What is a flower?
A flower is the sexual reproductive structure in plants commonly known as flowering plants.
Flowering plants compose the clade Angiospermae (from the Greek “angeion” = vessel, and “sperma” = sperm or seed) and have, besides flowers, ovules enclosed by an ovary (that become seeds enclosed in a fruit).
Angiosperms are one of two groups of plants that produce seeds, the other one being the gymnosperms. Although gymnosperms (from the Greek “gymnos” = naked and “sperma”) produce seeds, these are said to be “naked” because they are not enclosed by a fruit. This results from the ovule, which is not contained by an ovary either. Gymnosperms appeared earlier than angiosperms in plant evolution. They have cones formed by scales; ovules develop on the surface of the scales. Common gymnosperms are pines, cypresses, and cycads. A few gymnosperms (phylum Gnetophyta) have cones that look like groups of small flowers but are not real flowers, as they are not angiosperms.
Flower: the reproductive structure of angiosperms, composed of modified leaves, which bear the reproductive cells (gametes) and are the site where sexual reproduction occurs.
Angiosperm: a plant that produces flowers and fruits, and its ovules are enclosed in an ovary.
Sexual reproduction involves the fusion of two haploid gametes to produce a diploid zygote.
Parts of a Flower
No matter a flower’s size, shape, or colour, its function is the sexual reproduction of the plant, and thus it has structures that carry the plant gametes (eggs and sperm, which are the sexual reproductive cells).
A typical flower is composed of four floral structures:
- sepals,
- petals,
- stamens, and
- carpels
Flowers are also arranged in four distinct whorls: the outer calyx, which consists of sepals; the corolla, which consists of petals; the androecium, consisting of stamens; and the gynoecium, consisting of pistils, in the centre. You can visualise all these structures in the following section, which contains a labelled diagram.
Flower Structure in a Labelled Diagram
Flower structures are complex, so just knowing the names of each one might not solve all your questions. To get a better idea of what each part of a flower looks like, have a look at the labelled diagram below.
Fig. 1 - Diagram of a complete flower showing the four main floral parts and their components (left). The floral parts are arranged in whorls (right)
Flower Structure and Function
Flowers are present in a great variety of sizes, shapes, and colours. They can develop as single flowers or in groups called inflorescences.
Flowers are part of the plant shoot system (including non-reproductive leaves, stems, and reproductive flowers) and develop from a stalk called a peduncle.
In inflorescences, each flower has a pedicel that attaches them to the main axis. The tip of a peduncle or a pedicel is the receptacle, which contains the flower parts.
Tulips, roses, and magnolias are relatively big individual flowers. Snapdragons and lilac are inflorescences.
What about sunflowers? They look like big individual flowers; however, sunflowers are a type of inflorescence! Sunflowers, daisies, dandelions, and many others belong to the Asteraceae family, in which the floral receptacle contains numerous tiny flowers.
The flowers at the border that look like yellow petals are small incomplete, and sterile flowers (they do not have stamens or pistils). These surround hundreds of smaller flowers in the centre containing reproductive components. Besides, the tiny flowers develop on a big receptacle and do not have individual pedicels.
Fig. 2 - Flower diversity. From left to right: a rose develops as an individual flower; the next three images show different types of inflorescences: lilac, sunflower, and grass.
Flower Structure: Non-Reproductive Parts
Sepals and petals form the perianth and are non-reproductive components that either protect the reproductive components or attract pollinators.
The sepals are typically leaf-like and smaller than the petals. The group of sepals forms the calyx. These are the ones you see covering and protecting an unopened flower bud. The calyx is the outermost whorl on the receptacle. Petals are typically the conspicuous and coloured structures of the flower that attract pollinators. The group of petals forms the corolla. The corolla whorl lies within the calyx.
Flower Structure: Female Structure of a Flowering Plant
The reproductive components of a flower include the carpels that form the female structure called the gynoecium (the innermost whorl of the flower). The carpels are commonly called pistils.
A pistil can be formed by one (single pistil) or more fused carpels (compound pistil, for example, the right diagram in Figure 2 shows three fused carpels).
A carpel has three parts:
- the ovary in the base is the sac-like structure that contains the ovules (with an egg cell inside).
- the style is an elongated stalk that connects the ovary and the stigma. Because only compatible pollen can grow a pollen tube, the style acts as a buffer against pollen contamination.
- the stigma, which is the tip of the style that is sticky, for capturing pollen.
Flower Structure: Male Structure of a Flowering Plant
The stamens, collectively called the androecium (the whorl that lies between the gynoecium and the corolla) correspond to the male structures of a plant. The stamens produce the pollen that contains the sperm. A flower usually has several stamens; each is composed of a filament (a long and thin stalk that holds the anther) and an anther (a sac-like structure that produces the pollen).
As part of the shoot system, all four flower parts are modified leaves. Each part forms a whorl in the floral receptacle, thus creating four concentric circles. Any of the four floral parts can be highly modified and have a different function than the typical one. For example, stamens can be sterile and colourful, taking the role of petals. In some plants, the petals are missing, and the colourful attractive structures for pollinators are the sepals.
A flower with these four components is called a complete flower. However, flowers exist in many combinations, and some are incomplete flowers that lack one of the components. They can also be classified as perfect - when they are bisexual, presenting both androecium and gynoecium - or imperfect - when they are unisexual (an only-female flower is called carpellate, male flowers are staminate). A plant can have bisexual, unisexual, or both types of flowers. A plant is monoicous when it bears both unisexual flowers and dioicous when an individual plant bears only female or only male flowers.
Flower Structure and Reproduction
For a plant to reproduce sexually, the male gamete has to reach and fertilize the female gamete. However, unlike animals, plants do not move.
How does the sperm get to the egg in flowering plants?
Plants have evolved diverse mechanisms to accomplish that the sperm get to the egg, and in most flowering plants, the flower is central to this task. Plants make use of pollinating agents to move the pollen among flowers. While some plants use wind or water for pollination, the great majority rely on animals (insects, birds, and mammals) to transport the pollen.
Pollination is the transfer of pollen from the male reproductive structure to the female reproductive structure in seed plants.
Angiosperms use flowers to attract pollinators. Most of a flower’s traits (size, colour, nectar, scent) are for attracting a type of pollinator (or not attracting it, if it uses wind for pollination), sometimes even a specific animal species.
Colourful flowers typically attract birds and some insects. But some plants have inconspicuous flowers. Have you seen grass flowers? They pass unnoticed by most people. Grass belongs to the family Poaceae, a big, diverse group that includes grass, pastures, cereal grasses, and bamboo. The wind or insects pollinate most grasses and thus, have typically very small, colourless, or beige-ish flowers (see figure 1, the last image at the right is from a grass). You will learn more about Pollination here.
What are the steps after pollination?
Irrespective of the pollinating agent, pollen grains are transferred from a flower anther to the stigma of another flower. Once on the stigma, the pollen grain germinates, developing a pollen tube. The pollen tube is a long and slender tube that goes all the way down through the style and reaches the interior of the ovary. The sperm (which is inside the pollen grain) moves through this tube to the ovary (Figure 3).
Figure 3. Pollen germination (1) and fertilization (2). Source: modified from LadyofHats, Public domain, via Wikimedia Commons.
The ovary contains one or more ovules. Each ovule contains an embryo sac with a few cells including one egg cell. One sperm fertilizes the egg cell forming a zygote. The zygote develops into the plant embryo, contained in the ovule that develops into the seed.
Fertilization in angiosperms is called double fertilization.
The seed or seeds are enclosed by the ovary that grows to become the fruit. The fruit helps in seed dispersal, and its characteristics (colourful and nutritious or inconspicuous and weightless) depend on the dispersal method. When the seed germinates, a new plant develops, and the cycle starts again.
Fruits can be colourful and nutritious (and delicious!) like apples, mangoes, guavas, and figs, to attract animals that eat them and disperse the seeds. On the other hand, some fruits are also dispersed by animals not by eating them but by transporting them in their fur or feathers. In this case, fruits have to be small and have shape modifications or structures that allow them to stick to an animal's fur or feathers when they walk by (like burs that have spikes or hooks). Likewise, seeds dispersed by wind are small, weightless, and modified to be able to “fly” with the wind (like a dandelion seed with its feathery fruit).
Flower Structure - Key Takeaways
- The flower is the sexual reproductive part of angiosperms that bears the gametes and commonly attracts pollinators.
- In a flower, the non-reproductive sepals and petals mainly protect the reproductive parts and attract pollinators.
- The reproductive stamens and carpels/pistils bear the sperm and eggs in a flower.
- In angiosperms, sexual reproduction occurs in the flower, when the pollen (which contains the sperm) reaches and fertilizes the egg inside the ovary.
- Flowering plants rely on pollinating agents (wind, water, or animals) to transport the pollen among flowers.
- Most of a flower’s traits (size, colour, nectar, scent) are related to the specific pollinating agent that the plant uses.
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Frequently Asked Questions about Flower Structure
Which structure of a flower traps pollen?
The structure of a flower called stigma is sticky and traps the pollen. The stigma is the tip of the female reproductive structure, the pistil or carpel.
Which structure is a stalk that supports a single flower?
The stalk that supports a single flower is the peduncle, however, it can also bear a group of flowers. In this case, each flower is supported by a pedicel.
What is the structure and function of a flower?
A flower structure includes four floral parts: the non-reproductive sepals and petals and the reproductive stamens and carpels/pistils. A flower bears the gametes, and its function is to facilitate the sexual reproduction of the plant by attracting pollinators.
What are the male structures of a flower?
The male structures of a flower are the stamens that produce the pollen. Stamens are collectively called the androecium.
What structure of the flower receives pollen?
The structure of the flower called the stigma receives pollen which then moves through the style to finally get to the ovary where it fertilizes the ovules.
What is a function of a flower?
Through the formation of male and female gametes, flowers carry out sexual reproduction in plants. Fertilisation of gametes results in seeds, which aid in plant reproduction.
What is the function of style in flower?
Pollen grows a tube, called a pollen tube, through the style when it reaches the stigma, eventually reaching the ovary. Pollen tubes can only grow from compatible pollen, so the style acts as a barrier against pollen contamination.
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