Flowering plants are heterosporangiate (which means they produce two types of reproductive spores) and the pollen (male spores) and ovules (female spores) are produced in different organs, but these are together in a bisporangiate strobilus that is the typical flower.
A flower is regarded as a modified stem with shortened internodes and bearing, at its nodes, structures that may be highly modified leaves.
In essence, a flower structure forms on a modified shoot with an apical meristem that does not continuously grow (growth is determinate).
The stem is called a pedicel, the end of which is 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 and working upwards) are:
- 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 insects that help in the process of pollination.
- 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 (female gametes). A pistil may consist of a number of carpels merged together, whereby 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, or the style becomes the pathway for pollen tubes to grow from pollen grains adhering to the stigma, to the ovules, carrying the reproductive material.
Plant species show a wide variety of modifications from this plan. These modifications have significance in the evolution of flowering plants and are used extensively by botanists to establish relationships among species.
In the majority of species, individual flowers have both pistils and stamens as described above. These flowers are described by botanists as being perfect, bisexual, or hermaphroditic. However, in some species the flowers are imperfect or unisexual: having only either male or female parts. In the latter case, if an individual plant is either male or female the species is regarded as dioecious. Where unisexual male and female flowers appear on the same plant, the species is considered monoecious.
Some flowers with both stamens and a pistil are capable of self-fertilization! This increases the chance of producing seeds but limits genetic variation. The extreme case of self-fertilization occurs in flowers that always self-fertilize, such as the common dandelion.
Conversely, many species of plants have ways of preventing this. Unisexual male and female flowers on the same plant may not appear at the same time, or pollen from the same plant may be incapable of fertilizing its own ovules. The latter flower types, which have chemical barriers to their own pollen, are referred to as self-sterile or self-incompatible. |