification of plants; they also indicate the degree of evolution of the plant. In general, the higher a plant is on the evolutionary scale, the greater is the flower's complexity. The basic number of parts differs from class to class and from family to family; in monocotyledonous plants the parts generally occur in groups of three or in multiples of three, and in dicotyledons more often in groups of two, four, and five. Flowers may be staminate (lack carpels), carpellate, or both; staminate and carpellate flowers may appear on the same plant, on separate plants, or in the same inflorescence. One type of inflorescence, characteristic of the parsley family, is the umbel, in which the tiny florets are borne on separate stalks radiating out from the stem tip. Sometimes the parts serve unusual purposes: the true flowers of the dogwood and the poinsettia are inconspicuous, and the showy “petals” are really modified leaves called bracts. In the jack-in-the-pulpit the florets are clustered on a spike canopied by a large bract, the spathe; the hood of the lady's-slipper, on the other hand, is a modified sterile stamen. Grass inflorescences are tiny spikelets sheathed by protective scales called glumes (the chaff or grain). Flowers have been cultivated and bred for their beauty and their perfume from earliest times and have accumulated a vast and intricate treasury of symbolic associations derived from legend and folklore. Individual flowers have been celebrated in heraldry (rose), in religion (lotus), and in politics (violet) and have become emblems for many countries, including Switzerland (edelweiss), France (fleur-de-lis), Scotland (thistle), the Netherlands (tulip), and the United States (see state flowers).

See S. Buchmann, The Reason for Flowers (2015).

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