Corolla of a Flower

The word “corolla” comes from the Latin word corona, which means “wreath or crown and represents the crowing portion of the flower. The corolla is the whorl of petals of flowers. The largest group of dicots, typically exhibit four(Tetramerous) or five (Pentamerous) petals, whereas those from monocots often feature three(Trimerous) or six petals. These petals, often modified leaves, encircle and protect the reproductive organs of the flower, including the stamens and pistils. The corolla is usually colourful and plays a significant role in attracting pollinators.

1.0Features of Corolla

Morphology: The corolla is the second whorl of the flower, located inside the outermost whorl called the calyx. It consists of one or more petals, which are modified leaves. Petals may vary in number, arrangement, shape, size, and colour, depending on the species and evolutionary adaptations of the flower.

Examples of separate instances of petal evolution

(A) Adaxially petaloid tepals in Sesuvium portulacastrum.

(B) Petaloid sepals in Portulaca oleracea. 

(C) Petaloid staminodes in Delosperma napiforme.

(D) Petaloid bracts in Opuntia humifusa.

(E) Petaloid staminodes in Stellaria media.

(F) Petaloid bracts in Bougainvillea glabra.

Coloration: Petal colouration is determined by the presence of pigments such as anthocyanins, carotenoids, and flavonoids. These pigments are synthesized in specialized cells called chromatophores. Petal colouration plays a crucial role in attracting pollinators and may be influenced by factors such as UV light exposure, temperature, and nutrient availability.

Microstructure: Petals exhibit diverse microstructures, including epidermal cells, stomata, trichomes, and papillae. These structures contribute to petal texture, surface area, and water retention, which may affect interactions with pollinators and environmental conditions.

Chemical Composition: Petals contain various secondary metabolites such as volatile organic compounds (VOCs), phenolics, and alkaloids, which contribute to petal fragrance, defence against herbivores and pathogens, and signalling to pollinators.

Ecological Function: The corolla functions primarily in attracting pollinators and protecting reproductive organs. Petal traits such as colour, shape, scent, and nectar production have evolved through natural selection to maximize pollination efficiency and reproductive success.

2.0Fusion of petals

There are two distinct conditions regarding the arrangement of petals in flowers

1. Polypetalous condition: In polypetalous flowers, the petals are separate and distinct from each other. Each petal is not fused to any other petals, allowing them to be easily distinguished. Example: China rose,Rose, Mustard.

2. Gamopetalous condition: Gamopetalous flowers have fused petals, where two or more petals are joined together to form a single structure. This fusion may be partial or complete, resulting in a variety of shapes. Example: Datura, Tomato.

3.0Aestivation (Mode of arrangement of petals)

Aestivation is the arrangement of either the sepals or the petals in a flower bud in relation to one another as vernation proper is the arrangement of young leaves in the vegetative bud. Aestivation is of considerable importance in the classification of plants. This may be of the following types:

Valvate: Floral leaves in a whorl may just touch one another at the margins without overlapping as seen in the family Anonaceae or the subfamily Mimoseae and Calotropis.

Twisted or Contorted : When overlapping is regular in one direction so that one margin overlaps the next member on one side while its other margin is overlapped by the one before, giving a twisted appearance to the bud. This is seen in Malvaceae (china-rose, ladyfinger, cotton, etc.) and Apocynaceae (Nerium). 

Imbricate: When the margins overlap one another but not in any particular order as in the subfamily Caesalpinieae. In imbricate flowers the petals do not actually lie in a single whorl in Cassia and Gulmohur.

Quincuncial: The floral leaves are not in a whorl but spirally arranged . Leaves 1 and 2 are external, 3 partly external, 4 and 5 internal. This is seen in guava (Psidium guajava of Myrtaceae), etc. 

Vexillary: This is the typical aestivation of the papilionaceous corolla. The posterior vexillum overlaps the two alae which again overlap the paired anterior carina.In vexillary, the largest posterior petal (vexillum or standard) overlaps two lateral petals (alae or wings) which in turn overlaps the two smallest, anterior but united petals (keel or carina), e.g., Pea, Bean etc.

4.0Forms of Corolla

Asymmetrical flower Canna

1. Actinomorphic /RegularCorolla: This type of corolla has radial symmetry, meaning it can be divided into equal halves along multiple planes. Flowers with regular corollas often have petals that are similar in size and shape, forming a symmetrical arrangement. 

Actinomorphic Polypetalous Corolla:

• Cruciform: In the cruciform corolla, four separate petals are arranged crosswise. Each petal consists of a lower narrow part called the claw and an upper broad part called the limb. Examples of plants with cruciform corollas include radish and mustard (Cruciferae family).

Actinomorphic Gamopetalous Corolla:

• Campanulate/Bell-shaped: In this type of corolla, five fused petals are present, forming a shape similar to a bell. Examples of plants with campanulate corollas include tobacco, raspberry, and Campanula species.

• Tubular: In a tubular corolla, five fused petals form a tubular or cylindrical structure. This type of corolla is commonly found in disc florets of flowers like the sunflower.

• Funnel-shaped or Infundibuliform: This type of corolla consists of five fused petals, resembling a funnel. Examples of plants with funnel-shaped corollas include Datura, railway creeper, and Petunia species.     

• Wheel-shaped/Rotate: In a wheel-shaped corolla, five gamopetalous petals are present, with the fused parts forming a small tube. The petals are arranged in a whorl above the tube. Brinjal is an example of a plant with a rotate corolla.

3. Zygomorphic/Irregular Corolla: Flowers with an irregular corolla have bilateral symmetry, meaning they can be divided into two mirror-image halves along only one plane.

Zygomorphic Polypetalous Corolla:

• Papilionaceous (Butterfly-shaped corolla): In the papilionaceous corolla, five distinct petals are present. The largest posterior petal is known as the standard or vexillum, which covers two lateral petals called wings or alae. The innermost anterior petals are fused to form a keel or carina, and both lateral petals cover the keel. Examples of plants with papilionaceous corollas include pea, bean, and gram (Papilionatae family).

Zygomorphic Gamopetalous Corolla:

• Bilabiate: In the bilabiate corolla, the petals of the gamopetalous corolla are divided into two lips. The space between the two lips is referred to as the corolla mouth. Examples of plants with bilabiate corollas include Ocimum (holy basil, also known as tulsi) and Salvia (Labiatae family).

• Ligulate: In the ligulate corolla, the upper part of the corolla is elongated, flattened, and tongue-like, attached to a short narrow tube. An example of plants with ligulate corollas includes the ray florets of the sunflower.

5.0Functions of Corolla

• Attraction of Pollinators: One of the primary functions of the corolla is to attract pollinators, such as bees, butterflies, birds, and bats. The colorful and often fragrant petals of the corolla serve as visual and olfactory cues that signal the presence of nectar and pollen, enticing pollinators to visit the flower.
• Protection of Reproductive Organs: The corolla provides protection to the reproductive organs of the flower, including the stamens (male reproductive organs) and pistil (female reproductive organ). By enclosing these delicate structures within its petals, the corolla helps shield them from physical damage, adverse weather conditions, and herbivores.
• Facilitation of Pollination: The structure of the corolla can play a role in facilitating pollination. For example, certain corolla shapes, such as tubular or funnel-shaped, may guide pollinators towards the flower's reproductive organs, ensuring effective transfer of pollen between flowers.