The Flower

• The flowers are the reproductive unit in the angiosperms and is meant for sexual reproduction.

• A typical flower has four different kinds of whorls ie calyx, corolla, androecium and gynoecium, set consecutively on the swollen end of the stalk or pedicel, called thalamus or receptacle. The accessory organs are Calyx and corolla, while androecium and gynoecium are reproductive organs.

• Flowers like lily, the calyx and corolla are not distinct and are termed as perianth. Flowers with androecium and gynoecium, are bisexual and similarly flowers with either only stamens or carpels only is unisexual.

• In symmetry, the flowers may be actinomorphic (radial symmetry) or zygomorphic (bilateral symmetry). If a flower can be divided into two equal radial halves in any radial plane passing through the centre, it is actinomorphic, ex., mustard, datura, chilli and when it can be divided into two similar halves only in one particular vertical plane, it is zygomorphic, ex., pea, gulmohur, bean, Cassia.

• If a flower cannot be divided into two similar halves by any vertical plane passing through the centre, then it is asymmetric (irregular), ex., canna.

• The flowers are categorised as trimerous, tetramerous or pentamerous, depending on the floral appendages availability in multiples of 3, 4 or 5, respectively. 

• Flowers with bracts-reduced leaf found at the base of the pedicel- are termed as bracteate and those without bracts, ebracteate.

• The flowers are described as hypogynous, perigynous and epigynous, based on the position of calyx, corolla and androecium in respect of the ovary on thalamus.

• In the hypogynous flowers the gynoecium occupies the highest position whereas the other parts are placed below it. The ovary in such flowers are superior, ex., mustard, China rose and brinjal.

Position of floral parts on thalamus : Hypogynous

• If gynoecium is situated in the centre and other parts of the flower are located on the rim of the thalamus almost at the same level, it is perigynous. The ovary in these are half inferior, ex., plum, rose, peach.

Position of floral parts on thalamus : Perigynous

• In epigynous flowers, the margin of thalamus ascends enclosing the ovary completely and getting fused with it, the other parts of flower arise above the ovary. Hence, the ovary is said to be inferior, ex., flowers of guava and cucumber, and the ray florets of sunflower.

Position of floral parts on thalamus : Epigynous

Parts of a Flower:

Each flower normally has four floral whorls, viz.,

· Calyx

· Corolla

· Androecium

· Gynoecium

Parts of a flower

Calyx:

This forms the outermost whorl of the flower and the members are called sepals. The sepals are greenish, leaf like and protect the flower in the bud stage, generally. The calyx may be gamosepalous (sepals united) or polysepalous (sepals free).

Corolla:

It is composed of petals, which are usually brightly coloured to attract insects for pollination. Corolla may be also united gamopetalous (petals united) or polypetalous (petals free) just like Calyx. Its shape and colour vary greatly in plants and it may be tubular, bell shaped, funnel-shaped or wheel-shaped.

Aestivation: The manner of alignment of sepals or petals in floral bud with respect to the other members of the same whorl is known as aestivation.

Its main types are:

· Valvate

· Twisted

· Imbricate

· Vexillary

When sepals or petals in a whorl just touch one another at the margin, without overlapping, it is said to be valvate, ex., Calotropis.

Valvate

If one margin of the appendage overlaps that of the next one and so on it is called twisted, ex., China rose, lady’s finger and cotton.

Twisted

If the margins of sepals or petals overlap one another but not in any particular direction, the aestivation is called imbricate, ex., Cassia and gulmohur.

Imbricate

If there are five petals, the largest (standard) overlaps the two lateral petals (wings) which in turn overlap the two smallest anterior petals (keel); this type of aestivation is vexillary or papilionaceous, ex., pea and bean flowers.

Vexillary

Androecium:

• This is composed of stamens. Each stamen, represents the male reproductive organ contains, a stalk or a filament and an anther. Each anther is usually bi lobed and each lobe has two chambers, the pollen-sacs.

• The pollen grains are produced in pollen-sacs. A sterile stamen is called staminode.

• Stamens of flower may be united with other members such as petals or among themselves. When stamens are attached to the petals, they are epipetalous, ex., brinjal, or epiphyllous when attached to the perianth, ex., flowers of lily.

• The stamens in a flower may either remain free (polyandrous) or may be united in varying degrees. The stamens may be united into one bunch or one bundle (monoadelphous) ex., China rose, or two bundles (diadelphous) ex., pea, or into more than two bundles (polyadelphous) ex., citrus.

• Flower may have a variation in the length of filaments within a flower, ex., Salvia and mustard.

Gynoecium:

• It is the female reproductive part of the flower and is made up of one or more carpels and each carpel consists of three parts namely stigma, style and ovary.

• Ovary is the enlarged basal part, on which lies the elongated tube, the style.

• The style connects the ovary to the stigma.

• The stigma is usually at the tip of the style and is the accessible surface for pollen grains.

• Each ovary bears one or more ovules attached to a flattened, cushion-like placenta.

• When more than one carpel is present, they may be free (lotus and rose) and are called apocarpous.

• When carpels are fused, they are termed syncarpous, ex., mustard and tomato.

• After fertilisation, the ovules develop into seeds and the ovary matures into a fruit.

Placentation: The arrangement of ovules within the ovary is known as placentation.

The placentation are of different types namely,

· Marginal

· Axile

· Parietal

· Basal

· Free central

In marginal placentation the placenta forms a ridge along the ventral suture of the ovary and the ovules are borne on this ridge forming two rows, ex., pea. 

Marginal

When the placenta is axial and the ovules are attached to it in a multilocular ovary, the placentation is axile, ex., China rose, tomato and lemon.

Axile

In parietal placentation, the ovules develop on the inner wall of the ovary or on peripheral part. Ovary is one-chambered but it becomes two chambered due to the formation of the false septum, ex., mustard and Argemone.

Parietal

When the ovules are borne on central axis and septa are absent, ex., Dianthus and Primrose the placentation is called free central.

Free central

In basal placentation, the placenta develops at the base of ovary and a single ovule is attached to it, ex., sunflower, and marigold.

Basal