Flower is the reproductive portion of any plant in the
division Magnoliophyta (Angiospermae), commonly called flowering plants or
angiosperms. As popularly used, the term “flower” especially applies when part
or all of the reproductive structure is distinctive in colour and form.
In their range of colour, size, form, and anatomical arrangement, flowers present a seemingly endless variety of combinations. They range in size from minute blossoms to giant blooms. 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 , calla lily, and lilac, the individual flowers may be very small and are borne in a distinctive cluster known as an inflorescence (q.v.). Regardless of their variety, all flowers have a uniform function, the reproduction of the species through the production of seed. The flower is the characteristic structure of the evolutionarily highest group of plants, the angiosperms.
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 centre is (4) the gynoecium, consisting of the pistils.
petals together make up
perianth, or floral
envelope. The sepals are usually greenish and often resemble reduced leaves,
while the petals are usually colourful 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, comprise the
pistils, each of which
consists of an
ovary, with an upright
extension, the style, on the top of which rests 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.
A flower having sepals, petals, stamens, and pistils is
complete; lacking one or
more of such structures, it is said to be
incomplete. Stamens and
pistils are not present together in all flowers. When both are present the
flower is said to be
perfect, or bisexual,
regardless of a lack of any other part that renders it incomplete (see
photograph). A flower
that lacks stamens is
pistillate, or female,
while one that lacks pistils is said to be
staminate, or male. When
the same plant bears unisexual flowers of both sexes, it is said to be
monoecious (e.g., tuberous begonia, hazel, oak, corn); when the male and
female flowers are on different plants, the plant is dioecious (e.g.,
date, holly, cottonwood, willow); when there are male, female, and bisexual
flowers on the same plant, the plant is termed polygamous.
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.
Neither the calyx nor the corolla is necessary for reproduction. The stamens and pistils, on the other hand, 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. This is of two chief kinds: (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 (see photograph) generally can be recognized by their lack of colour, odour, or nectar, while insect-pollinated flowers (see photograph) are conspicuous by virtue of their structure, colour, or the production of scent or nectar.
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 they fertilize the egg cell, which becomes an embryo. (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.
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, other family members, and friends; as decorations at weddings and other
ceremonies; as tokens of respect for the deceased; as cheering gifts to the
bedridden; or as expressions of thanks to hostesses and other social contacts.
Most flowers bought by the public are grown in commercial greenhouses and then
sold through wholesalers to retail florists. See also articles on
individual flowers, e.g.,
Excerpt from 1994-2002 Encyclopædia Britannica, Inc.