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| Structure |
Orchids and their allies are distinguished from
other orders of flowering plants by a combination of floral
characteristics rather than by a single characteristic unique to the group.
Orchid flowers are borne on stalks called pedicels, as are other flowers.
During the growth and development of the flower, however,
the pedicel rotates 180°, so that the mature orchid flower is borne
upside down.
Of the flower's three sepals (outer floral whorls)
and three petals (inner floral whorls), all the sepals and the two
lateral petals are usually similar to one another in color and shape.
The remaining petal, always distinct from them, is called the labellum,
or lip; it is usually larger and different in color and shape,
often being lobed or cupped.
The labellum, which often acts as a landing platform for the orchid
pollinator, may attract the pollinator to the flower through
particular color patterns and shapes to which the pollinator
responds in particular ways.
The sexual organs (pistil and stamens) of the orchid flower are fused
together into a structure called the column, which lies opposite the lip.
Orchids have only one stamen (male floral organ), and in
most orchids it bears only one anther (pollen-producing structure);
in a few orchids, however, two anthers are produced.
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The pollen is not granular, as it is in most flowering plants, but is
aggregated together in a number of masses, or sacs, that vary in
texture from mealy to horny.
Three stigmatic lobes (pollen-receptive areas) are usually present
and located near the anther, although usually only two are functional.
The ovary is below the other flower parts and is surrounded by pedicel tissue.
Tripartite, it contains numerous ovules (egg-bearing structures) that
mature into seeds. The seeds are small, with only an undifferentiated embryo.
As many as 2 million seeds may be produced from a single orchid seedpod.
Unlike most other flowering plants, orchids have no food-storage tissue.
Orchid flowers are pollinated by a great variety of flying animals,
and their great diversity in floral structure has resulted from
adaptations to various pollinators.
About half the orchid species are pollinated by bees; moths,
butterflies, flies, birds, and other agents pollinate the rest.
Many orchid flowers are adapted for pollination by a single species
of insect.
Orchids do not vary as much vegetatively as they do in floral structure,
but a great variety of forms exists, reflecting the wide range of habitats
they occupy.
About half are epiphytic, growing on other plants for support only,
but some are parasitic and others saprophytic (living on decaying vegetation).
A few Australian species complete their life cycles entirely underground.
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