The flowers,
which are the reproductive organs of flowering plants, are the most
remarkable feature distinguishing them from other seed plants. Flowers aid
angiosperms by enabling a wider range of adaptability and broadening the
ecological niches open to them. This has allowed flowering plants to largely
dominate terrestrial ecosystems. Stamens are much lighter than the
corresponding organs of gymnosperms and have contributed to the
diversification of angiosperms through time with adaptations to specialized
pollination syndromes, such as particular pollinators. Stamens have also
become modified through time to prevent self-fertilization, which has
permitted further diversification, allowing angiosperms eventually to fill
more niches.
The transition to
flowering is one of the major phase changes that a plant makes during its
life cycle. The transition must take place at a time that is favorable for
fertilization and the formation of seeds, hence ensuring maximal
reproductive success. To meet these needs a plant is able to interpret
important endogenous and environmental cues such as changes in levels of
plant hormones and seasonable temperature and photoperiod changes. Many
perennial and most biennial plants require verbalization to flower. The
molecular interpretation of these signals is through the transmission of a
complex signal known as Florien, which involves a variety of genes,
including CONSTANTS, FLOWERING LOCUS C and FLOWERING LOCUS T. Florien is
produced in the leaves in reproductively favorable conditions and acts in
buds and growing tips to induce a number of different physiological and
morphological changes. The first step is the transformation of the
vegetative stem primordial into floral primordial. This occurs as
biochemical changes take place to change cellular differentiation of leaf,
bud and stem tissues into tissue that will grow into the reproductive
organs. Growth of the central part of the stem tip stops or flattens out and
the sides develop protuberances in a whorled or spiral fashion around the
outside of the stem end. These protuberances develop into the sepals,
petals, stamens, and carpals. Once this process begins, in most plants, it
cannot be reversed and the stems develop flowers, even if the initial start
of the flower formation event was dependent of some environmental cue. Once
the process begins, even if that cue is removed the stem will continue to
develop a flower.
Many flowers have close relationships with one or a few specific pollinating
organisms. Many flowers, for example, attract only one specific species of
insect, and therefore rely on that insect for successful reproduction. This
close relationship is often given as an example of convolution, as the
flower and pollinator are thought to have developed together over a long
period of time to match each other's needs.
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This close relationship compounds the negative effects of extinction. The
extinction of either member in such a relationship would mean almost certain
extinction of the other member as well. Some endangered plant species are so
because of shrinking pollinator populations. |
