The Flower ~ Structure, Types, Examples, Definitions

A flower is a specialized shoot in which the leaves are modified to form floral structures.

Complete and Incomplete Flower

A flower which has all the four whorls, calyx, corolla, androecium and gynoecium, is called a complete flower. Examples: Hibiscus, rose, sunflower etc.

A flower in which one or more sets of floral whorls are absent is called an incomplete flower. Examples: Rue anemone, American elm, black walnut etc.

Essential & Non-essential Whorls

The essential whorls of a flower are the whorls that are directly associated with the process of reproduction. Examples: Stamen and carpel.
The non-essential whorls of a flower are the helping parts which either protect the reproductive parts of the flower or make them attractive for pollination. Examples: Calyx and corolla.
Also Read Respiration in Plants

Perianth of a Flower

In plants, such as wheat and grasses, the petals and sepals are undifferentiated and together compose the perianth.
In some plants, the sepals are not green but appear brightly coloured like the petals. These sepals are called petaloid sepals, and the perianth is called a petaloid perianth. Example: Orchid etc.
In some plants, the petals appear green in colour and are called sepaloid petals. The perianth is called a sepaloid perianth. Examples: Viscum, Mistletoe etc.

Bracts are modified leaves arising from the axil of a small leaf-like structure. They may be green-colored like the leaves or different colored like the petals. Examples: Hibiscus, Bougainvillea etc.

The nectar in the flowers is secreted by a group of cells, present either at the base of the pistil or on the petals. These groups of cells are called nectaries. Example: Nasturtium etc.

Unisexual and Bisexual Flower

Flowers that contain both male and female reproductive structures are called a bisexual flower. Example: Hibiscus etc.
Flowers that have only one reproductive structure, either the male or female, are called a unisexual flower. Examples: Papaya, palm etc.
Unisexual flowers which contain only the androecium are called a male or staminate flowers. Example: Eastern cottonwood etc.
Unisexual flower which contain only the gynoecium are called a female or pistillate flowers. Example: Date palm etc.
Flowers in which both male and female reproductive organs are absent are called a neuter flowers. Example: Ray florets of sunflower etc.
If the male and female flowers grow on the same plant, then the plant is said to be a monoecious plant. Examples: Pumpkin, maize, cucumber etc.
If the male and female flowers grow on separate plants, then the plant is said to be a dioecious plant. Examples: Palm, papaya etc.

Calyx of a Flower

The calyx is the outermost whorl of a flower. It is made of small, green leaf-like structures called sepals.
The sepals completely cover the flower at the bud stage and provide protection to the inner whorls of the flower. The sepals, if green in colour, perform photosynthesis, thereby contributing to the production of food in the plant. The non-green sepals sometimes perform the function of attracting pollinators.
If sepals are joint or fused together, then the calyx is said to be gamosepalous. Example: Hibiscus etc.
If sepals are free and not joined together, then the calyx is said to be polysepalous. Examples: Rose, mustard etc.
In Hibiscus, the bracts form a whorl called epicalyx, just below the calyx.

Corolla

The corolla is the second whorl from the outside of a flower. It consists of units called petals.
If the petals of a flower are fused or united, forming a tube-like structure, then they are called gamopetalous. Examples: Nerium, Ipomea etc.
If the petals of a flower are not fused and are free from each other, then they are called polypetalous. Examples: Rose, mustard etc.
Petals are generally arranged in a single whorl, but in some plants, they are arranged in double whorls. Example: Poppy etc.
In some plants, the petals are spirally arranged. Example: Water lily etc.
In some flowers, the number of petals is equal to the number of sepals. Example: Buttercup flower etc.
In some flowers, the number of petals is not equal to the number of sepals. Example: Rose etc.
The corolla of a flower performs two main functions—pollination and protection.

Stamens

The androecium is the third whorl from the outside and represents the male reproductive part of the flower. Stamens together constitute the androecium.
When the stamens of a flower are free, the condition is called polyandrous. Example: Petunia etc.
When the stamens of a flower are fused, the condition is called adelphous. Example: Hibiscus etc.

Filaments

When the filaments of the anthers in a flower are fused into one group, the condition is called monadelphous. Example: Hibiscus etc.
When the filaments of the anthers in a flower are fused into two groups, the condition is called diadelphous. Example: Pea etc.
When the filaments of the anthers in a flower are fused into more than two groups, the condition is called polyadelphous. Example: Bombax etc.

Carpel

The gynoecium or pistil is the female reproductive part of the plant. The pistil is made of units called carpels.
The pistil consists of either a single carpel or many carpels.
If the carpels of a flower are free, then the gynoecium is said to be apocarpous. Example: Buttercup flower etc.
If the carpels of a flower are fused, then the gynoecium is said to be syncarpous. Example: Orchid etc.
A bicarpellary ovary has two carpels.
A tricarpellary ovary has three carpels.

A superior ovary is an ovary attached to the receptacle above the attachment of other floral parts such as the thalamus and the calyx. Example: Onion etc.
An inferior ovary lies below the attachment of other floral parts such as the thalamus and the calyx. Example: Rose etc. An inflorescence is a cluster or a group of flowers arranged on the plant stem.

Placentation

The arrangement of ovules inside the ovary is called placentation.
In flowers showing marginal placentation, the ovary is monocarpellary and single chambered. The placenta, along with the ovules, develops along the margins of the carpel. Example: Pea etc.
In flowers showing axile placentation, the ovary is two chambered or many chambered, in which the margins of the carpels fuse together. The fused margin grows inwards to meet in the centre of the ovary, forming a central axis. The placenta, along with the ovules, develops from this axis. Examples: China rose, lemon, orange etc.
In flowers with parietal placentation, the ovules are attached to the walls of a unilocular ovary. The ovules are borne on the inner surface of the ovary wall. Examples: Mustard, Argemone etc

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