Calyx: Structure, Location, Features, Types, Functions, Practice Problems, and FAQs

We all like flowers as they look so beautiful. These are the ornaments of the plant. We use them for decorations, poojas and in almost all the ceremonies like weddings, housewarmings, funerals etc. At your home you may have some flower pots, right?. Have you ever observed that there are some green colour leaf-like structures present under the colourful petals in a flower like Hibiscus? Tell me what these structures are?

When you were a foetus, you were protected inside the womb of the mother in amniotic fluid. It used to protect you from all dangers.

Fig: Mature foetus in the amniotic sac

Those green colour structures present below the petals are sepals. In the same way the foetus is protected inside the womb, a flower bud is protected by the calyx. It is the whorl of sepals. These parts are not always very attractive like petals, but are important for flowers, especially in the bud stage.

In flowers, different types of calyxes are present. In some flowers, these are green in colour but in some flowers, they are also modified and colourful. Now let’s take a deep dive into the details of calyx in this article.

Table of contents

• Calyx
• Location of calyx
• Types of flowers based on united or free sepals
• Types of flowers based on the number of sepals
• Types of sepals based on colour
• Types of sepals based on persistence and loss
• Types of sepals based on shapes
• Functions of calyx
• Practice Problems
• FAQs

Calyx

The calyx is described as the outermost whorl of sepals. These are normally green leaf-like structures. As it is the outer whorl, it provides protection to all the interior parts of a flower. It also prevents rapid transpiration from the flower's inner parts. The calyx is represented by the symbol ‘K’.

Fig: Calyx

Odd sepal

The sepal which is seen in line with the mother axis is called an odd sepal. This sepal is only one and it appears like a leaf-like structure. Usually, the odd sepals are present posteriorly but in the Leguminosae or pea family, it is present anteriorly.

Fig: Butterfly pea flower

Location of the calyx

At the base of a flower, on top of the pedicel (the stalk of the flower that connects it to the plant), calyxes are located. A flower is supported and protected during the developmental stages by the flexible structures of the calyx. To protect the flower from attack and environmental harm, some calyxes have undergone modifications. Some flowers lose their calyx after they bloom, causing the sepals to fade or separate from the flower.

Fig: Location of sepals

Features of the calyx

The calyx shows the following characteristics:

• It is the lowermost whorl.
• Epicalyx develops from the base of the calyx. It is a whorl of bracteoles. Examples include Hibiscus.

Fig: Epicalyx in Hibiscus

• The sepals are normally green and leaf-like.
• These appear thicker and harder than the leaves and show veins.
• It can be fused or free.
• Sometimes sepals undergo modifications and form several appendages. It depends on the environmental conditions.
• In most of the plants, sepals fall off as the flower blooms.

Fig: Blooming of flowers

• In a few plants, sepals remain intact till the petals fall off.
• In a few other plants sepals remain permanently intact with the fruit.

Types of flowers based on united or free sepals

The flowers are categorised into two types on the basis of the presence of united or free sepals. These are as follows:

• Gamosepalous or synsepalous flower
• Polysepalous flower

Gamosepalous or synsepalous flower

The sepals of the calyx are fused with its lateral margins. Examples include Petunia.

Fig: Gamosepalous flower

Polysepalous flower

Polysepalous flowers consist of a calyx with free sepals. An example of this condition is Rosa rubiginosa.

Fig: Polysepalous flower

Types of flowers based on the number of sepals

The flowers are divided into three categories based on the number of sepals. These are listed below:

• Trimerous
• Tetramerous
• Pentamerous

Trimerous

In this condition, the sepals are present in three or in a multiple of three. Examples include onions (Allium cepa), lilies etc.

Fig: Trimerous flowers

Tetramerous

In this condition, the sepals are present in four or a multiple of four. Examples include the flowers from the family Brassicaceae.

Fig: Tetramerous flowers

Pentamerous

In pentamerous conditions, the sepals present are five or a multiple of five. Examples include Hibiscus.

Fig: Pentamerous flowers

Types of sepals based on colour

The sepals are divided into two categories based on their colour. These are as follows:

• Sepaloid
• Petaloid

Types of sepals based on persistence and loss

According to the time sepals fall off, there are three types as follows:

• Caducous
• Deciduous
• Persistent

Caducous

In this condition, the sepals fall off just before the flowers open perfectly. Examples include Papaver somniferum (poppy).

Fig: Poppy

Deciduous

In this condition, sepals fall off at the time of the withering of the flower after fertilisation. Examples include Brassica.

Fig: Mustard (Brassica nigra) flower

Persistent

Sepals persist in this condition till the fruit formation. The sepals are present even in the mature fruit. The persistent sepals can be of two types as follows:

Accrescent

In this condition the sepals grow in size along with the fruit and remain green. It may cover the fruit partially or completely. Examples include brinjals (Solanum melongena), chilies (Capsicum frutescens), and tomatoes (Solanum lycopersicum L.).

Fig: Accrescent calyx

Marcescent

The sepals in this condition do not grow along with the fruit. Therefore, they become dry. Examples include guava (Psidium guajava) and pepper (Piper nigrum).

Fig: Marcescent calyx in guava

Types of sepals based on shapes

There are different types of sepals based on shapes as follows:

Function of calyx

The calyx serves a number of purposes to ensure the flower's survival while it grows and blooms. The following are the major functions of the calyx:

• Protection of the flower
• Photosynthesis
• Defence against insects

Protection of the flower

A flower bud is enclosed in the calyx as it develops. This forms a leafy outer layer. The developing bud is protected from harmful elements like heat, wind, and radiation by this strong outer whorl. The growing flower is kept hydrated by its protective coating. It is ideal to think of the calyx as a leafy home with walls that protect the flower both during development and the process of flowering.

Fig: Protection of bud

Photosynthesis

For the survival of plants, the process of photosynthesis is important. Photosynthesis occurs in chloroplasts which possess chlorophylls. Photosynthesis is the process by which plants use water and carbon dioxide in the presence of sunlight and chlorophylls to create oxygen and energy in the form of sugar. Stomata are found mainly on the outside of the plant, notably in the leaves and stems. But it is also present on the sepals of the calyx. These are the structures that help in the exchange of gases such as carbon dioxide and oxygen. For example, some apple species have a calyx stomatal density that is near to but less than the density of the leaves and stems. Flowering plants benefit from calyx photosynthesis also because of the requirement of the tremendous amount of energy needed to produce flowers and seeds.

GIF: Stomata

Defence against insects

Insects and herbivores prey on the delicate tissue of a developing bud and the sweet nectar of a flower. The world of flowers has evolved defence systems, such as structural and chemical adaptations, for protection. Many modified calyxes of angiosperms contain thorns or are covered with hairs. These protrusions discourage insects or produce a crowded surface that restricts movement of insects across the calyx surface. For example, the tomatillo plant's calyxes have enormous thorns that aid in protection from insects and herbivores. The calyx hairs of some flowers release a viscous liquid that acts as an effective glue to hold the insect to the calyx. As demonstrated in some kinds of passion flowers, digestive enzymes rapidly liquefy the dead insects, preventing decomposers from invading the flower buds.

Practice Problems

1. A flower without the unnecessary whorls is referred to as ________________.

1. dioecious
2. monoecious
3. hermaphrodite
4. achlamydeous

Solution: The calyx and the corolla are the non-essential whorls of a flower because they are not involved in producing gametes and seeds. The term ‘achlamydeous’ refers to a flower that lacks both the non-essential whorls (calyx and corolla). Examples include Betel (Piper betle). Hence, the correct option is d.

2. Identify the structures that make a complete flower.

1. Calyx, corolla, androecium and gynoecium
2. Calyx and corolla
3. Androecium and gynoecium
4. Corolla, androecium and gynoecium

Solution: A flower which possesses all of the floral whorls or parts, including the calyx (whorl of sepals), corolla (whorl of petals), androecium (whorl of stamens), and gynoecium (whorl of carpels or pistils), is called a complete flower. Rose and Hibiscus are two common examples. Hence, the correct option is a.

Fig: Complete flower

3. What is epicalyx?

1. a whorl of bracts
2. a whorl of bracteoles
3. a whorl of petals
4. a whorl of androecium

Solution: The calyx of a flower is surrounded by an extra whorl called an epicalyx. This outer whorl is made up of a bunch of bracteoles. On the flower stalk, bracteoles are little, scaly, or leafy structures. It exists between the bracts and the calyx. Examples include Hibiscus. Bracts are emerald-green, leaf-like appendages that shield flower buds. A whorl of bracts is called involucre. A whorl of petals is called corolla. Hence, the correct option is b.

Fig: Epicalyx in Hibiscus

4. Identify the flower in which the odd sepal is enlarged and leaf-like?

1. Rose
2. Smilax
3. Mussaenda
4. Bougainvillaea

Solution: The sepal which is seen in line with the mother axis is called an odd sepal. This sepal is only one and it appears like a leaf-like structure. Mussaenda has sepals that enlarge and take on a leaf-like shape. It is an odd sepal. It could be white or vividly coloured. It attracts insects for pollination. Hence, the correct option is c.

Fig: Mussaenda

FAQs

1. Which flower is considered as the single largest flower in the world?

Answer: The largest individual flower in the world, measuring more than three feet in diameter and weighing 20 pounds, belongs to the Rafflesia arnoldii species.

2. What type of calyx is present in the Cannabis plant?

Answer: In Cannabis, the calyx is a translucent layer at the base of the flower and it is not visible to the naked eye.

Fig: Cannabis

3. What does swollen calyx mean?

Answer: Farmers consider the swollen calyxes as a sign of maturity. It indicates the readiness for harvesting. Growers normally prefer a phenotype which has a high calyx to leaf ratio.

4. What is the difference between sepal and bract?

Answer: The main difference between the sepal and bract is as follows:

A whorl of sepal is called the calyx. It protects the flower bud. Bract is a comparatively smaller and leaf-like structure. Bracts are emerald-green that shield flower buds. A whorl of bracts is called involucre.

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