Calyx and Corolla: Types, Cohesion, Aestivation, Perianth, Practice problem and FAQs

Flowers always lift my mood up. They can brighten up any person or any place with their bright and beautiful colours, fragrance and beautiful shapes. Which is the most attractive part of a flower? Needless to say that it is the bright and attractive petal, right? But do you know that there is more to a flower than just petals? In fact, a flower has 4 different parts arranged in four different whorls and each whorl has its own specific function. In fact, you will be amazed to know that petals are considered to be one of the non-essential whorls of a flower. Then why are they the most attractive parts of a flower? What purpose do they serve? Don’t worry, you will get to know as you read along. 

There’s another non-essential part of a flower that you may have noticed as the green leaf-like structures present outside the petals at the base, attached to the tip of the flower stalk. These are the sepals.

Do you think all flowers have similar arrangement, shape and types of sepals and petals? Of course not! Or else wouldn’t they have looked identical to each other?So how do different flowers vary in terms of their sepals and petals? Come let’s learn more about this.

Table of contents

• Flower
• Calyx
• Corolla
• Perianth
• Aestivation
• Practice Problems
• FAQs

Flower

Flowers are highly adapted to attract their specific pollinators such as bees, flies, moths, hummingbirds, and bats just to name a few. This is the reason flowers are the beautiful and attractive parts of a plant with brightly colored petals, fragrance and sweet nectar. Flower is a reduced shoot which is the reproductive structure of angiosperms. Flowers are attached to the main stem by a stalk called pedicel. It is a unique feature of angiosperms facilitating sexual reproduction in them. Plants are also classified into different categories depending on floral attributes. 

A typical flower has four parts - sepals, petals, stamens and pistils/carpels. Each part is arranged as a distinct whorl. Sepals collectively form the calyx, petals form the corolla, the stamens form the androecium or the male reproductive whorl and the pistils form the gynoecium or the female reproductive whorl. The calyx and corolla are known as the non-essential whorls of a flower as they are not directly involved in the process of reproduction. Let us discuss more about these non-essential whorls of a flower.

Calyx

Calyx is the outermost leafy whorl of the flower. It is the collective term used for sepals. It protects the flower in the bud stage. It is represented by K.

Sepals are usually green in colour but they can also be non-green, in which case they are known as petaloid.

                                  Fig: Calyx

Normally sepals are deciduous and fall down at the time of withering of the flower, post fertilisation. In some cases, sepals are caducous and fall down when the flower opens. Sepals can also be persistent and remain attached to the fruit, as seen in tomato, brinjal, guava, etc.

Classification of flowers based on cohesion of sepals in the calyx

Based on whether the sepals in a flower remain free or fused to each other, the calyx can be of two types - polysepalous and gamosepalous.

Polysepalous calyx

Flowers with free sepals, as is seen in a rose, are said to have polysepalous calyx. A flower having five free sepals is represented as K5.

               Fig: Polysepalous flower with free sepals

Gamosepalous calyx

Flowers in which the sepals are fused, as seen in Hibiscus, are said to have gamosepalous calyx. A flower having five fused sepals is represented as K(5).

                                     Fig: Gamosepalous flower with fused sepals

The fusion between the sepals can be toothed (fused almost to the top and has small pointed outgrowths), fid (fused to half length), partite (fused to less than half the length) and connate (fused only at the base).

Gamosepalous calyx can of different types based on its structure - 

• Campanulate - bell like, e.g, Petunia.
• Infundibuliform - funnel shaped, e.g., Atropa.
• Tubular - tube like, e.g, Verbena.
• Cupulate - cup like, e.g, Gossypium.
• Urceolate - urn shaped, e.g, Dianthus.
• Hooded - one sepal enlarged to form a hood, e.g., Aconitum.
• Bilabiate - having two lips, e.g, Ocimum.
• Spurred - drawn out to form a beak, e.g, Delphinium ajacis.
• Spinous - sepals turned into spines, e.g, Trapa.
• Pappus - hairy or feathery, e.g, Sonchus.

                                        Fig: Different types of calyx

Epicalyx

It is an extra whorl of green sepal-like structures that lie below and outside the calyx. It is normally seen in china roses and in flowers of some rosaceous plants such as strawberries.

                                             Fig: Epicalyx

Corolla

Petals are united to form the second whorl of the flower, that is, corolla.

                                                               Fig: Corolla

Corolla is usually brightly coloured to attract insects for pollination. It is represented by the letter ‘C’. Green coloured petals are said to be sepaloid.

Classification of flowers based on cohesion of petals

Based on whether the petals are free or fused, corolla can be gamopetalous or polypetalous.

Polypetalous corolla

The petals in such flowers remain free as seen in Hibiscus. A flower having five free petals is represented as C5.

Polypetalous corolla can be of the following shapes -

• Cruciform - Four unguiculate or clawed, free petals are arranged like a cross, e.g, Brassica.
• Caryophyllaceous - Corolla has five unguiculate petals with long claws and horizontally bent limbs, e.g, Dianthus.
• Rosaceous - sessile petals spread horizontally, e.g, rose.
• Papilionaceous - five free and unequal petals arranged like a butterfly with a large bilobed, posterior standard or vexillum petal. The standard petal overlaps the adjacent margins of two smaller wings which in turn overlap the margins of two anterior petals which remain fused at the upper anterior margins to form a boat like structure called a keel or carina, e.g., pea flower.

                              Fig: Different types of polypetalous corolla

Gamopetalous corolla

Petals are united in such a corolla. A flower having five fused petals is represented as C(5).

Based on shape, gamopetalous corolla can be of the following types -

• Campanulate - bell like, e.g, Campanula.
• Infundibuliform - funnel shaped, e.g., Petunia.
• Tubular - tube like, e.g, disc florets of sunflower.
• Urceolate - urn shaped, e.g, Bryophyllum.
• Hypocrateriform or Salver shaped - long tubular with spreading lobes, e.g., Clerodendron.
• Rotate - Short tubular with horizontal limb to appear wheel-like, e.g., Solanum nigrum.
• Bilabiate - having two lips, e.g, Ocimum, Salvia.
• Spurred - having a beak like outgrowth, e.g, Delphinium ajacis.
• Ligulate - strap like with a tubular base, e.g, ray florets of sunflower.

                      Fig: Different types of gamopetalous corolla

Perianth

If sepals and petals cannot be distinguished, they are collectively known as the perianth. The term tepals is used for the individual units of the perianth. Tepals are sepaloid if green and petaloid if colourful. It is represented by the letter ‘P’ and is found in certain monocots. Perianth is polytepalous if tepals are free and gamotepalous if tepals are fused.

                      Fig: Perianth of tulips

Aestivation

The arrangement or placement of accessory floral whorls, i.e., sepals or petals in the floral bud with respect to other members of the same whorl in a bud before it opens up to become a flower is termed as aestivation. 

Types of aestivation 

Valvate Aestivation

In valvate aestivation, adjacent sepals or petals of a whorl contact at the margins without any overlaps. Example - Calotropis.

                Fig: Valvate aestivation

Twisted Aestivation

One margin of the appendage overlaps that of the next one while the other margin remains overlapped. Examples - Lady’s finger flower, Cotton flower, China rose (Hibiscus).

                   Fig: Twisted aestivation

Ascending Imbricate Aestivation

In ascending imbricate aestivation the posterior petal is the innermost and it is overlapped on the margins by the lateral petals which are in turn overlapped by the anterior petals. Example - Gulmohar.

               Fig: Ascending imbricate aestivation

Vexillary/ Papilionaceous/ Descending Imbricate Aestivation

In descending imbricate aestivation, the largest petal (called standard) overlaps the two lateral petals (called wings) which in turn overlaps the two smallest petals (called keel) situated anteriorly. Example - pea flower.

         Fig: Vexillary/ Papilionaceous aestivation

Practice problems

Q 1. A flower with fused green leafy structures in the outermost whorl is called

a. polysepalous
b. polypetalous
c. gamosepalous
d. gamopetalous

Answer: Green leaf-like structures in the outermost whorl of a flower are the sepals. A flower with fused sepals is said to be gamosepalous and with free sepals is said to be polysepalous. A flower with fused petals is known as gamopetalous and the one with free petals is called polypetalous.

Thus, the correct option is c.

Q 2. A flower without the non-essential whorls is called

a. Dioecious
b. Monoecious
c. Hermaphrodite
d. Achlamydeous

Answer: The non-essential whorls of a flower are calyx and the corolla since they are not responsible for the formation of gametes and are not directly involved in reproduction. When both non-essential whorls (calyx and corolla) are absent then the flower is described as achlamydeous. When both the non-essential whorls are present then the flower is described as dichlamydeous. If only one non-essential whorl is present then the flower is described as monochlamydeous. When a single plant has separate male and female flowers, then this condition is called monoecy. When male and female flowers are present in different plants in a species, then, the condition is called dioecy. 

Thus, the correct option is d.

Q 3. When the margins of sepals or petals overlap one another without any particular direction, the condition is termed as:

a. Imbricate
b. Twisted
c. Valvate
d. Vexillary

Answer: In imbricate aestivation, margins of sepals or petals overlap one another but not in any particular direction. Example: Cassia and gulmohur.

In valvate condition, sepals or petals in a whorl just touch one another at the margin without overlapping. Example: Calotropis. In twisted aestivation, the margin of sepal or petal overlaps that of the next one. Example: China rose, lady’s finger, cotton. If the largest petal (standard) overlaps with the two lateral petals (wings) which in turn overlaps two smaller anterior petals (keel) then the aestivation is known as vexillary or papilionaceous. Example: Pea and bean flower. 

Q 4. Which of the following flowers has petaloid tepals?

a. Tulip
b. China rose
c. Petunia
d. Pea flower

Answer: Tulip has a perianth in which the petals and sepals are indistinguishable and non-green in colour. Each individual unit of the perianth is said to be a tepal and are considered to be petaloid due to their non-green nature.

Thus, the correct option is a.

FAQs

Q 1. The pappus calyx is a characteristic feature of which floral family?
Answer: The members of family Asteraceae have hairy modified calyx known as pappus which helps them in wind dispersal of seeds.

Q 2. What is meant by marcescent calyx?
Answer: Calyx which persists in the fruit, post fertilisation, in a dry and withered form is known as marcescent calyx, for example, in guava.

Q 3. Descending imbricate aestivation of petals is characteristic of which floral family?.
Answer: The Fabaceae or Leguminosae family includes flowers which show a descending imbricate or vexillary aestivation consisting of 5 free petals arranged in a 1+2+2 fashion. The largest petal (called standard) overlaps the two lateral petals (called wings) which in turn overlaps the two smallest petals (called keel) situated anteriorly.

Q 4. What is the name given to a flower with five petals?
Answer: A flower with five petals is said to be a pentamerous petal. It is gamopetalous if the petals are fused and polypetalous if the petals are free.

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