The part of an angiosperm (flowering plant) which is concerned with reproducing the species. A flower is essentially an unlengthened shoot whose leaves are modified to form the floral organs (petals, etc).
Close-up of a buttercup
Below the flower there may be one or two tiny leaves (bracteoles) on the pedicel. These are the very earliest of the floral leaves. The pedicel tip continues to grow after forming them but its growth almost ceases when the sepals are formed so that the floral organs are close together on the receptacle. In a flower such as the snowdrop the bracteole protects the flower until it opens. It can be seen as a green scale at the back of the flower. A leaf at the base of the flower stalk is called a bract.
Varieties of flower structure
A monoecious plant is a plant in which in which the male and female organs are borne on the same plant, but in separate flowers. The male flowers bear stamens, and the female bear one or more carpels. Examples are oak, corn (Zea mays), and walnut. A dioecious is one in which in which the male and female organs are borne in separate flowers which are on separate plants. Examples are willow, hemp, and asparagus.
Any type of organ may may be absent from a flower. If there are no petals the sepals are often brightly colored (e.g., the marsh marigold). Flowers, especially those of trees (e.g. hazel), may be unisexual having only male or female parts in each individual flower. The stamens do not vary a great deal but may be joined to each other (e.g. sweet pea) or to the petals (e.g., primrose). The carpels may contain one or more ovules, each of which gives rise to a seed. The pea pod, for instance, is derived from a single carpel with several ovules. Carpels may be joined as in the bluebell or free as in the buttercup. The flowering plants are classified mainly according to the flower structure.
The inflorescenceFlowers are sometimes borne singly (e.g., anemone, tulip) but more frequently they occur in a group, called an inflorescence, whose appearance depends upon the amount and type of branching. There are however two basic patterns. In one – the racemose pattern – the main growing point of the stem goes on growing – or at least does not produce a flower. The flowers are produced laterally (on the sides) e.g., the bluebell, and the inflorescence is termed a raceme. If the flowers are not stalked it called a spike. An umbel is a special raceme in which the main tip stops growing and all the flower stalks develop at one level producing the familiar head of flowers such as is found in the hogweed and other hedgerow plants. The umbel must not be confused with the corymb, however. This is a raceme in which the pedicels are of different lengths so that the flowers all appear at one level. The grouping of the small flowers in this way makes them more attractive to insects. In the second basic pattern – the cymose pattern – the stalk does end in a flower after giving off one or two branches which also end in a flower after branching. The stitchwort is an example of a cyme.
Flowers of the family Compositae are very specialized. The dandelion "flower" is really a collection of tiny flowers (florets) on a flat disk or capitulum. Each floret contains sexual organs and is a complete flower. The calyx is represented by fine hairs which later develop and carry away the seed. Each dandelion floret has a flat blade or ligule but thistles have only tubular florets. Daises have both types – the outer florets have colored ligules to attract insects while the inner florets are tubular and produce pollen and nectar. The Compositae is a very widespread and successful family of plants.
Floral formulae and floral diagrams
The letters K, C, A, and G stand for calyx, corolla, androecium, and gynoecium, respectively. P (for perianth) is used if the sepals and petals are alike. The formula for a buttercup is K5 C5 A∞ G∞, where ∞ means "numerous". A line under the carpel figure means that the flower is hypogynous. Above the figure it indicates epigyny. Where parts are joined, brackets surround the figure. The formula does not give a complete description. A floral diagram and a section cut through the flower are required to make the structure quite clear.
The floral diagram consists of a plan view of the flower with the organs arranged on circles or spirals, showing the degree of overlapping, any fusion of parts or irregularity and the position relative to the main stem of the plant (indicated by a small circle). Bracts and bracteoles are also shown. The longitudinal section is necessary to show the degree of perigyny if any.
Related category BOTANY
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