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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

Longitudinal section of a buttercup, an hypogynous flower.
The buttercup is a convenient, simple flower to study. The stalk of the flower is called the pedicel and is swollen at the tip forming the receptacle. During development of the flower the rapidly dividing cells of the growing tip produce the floral leaves in the same way that leaves are produced on ordinary stems. The floral leaves develop in more or less concentric circles (whorls), but in some of the more primitive flowers, such as the water-lily, the petals and other organs are arranged in a spiral on the receptacle. The first formed organs of the flower are the five sepals. These are on the lowest part of the receptacle and form the calyx. The sepals are green, leaf-like organs whose chief purpose is to protect the developing flower. Above the sepals are the five yellow petals each with a small pocket at its base which produces nectar and is called a nectary. The group of petals is called the corolla and, with the calyx, forms the perianth. The petals and nectaries attract insects and other animals to the flower and also help to protect the essential sexual organs within. These are two types of reproductive organs in the buttercup flower – the stamens and the carpels. The stamens are the male, pollen-producing organs and form the androecium. Each stamen consists of a stalk (filament) and an anther which is the pollen sac at the tip. The carpels form the gynoecium. Each carpel contains a female egg-cell or ovule which gives rise to a seed when fertilized by a male cell from a pollen grain. The stigma is the tip of the carpel through which the pollen grain gains access to the ovule. The buttercup flower has numerous stamens and carpels but this is not the case in all flowers.

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

plum flower and apple
Plum flower (left). Although the receptacle has grown up around the carpel, the flower is still perigynous. The receptacle of the apple (right) has enclosed the carpels producing an epigynous flower.
The buttercup as we have seen has all four types of floral organ and the parts are arranged in a regular manner. The petals are all of the same size shape and a section cut down through the flower (a longitudinal section) will always produce two similar halves. There are, however, many variations on this structure in the plant kingdom. Many are concerned with pollination which is the transference of pollen from flower to flower. Pea-flowers and many orchids are highly irregular with very oddly shaped petals. There is in such cases only one line along which a section will produce two similar halves. Frequently the petals (and the sepals) join to form a tube which gives added protection and may also serve to hold nectar. Examples are the primrose and bluebell. The carpels are usually above the rest of the flower because of the manner of growth. This is the hypogynous condition but it is not universal. In some flowers the receptacle spreads at the top so that the petals, etc., surround the carpels (perigynous condition) while in others (e.g., the apple) the receptacle grows up around the carpels so that they are below the other parts (epigynous condition). 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 inflorescence

Flowers 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 obe 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

floral formulae and diagrams
Botanists do not need to have a lengthy description of a flower to understand its structure. They use a simple expression – the floral formula. This tells them the number of parts and a good deal about their arrangement.

The letters K, C, A, and G stand for calyx, corolla, androecium, and gynecium, 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, brackers 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.

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