Aestivation in Plants: Definition, Types, Examples and Sample Questions

Aestivation in Plants is the arrangement of petals and sepals in floral buds before blooming. It is arranged with respect to the other members of the whorl. The term aestivation is often confused with vernation which basically refers to the arrangement of new leaves and scales in a leaf bud. The petals and sepals are collectively termed perianth. The arrangement of perianth inside a flower bud before it blooms is known as the aestivation of plants.

Key Terms: Aestivation in Plants, Valvate Aestivation, Imbricate Aestivation, Perianth, flowers, sepals, petals

What is Aestivation?

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Aestivation is the floral characteristic that demonstrates the arrangement of corolla (petals) and calyx (sepals) in relation to each other in a flower bud. Corolla and calyx are collectively known as the perianth. It is a property that is taken into consideration in order to show the relation between petals and sepals. For example, the petals and sepals or the perianth of the Hibiscus flower that belongs to the Malvaceae family show valvate and twisted aestivation. 

Aestivation is an important part of taxonomy as flowers of different species show different aestivation. The five different types of aestivation are:

1. Valvate Aestivation
2. Twisted Aestivation
3. Imbricate Aestivation
4. Vexillary Aestivation
5. Quincuncial Aestivation

• There is also a similar term known as vernation, which is slightly different from aestivation as it means the arrangement of leaves within a vegetative bud.
• Aestivation basically refers to the way sepals and petals or the perianth, are arranged in a floral bud before it blooms. 
• The calyx is the outermost whorl of the flower and its members are called sepals.
• Usually, sepals are green and they are responsible for protecting the flower while they are in the bud stage.
• The calyx may be gamosepalous, which means when the sepals are united, or polysepalous when the sepals are not united but free. 
• Corolla is a combination of petals and they are usually bright in colour to attract insects for pollination.
• Similar to the calyx, corolla can have jointed petals which are gamopetalous or free petals which are polypetalous.
• The corolla is built differently in different plants.
• They vary by shape, size and colour hugely in different plant species. They can be tubular, bell-shaped, funnel-shaped or wheel-shaped. 

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What is Perianth?

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The perianth is the non-reproductive floral part that envelops the sexual organs such as calyx or sepals, and corolla or petals. The term perianth is Greek in origin, taken from peri which means ‘around’, and anthos which means ‘flower’.

• In some plants, petals and sepals are non-differentiated and therefore are found as auxiliary whorls. For example, Flowers of Crinum, Polyanthus, etc.
• Sometimes, in the case of Gloriosa superba (Liliaceae) and Crinum asiaticum (Amaryllidaceae), the petals and sepals resemble each other and are brightly coloured. 
• Flowers belonging to the Amaranthaceae family have a membranous and persistent perianth type.
• The perianth or the outermost whorl is termed polyphyllous if the petals and sepals remain free as in the case of Gloriosa superba flowers.
• However, if the petals and sepals are fused, the perianth is known as gamophyllous as in the case of flowers of Polianthes tuberosa.

In flowering plants, the perianth is known as dichlamydeous or heterochlamydeous in which the corolla and calyx are clearly separate. There is also homochlamydeous perianth in which sepals and petals are not distinguishable.

• A two whorled perianth is known as biseriate.
• A green calyx is called a sepaloid while a brightly coloured one is called a petaloid. 
• The corolla and petals are usually prone to attracting insects for pollination, though some exceptions can be noticed in a few cases.

Read More: Sexual Reproduction in Flowering Plants

Types of Aestivation

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The positional organization of petals and sepals are of the following types - valvate aestivation, twisted aestivation, imbricate aestivation, vexillary aestivation and quincuncial aestivation. 

Valvate Aestivation

Valvate is open aestivation that includes flowers whose petals and sepals are only attached through the margins of a single whorl by leaving a free space within them. 

• The petals and the sepals of the whorl do not overlap each other.
• They meet at the edges, therefore valvate aestivation is referred to as open aestivation.
• The edges of the corolla and calyx are really close within a whorl of a flower but they do not intersect each other.
• Example: Mustard, Annona and Calotropis flower

Twisted Aestivation

Twisted Aestivation is also known as convolute or contorted aestivation. 

• Twisted Aestivation includes the flowers whose petals or sepals are organized in a manner in which one edge overlaps the next edge inside. 
• The other margin however is overlapped by the former petal or sepal edge.
• Therefore, they overlap the neighbouring members on one side regularly. 
• The overlapping can be regarded as clockwise or anticlockwise with regularity.
• The edge of an appendage overlaps the next, which gives it a different or distorted shape. Therefore, it is called ‘twisted aestivation’. 
• Example: Ladyfinger, Cotton, and Hibiscus flowers.

Imbricate Aestivation

In imbricate aestivation, the perianth or the outermost whorl includes petals and sepals that overlap each other in such a way that a few petals are inside entirely and some are entirely outside. 

• The outermost whorl or perianth consists of petals which are either entirely inside or entirely outside. 
• Therefore, the overlapping of corolla or calyx in imbricate aestivation does not occur in a specific position which means it has a non-specific structural arrangement.
• The two sub-types of imbricate aestivation are ascending imbricate and descending imbricate aestivation.
• Example: Flowers of Gulmohar.

Vexillary Aestivation

Vexillary aestivation is also known as descending imbricate or papilionaceous aestivation. 

• The aestivation includes overlapping of the petals from the bottom of the posterior end towards the anterior side.
• The overlapping starts from the posterior side and ends at the anterior side. 
• This includes flowers having an intersection of the odd posterior petal and the margin of 2 lateral petals.
• A pair of the smallest anterior petals are overlapped by the lateral wings or the petals.
• Hence, in this arrangement of petals, the odd or the largest posterior petal is entirely on the outside.
• The anterior pair of petals or the keel is on the inside.
• The overlapping pattern in vexillary aestivation is in a descending manner. 
• Example: Crotalaria, Dolichos, and Tephrosia flowers.

Quincuncial Aestivation

Quincuncial aestivation is also referred to as ascending imbricate aestivation.

• Quincuncial aestivation is the opposite of the vexillary or descending imbricate aestivation. 
• In the case of quincuncial aestivation, the petals start overlapping from the anterior end and continue till the posterior end. 
• The perianth whorl is not arranged in a single whorl but is built in a spiral form.
• Firstly, two lateral petals intersect the odd posterior petal. 
• Next, the whole arrangement is intersected by the anterior pair of petals. 
• Hence, it can be concluded that in this case, two petals are external, two are internal, and the fifth one has internal and external margins. 
• The odd posterior petal is completely on the inside, and one of the two anterior petals is completely on the outside.
• The two lateral petals are located on the partly external side. 
• Example: Delonix and Cassia flowers.

Read More: Lamina of a Plant Leaf

Examples of Aestivation

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The examples of aestivation of different types are noted below:

• Valvate aestivation: Hibiscus, Calotropis, mustard and Annona.
• Twisted aestivation: Cotton, ladyfinger, Hibiscus
• Imbricate aestivation: Legumes, flowers of Gulmohar
• Vexillary aestivation: Pea plant, flowers of Crotalaria, Dolichos and Tephrosia.
• Quincuncial aestivation: Guava, Delonix, Cassia.

Things to Remember

• Aestivation is defined as the specific arrangement of the members of the perianth, i.e. sepals and petals with respect to each other. 
• Flowers can be classified based on the type of aestivation.
• Different types of aestivation in plants are valvate, twisted, imbricate, vexillary, and quincuncial aestivation.
• The perianth is a non-reproductive floral part that envelops the sexual organs, namely calyx or sepals, and corolla or petals.
• Aestivation in plants is an important topic in plant taxonomy as different types of flowers of various species show different aestivation.

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Previous Year Questions

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