Inflorescence Flower

5.5 The Inflorescence

A flower is a modified shoot wherein the shoot apical meristem changes to floral meristem. The arrangement of flowers on the floral axis is termed as inflorescence. Depending on whether the apex is converted into a flower or continues to grow, two major types are recognised: racemose and cymose. In racemose, the main axis continues to grow; flowers are borne laterally in acropetal succession (older below, younger above). In cymose, the main axis terminates in a flower; further growth is limited; younger flowers below the older one (basipetal).

5.6 The Flower

The flower is the reproductive unit in angiosperms. It is meant for sexual reproduction. A typical flower has four kinds of whorls arranged successively on the swollen end of the stalk or pedicel, called thalamus or receptacle. These are: calyx (sepals), corolla (petals), androecium (stamens), and gynoecium (carpels).

Calyx and corolla are accessory organs; androecium and gynoecium are reproductive organs. In some flowers, calyx and corolla are not distinct and called perianth.

5.6.1 Symmetry

A flower is actinomorphic (radially symmetrical) when it can be divided into two equal radial halves through any radial plane (mustard, datura, chilli). It is zygomorphic (bilaterally symmetrical) when it can be divided into two similar halves only in one particular vertical plane (pea, gulmohur, bean, Cassia). A flower is asymmetric (irregular) if it cannot be divided into two similar halves by any vertical plane (canna).

5.6.2 Number of Floral Parts

Based on number of floral appendages, a flower may be trimerous, tetramerous, or pentamerous when the floral appendages are in multiples of 3, 4, or 5 respectively. Flowers with bracts (reduced leaf below pedicel) are bracteate; without bracts, ebracteate.

5.6.3 Position of Ovary

Based on the position of calyx, corolla and androecium with respect to the ovary on thalamus, the flowers are described as:

• Hypogynous flower — gynoecium occupies the highest position; other parts below it. Ovary is superior. e.g., mustard, china rose, brinjal.
• Perigynous flower — gynoecium situated in the centre; other parts at the same level. Ovary is half-inferior. e.g., plum, rose, peach.
• Epigynous flower — margin of thalamus grows upward enclosing the ovary completely; other floral parts arise above it. Ovary is inferior. e.g., guava, cucumber, ray florets of sunflower.

5.7 Parts of a Flower

5.7.1 Calyx

Calyx is the outermost whorl, members called sepals. Generally green, leaf-like; protect the flower in bud stage. May be gamosepalous (united) or polysepalous (free).

5.7.2 Corolla

Corolla is composed of petals. Brightly coloured to attract insects for pollination. Like the calyx, corolla may be gamopetalous (united) or polypetalous (free). Shape and colour vary greatly. Tubular, bell, funnel or wheel-shaped.

Aestivation

The mode of arrangement of sepals or petals in floral bud with respect to the other members of the same whorl is known as aestivation. Main types: valvate, twisted, imbricate, and vexillary.

5.7.3 Androecium

Androecium is composed of stamens. Each stamen has a stalk (filament) and an anther. Anther is usually bilobed and each lobe has two chambers — pollen sacs. 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 (brinjal); attached to perianth — epiphyllous (lily). Within the androecium, stamens may be united in one bundle (monoadelphous, china rose), two bundles (diadelphous, pea), or many bundles (polyadelphous, citrus).

5.7.4 Gynoecium

Gynoecium is the female reproductive part of the flower, made of one or more carpels. A carpel consists of three parts — stigma, style, and ovary. The ovary is the enlarged basal part on which lies the elongated style; the style connects ovary to stigma. The stigma is at the tip of the style and is the receptive surface for pollen grains.

When more than one carpel is present, they may be free (apocarpous, lotus, rose) or fused (syncarpous, mustard, tomato). Each ovary bears one or more ovules attached to a flattened, cushion-like placenta.

5.7.5 Placentation

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

• Marginal — placenta forms a ridge along ventral suture; ovules borne on this ridge in two rows. e.g., pea.
• Axile — placenta is axial; ovules attached in multilocular ovary. e.g., china rose, tomato, lemon.
• Parietal — ovules develop on inner wall or peripheral part of unilocular ovary. e.g., mustard, Argemone.
• Free central — ovules borne on central axis lacking septa. e.g., Dianthus, Primrose.
• Basal — placenta develops at the base of the ovary; single ovule attached. e.g., sunflower, marigold.

Worked Examples