A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants. The biological function of a flower is to mediate the union of male sperm with female ovum in order to produce seeds. The process begins with pollination, is followed by fertilization, leading to the formation and dispersal of the seeds.
For the higher plants, seeds are the next generation, and serve as the primary means by which individuals of a species are dispersed across the landscape. The grouping of flowers on a plant are called the inflorescence.
A flower structure forms on a modified shoot or axis with an apical meristem that does not grow continuously (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 is the outer whorl of sepals which are typically green, but are petal-like in some species.
The corolla is the whorl of petals, which are usually thin, soft and colored to attract insects that help the process of pollination.
Androecium is one or two whorls of stamens, each a filament topped by an anther where pollen is produced. Pollen contains the male gametes.
Gynoecium contain one or more pistils. The female reproductive organ is the carpel which 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.
In the majority of species individual flowers have both pistils and stamens as described above. These flowers are described by botanists as bisexual or hermaphrodite. However, in some species of plants the flowers unisexual, having only either male (stamens) or female (pistil) parts. In the latter case, if an individual plant is either female or male the species is regarded as dioecious. However, where unisexual male and female flowers appear on the same plant, the species is considered monoecious.