Flower is the reproductive structure found in the flowering plants. Flower is attached to the stems by a stalk called, the pedicel. The terminal portion of the pedicel is swollen and is known as thalamus. The pedicel in many flowers is condensed or reduced and such flowers are known as sessile. A flower having a pedicel is called pedicellate.
On the thalamus are borne the different floral parts which are arranged in a whorl. A typical flower consists of four whorls – calyx, corolla, androecium and gynoecium. Calyx is the outermost whorl and each individual member of the calyx is called sepal. Sepals are usually green in colour. The next whorl lying internal to calyx is corolla. Each individual member of this whorl is called petal. The petals are usually coloured and showy. The two outer whorls constitute the accessory whorls, which protect the inner whorls.
The third whorl consists of the reproductive organ known as androecium. The individual membranes are called stamens. Each stamen consists of filament and anther. The anther consists of four chambers called the pollen sacs. Within the pollen sacs pollen grains or male pores are present.
The fourth or the inner most whorl consists of the female reproductive organ known as gynoecium or pistil. Each member of this whorl is known as carpel. Gynoecium consists of three parts, the basal portion is swollen and is known as ovary. The ovary contains small egg-like structures. These are the female pores known as ovules. The portion lying above the ovary is pistil and the terminal portion of the pistil is stigma.
Parts of a flower
The thalamus of a flower is condensed and is a slightly swollen structure. The nodes and internodes lie close to one another. Sometimes the internode between corolla and androecium are found to be elongated and is known as androphore, example, Passiflora. Sometimes, the region between the androecium and gynoecium is elongated and is known as gynophore, for example, Pterospermum. In some flowers, the internodes between corolla and androecium (androphore) and between the androecium and gynoecium (gynophore) are elongated and they together constitute gynandrophore, for example, Gynandropsis. The thalamus sometime elongates upwards into a slender axis and the carpel remains attached. Such an axis is called carpophore for example, Impatiens and Coriandrum.
Relative position of the floral whorl on the thalamus: On the basis of gynoecium and other floral whorls, the flowers may be:
- Hypogynous: When the ovary occupies the highest position and the stamens, petals and sepals are developed below the ovary, the condition is known as hypogyny (hypo=below, gynoe= gynoecium) and the flowers are called hypogynous. The ovary is described as superior in a hypogynous flower, for example, Brassica (mustard), Solanum melongena (brinjal), Lycopersicum (tomato) etc.
- Perigynous: Sometimes the thalamus forms a cup-like structure around the ovary and the sepals, petals and stamens are raised to a certain height so that they do not remain below the ovary. Such a condition is called perigyny (peri=in between, gynoe= gynoecium) and the flowers are said to be perigynous. The ovary is described as semi-inferior in a perigynous flower, for example, Rosa (rose).
- Epigynous: In some flowers, the thalamus grows upward completely enclosing the ovary and being fused with it, so that the sepals, petals and stamens are developed from a region of the thalamus above the ovary and the condition is said to be epigyny (epi=upon, gynoe= gynoecium) and the flowers are said to be epigynous. The ovary is described as inferior in an epigynous flower, for example, Tagetes (marigold), Helianthus (sunflower), Cucurbita (gourd).
The green leaf-like structures developed at the axil from which the flowers are developed are known as bracts. They lie at the base of the pedicel of the flowers. When a small leaf-like structure is developed on the pedicel of a flower, it is known bracteole. Depending on their size, shape and colour, the bracts or bracteoles are of the following types.
- Leafy bracts: Bracts look like typical leaves and are green in colour, they are called leafy bracts, for example, Gynandropsis, Acalypha etc.
- Spathe: When a large-coloured bract encloses inflorescence or a group of flowers, it is known as spathe, for example, Musa (banana), Colocasia, Zea mays (maize), Cocos (coconut) etc.
- Involucre: When the bracts are arranged in one or more whorls and they surround a cluster of flowers, the group of bracts is known as involucre, for example, Tagetes (marigold), Helianthus (sunflower), Coriandrum
- Glumes: The dry and scaly bracts found in the flowers of many grasses and rice family are called glumes, for example, Oryza (rice), Triticum (wheat).
- Scaly bracteoles: In some flowers, beside the scaly bracts, very small scaly bracteoles are developed at the base of the florets, for example, Helianthus
- Petaloid bracts: These bracts are usually large and coloured and appear like petals for example, Bougainvillea, Poinsettia.
- Epicalyx: When a whorl of bracteoles is developed below the calyx, they are called epicalyx, for example, Hibiscus, Abelmoschus (lady’s finger), Gossypium (cotton) etc.
Calyx is the first whorl of a flower and each individual of this whorl is called sepal. They may be arranged in a regular, irregular or zygomorphic manner. When the sepals are green in colour, they are described as sepaloid and when they are brightly coloured, they are called petaloid as in Caesalpinia. The sepals may become scaly or hair-like, for example, Helianthu, Tagetes and such modified sepals are known as pappus. They are persistent and help in the dispersal of fruits. Sepals of trapa are modified into spines which act as a defence organ. In case of Impatiens, the calyx is modified into a beak-like projection and in case of Mussaenda, one of the sepals become enlarged, leafy and showy to attract insects for pollination.
The calyx is described as polysepalous when the sepals are free from one another, for example, Brassica, etc. The calyx is said to be gamosepalous when the sepals are united or fused with one another, for example, Hibiscus, Gossypium etc. The calyx is said to be bilabiate when the sepals are united and unequal in size and form a lip-like structure, for example, Ocimum etc. When the sepals fall off as soon as the flower bud opens, they are called caducous, for example, Argemone . The sepals are called deciduous when they survive till the withering of petals, for example, lotus. The sepals are called persistent when they remain for a longer period and remain attached to the fruits, for example Solanum melongena etc).
It is the second whorl inner to calyx. The individual members of this whorl are called petals. The petals are coloured and scented. The petals protect the inner whorls and attract the insects for pollination. Each petal has a narrow basal portion known as the claw and upper portion called the limb. When the petals are free from one another, they are described as polypetalous. When they are united with one another, they are known as gamopetalous. In a gamopetalous corolla, the united basal portion is known as corolla tube and the upper free portions are known as corolla lobes. The junction between the corolla tube and the lobes is called throat. Tube-like appendages are developed from the corolla to store nector called spurs, for example, Impatiens. In some cases, additional structures are developed from the throat of the corolla in the form of lobes, scales or hairs forming the corona, for example, Passiflora, Vinca etc.