flower, sometimes known as a bloom or blossom, is the reproductive structure
found in flowering plants (plants of the division Magnoliophyta, also called
angiosperms). The biological function of a flower is to mediate the union of
male sperm with female ovum in order to produce seeds. The process begins with
pollination, is followed by fertilization, leading to the formation and
dispersal of the seeds. For the higher plants, seeds are the next generation,
and serve as the primary means by which individuals of a species are dispersed
across the landscape. The grouping of flowers on a plant is called the
inflorescence.
In addition to serving as the reproductive organs of flowering plants, flowers
have long been admired and used by humans, mainly to beautify their environment
but also as a source of food.
Flowering plants usually face selective pressure to optimise the transfer of
their pollen, and this is typically reflected in the morphology of the flowers
and the behaviour of the plants. Pollen may be transferred between plants via a
number of 'vectors'. Some plants make use of abiotic vectors - namely wind
(anemophily) or, much less commonly, water (hydrophily). Others use biotic
vectors including insects (entomophily), birds (ornithophily), bats (chiropterophily)
or other animals. Some plants make use of multiple vectors, but many are highly
specialised.
Cleistogamous flowers are self pollinated, after which they may or may not open.
Many Viola and some Salvia species are known to have these types of flowers.
The flowers of plants that make use of biotic pollen vectors commonly have
glands called nectaries that act as an incentive for animals to visit the
flower. Some flowers have patterns, called nectar guides, that show pollinators
where to look for nectar. Flowers also attract pollinators by scent and color.
Still other flowers use mimicry to attract pollinators. Some species of orchids,
for example, produce flowers resembling female bees in color, shape, and scent.
Flowers are also specialized in shape and have an arrangement of the stamens
that ensures that pollen grains are transferred to the bodies of the pollinator
when it lands in search of its attractant (such as nectar, pollen, or a mate).
In pursuing this attractant from many flowers of the same species, the
pollinator transfers pollen to the stigmas—arranged with equally pointed
precision—of all of the flowers it visits.
Anemophilous flowers use the wind to move pollen from one flower to the next.
Examples include grasses, birch trees, ragweed and maples. They have no need to
attract pollinators and therefore tend not to be "showy" flowers. Male and
female reproductive organs are generally found in separate flowers, the male
flowers having a number of long filaments terminating in exposed stamens, and
the female flowers having long, feather-like stigmas. Whereas the pollen of
animal-pollinated flowers tends to be large-grained, sticky, and rich in protein
(another "reward" for pollinators), anemophilous flower pollen is usually
small-grained, very light, and of little nutritional value to animals.