The flower is the
reproductive structure of angiosperms, or flowering plants. Compared
to the reproductive structures
of other plants,
the flower is unique in several ways. It consists of four kinds
of modified leaves, two of which (stamens and carpels, the latter
sometimes called pistils) bear pollen and seeds. Several non-flowering
plants also produce pollen and seeds on modified leaves, but in
angiosperms the modified leaf called the carpel forms an ovary
that completely encloses the ovule, which becomes the seed. In
the gymnosperms, ovules are borne on open modified leaves, such
as the scale of a pinecone. The term angiosperm, derived from the
Greek, means "seed in a vessel." Gymnosperm means "naked
seed".
According to the
fossil record, flowering plants appeared only about 140 million
years ago, although some recently
found
fossil evidence suggests that they appeared 80 million years
before that. (The earliest land plants, blue-green algae, appeared
perhaps
1.2 billion years ago.) The angiosperms now dominate the world's
vegetation. Only the gymnosperms offer any substantial competition.
There may be more than 250,000 angiosperm species, compared to
fewer than 1,000 gymnosperm species and fewer than about 40,000
other types of vascular plants (ferns and their relatives) and
bryophytes (liverworts, mosses, hornworts). There are fewer than
15,000 species of algae, and perhaps more than 100,000 species
of fungi and bacteria.
More than any other
of the major plant groups, flowering plants are ecologically
related to animals.
Modern animals, including
humans, and flowering plants are equally dependent upon each
other. Most flowering species rely on animals for reproduction.
Insects
carry pollen from the stamens to the carpels; bats and birds
participate in Pollination of some species.
Animals attracted
to their colorful
and aromatic flowers and tasty fruits further ensure the
dispersal and growth of the seeds. Many fruits and seeds (the
exclusive
products of angiosperms) are also collected and consumed
by humans, and
the seeds are planted in extensive systems of agriculture.
Almost all plants used in agriculture are angiosperms. (Mushrooms,
fern
fiddleheads, and pine nuts are exceptions.) In another relationship
between plants and animals, only the special growing cells
at the base of a grass (angiosperm) leaf seem well adapted
to animal
grazing.
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