Flowering plants are called heterosporangiate, which means that they produce two types of reproductive spores. The pollen (called male spores) and ovules (called as female spores) are produced in different organs. But a typical flower is called as a bisporangiate strobilus in because it contains both organs.
A flower is regarded as a modified stem with shortened internodes and bearing. at its nodes there as structures that may be highly modified leaves. In actual essence, the structure of a flower forms on a modified shoot or axis with an apical meristem. This does not grow continuously (meaning that growth is determinate). Flowers may be attached to the plant in a few ways. If the flower has no stem but forms in the axil of a leaf, it is called sessile. When one flower is produced, the stem holding the flower is called a peduncle. If the peduncle ends with groups of flowers, each stem that holds a flower is called a pedicel. The flowering stem forms a terminal end which is called the torus or receptacle. The parts of a flower are arranged 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:
* Calyx: 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 animals that help the process of pollination. The coloration may extend into the ultraviolet, which is visible to the compound eyes of insects, but not to the eyes of birds.
* Androecium (from Greek andros oikia: man's house): one or two whorls of stamens, each a filament topped by an anther where pollen is produced. Pollen contains the male gametes.
* Gynoecium (from Greek gynaikos oikia: woman's house): one or more pistils. The female reproductive organ is the carpel: this contains an ovary with ovules (which contain 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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