flower

flower, the characteristic reproductive structure of angiosperms. As popularly used, the term “flower” especially applies when part or all of the reproductive structure is distinctive in color and form. Given that there over 352,000 species of flowering plants known to science, flowers have a seemingly endless array of colors, sizes, forms, and anatomical arrangement. They range in size from minute blossoms to the giant blooms of Rafflesia and the titan arum. In some plants, such as poppy, magnolia, tulip, and petunia, each flower is relatively large and showy and is produced singly, while in other plants, such as aster, snapdragon, and lilac, the individual flowers may be very small and are borne in a distinctive cluster known as an inflorescence. Regardless of their variety, all flowers have a uniform function, the reproduction of the species through the production of seed.

Form and types

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Basically, each flower consists of a floral axis upon which are borne the essential organs of reproduction (stamens and pistils) and usually accessory organs (sepals and petals); the latter may serve to both attract pollinating insects and protect the essential organs. The floral axis is a greatly modified stem; unlike vegetative stems, which bear leaves, it is usually contracted, so that the parts of the flower are crowded together on the stem tip, the receptacle. The flower parts are usually arrayed in whorls (or cycles) but may also be disposed spirally, especially if the axis is elongate. There are commonly four distinct whorls of flower parts: (1) an outer calyx consisting of sepals; within it lies (2) the corolla, consisting of petals; (3) the androecium, or group of stamens; and in the center is (4) the gynoecium, consisting of the pistils.

The sepals and petals together make up the perianth, or floral envelope. The sepals are usually greenish and often resemble reduced leaves, while the petals are usually colorful and showy. Sepals and petals that are indistinguishable, as in lilies and tulips, are sometimes referred to as tepals. The androecium, or male parts of the flower, comprise the stamens, each of which consists of a supporting filament and an anther, in which pollen is produced. The gynoecium, or female parts of the flower, comprises one or more pistils, each of which consists of an ovary, with an upright extension, the style, on the top of which rests the stigma, the pollen-receptive surface. The ovary encloses the ovules, or potential seeds. A pistil may be simple, made up of a single carpel, or ovule-bearing modified leaf; or compound, formed from several carpels joined together.

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• Complete: a flower having all four whorls of sepals, petals, stamens, and pistils
• Incomplete: a flower lacking one or more of these whorls
• Perfect, or bisexual: a flower with both stamens and pistils; can be complete or incomplete
• Pistillate, or female: an imperfect (unisexual) flower that lacks stamens
• Staminate, or male: an imperfect (unisexual) flower that lacks pistils
• Monoecious: a species in which an individual plant that bears both pistillate and staminate flowers (e.g., tuberous begonia, hazel, oak, corn)
• Dioecious: a species in which an individual plant that bears only pistillate or staminate flowers (e.g., date, holly, cottonwood, willow)
• Polygamous, or polygamomonoecious: a species in which an individual plant that bears perfect, pistillate, and staminate flowers (e.g., horse chestnut)

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A flower may be radially symmetrical (see photograph), as in roses and petunias, in which case it is termed regular or actinomorphic. A bilaterally symmetrical flower, as in orchids (see photograph) and snapdragons, is irregular or zygomorphic.

Pollination

The stamens and pistils are directly involved with the production of seed. The stamen bears microsporangia (spore cases) in which are developed numerous microspores (potential pollen grains); the pistil bears ovules, each enclosing an egg cell. When a microspore germinates, it is known as a pollen grain. When the pollen sacs in a stamen’s anther are ripe, the anther releases them and the pollen is shed. Fertilization can occur only if the pollen grains are transferred from the anther to the stigma of a pistil, a process known as pollination.

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There are two chief kinds of pollination: (1) self-pollination, the pollination of a stigma by pollen from the same flower or another flower on the same plant; and (2) cross-pollination, the transfer of pollen from the anther of a flower of one plant to the stigma of the flower of another plant of the same species. Self-pollination occurs in many species, but in the others, perhaps the majority, it is prevented by such adaptations as the structure of the flower, self-incompatibility, and the maturation of stamens and pistils of the same flower or plant at different times. Cross-pollination may be brought about by a number of agents, chiefly insects and wind. Wind-pollinated flowers generally can be recognized by their lack of color, odor, or nectar, while animal-pollinated flowers are conspicuous by virtue of their structure, color, or the production of scent or nectar. See also major types of pollinators.

After a pollen grain has reached the stigma, it germinates, and a pollen tube protrudes from it. This tube, containing two male gametes (sperms), extends into the ovary and reaches the ovule, discharging its gametes so that one fertilizes the egg cell, which becomes an embryo, and the other joins with two polar nuclei to form the endosperm. (Normally many pollen grains fall on a stigma; they all may germinate, but only one pollen tube enters any one ovule.) Following fertilization, the embryo is on its way to becoming a seed, and at this time the ovary itself enlarges to form the fruit.

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Cultural significance

Flowers have been symbols of beauty in most civilizations of the world, and flower giving is still among the most popular of social amenities. As gifts, flowers serve as expressions of affection for spouses and partners, other family members, and friends; as decorations at weddings and other ceremonies; as tokens of respect for the deceased; as cheering gifts to someone ill or hospitalized; and as expressions of thanks or appreciation. Most flowers bought by the public are grown in commercial greenhouses or horticultural fields and then sold through wholesalers to retail florists. See also articles on individual flowers (e.g., carnation; lotus; petunia; rose; tulip).