Tuesday, March 20, 2007

Flower anatomy

Flowering plants are heterosporangiate (producing two types of reproductive spores) and the pollen (male spores) and ovules (female spores) are created in different organs, but these are jointly in a bisporangiate strobilus that is the typical flower.
A flower is regarded as a customized stem with shortened internodes and bearing, at its nodes, structures that may be very modified leaves. In essence, a flower structure forms on a modified shoot or axis with an apical meristem that does not grow continuously (growth is determinate). The stem is called a pedicel, the end of which is the torus or receptacle. The parts of a flower are set in whorls on the torus. The four main parts or whorls (starting from the base of the flower or lowest node and working upwards) are as follows:
Poppycalyx – the outer whorl of sepals; typically these are green, but are petal-like in some species.
corolla – the whorl of petals, which are usually thin, soft and colored to attract insects that help the process of pollination.
androecium– one or two whorls of stamens, each a filament topped by an anther where pollen is produced. Pollen contains the male gametes.
gynoecium– one or more pistils. The female reproductive organ is the carpel: this contains an ovary with ovules (female gametes). A pistil may consist of a number of carpels merged together, in which case there is only one pistil to each flower, or of a single individual carpel (the flower is then called apocarpous). The sticky tip of the pistil, the stigma, is the receptor of pollen. The supportive stalk, the style becomes the pathway for pollen tubes to grow from pollen grains adhering to the stigma, to the ovules, carrying the reproductive material.

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