Calyx in plants merely refers as an outermost whorl that is sterile or non-reproductive structure constituting perianth. Group of sepals unitedly refer as the calyx. Thus, sepals are nothing but the modified leaves that account for the formation of the flower’s outermost whorl. It can be regular or irregular in size and shape, and the number may vary based on different plant species.
The sepals are sessile, leafy and usually green in colour but few flowers may consist of coloured sepals. The fundamental role of sepals is to ensure proper maturation of flower and protection against drying out condition.
Content: Calyx in Plants
Definition of Calyx
The calyx is the sterile, outermost whorl that originates directly upwards the stem and encloses the petals along with other floral parts inside. It is distinctively referred to as “Sepal”. During bud formation, the sepal remains intact protecting the bud till it attains maturity. Once the bud blooms into a flower, then the whorl of calyx extends from the flower base.
The calyx shows the following characteristic features:
- It is the lowermost axillary whorl.
- “Epicalyx” is another structure, which develops from the base of calyx as a whorl of bracteoles in few flowers.
- The sepals are usually green and leaf-like.
- It is quite thick, hardy than the leaves but shows veins and consists of stomata like in ordinary leaves.
- The calyx can be free or fused in origin.
- In the fused calyx, sepals form a calyx tube when the fusion occurs towards the base, and the remaining free portion refers as calyx limbs.
- When the sepals appear like petals, they are called as “Tepals”.
- A group of sepal sometimes undergo modifications where it develops several appendages depending upon the environmental adaptation.
- In the majority of plants, sepal drops off as the flower blooms, but in few, it remains intact till the petals fall off and few remains intact permanently with the fruit.
Types of Calyx
In different plants, the calyx may differ in size, structure, loss or persistence etc. depending upon which the calyx classifies into the following types.
Depending upon the size of calyx
Regular: The sepals of the calyx exhibits the same size, as in China rose.
Irregular: Here, the sepals exhibit different sizes, as in Cliteria sp.
Depending upon the structure of calyx
Tubular: The sepals appear tube-like, as in Nicotiana.
Infundibuliform: Calyx resembles a funnel shape, like in Atropa belladonna.
Urceolate: The sepals appear as urn shape, like in Hyoscyamus.
Bilabiate: It consists of two lips, as in Ocimum, Salvia etc.
Campanulate: Calyx resembles a bell-shape, as in Lathyrus odoratus.
Cupulate: The calyx appears cup-like, as in Gossypium.
Depending upon the fusion of sepals
Gamosepalous: In this type, the sepals are fused. Examples: Datura, Hibiscus etc.
Polysepalous: In this type, the sepals are free. Examples: Anona, Tomato etc.
Depending upon the loss and persistence of a calyx
Caducous: In this type, sepals fall down as the flower blooms, like in poppy plant.
Deciduous: In this type, sepals fall down along with the petals after its fertilization, as in the mustard plant.
Persistent: In this kind, the sepals remain intact with the fruits, like in brinjal, chillies etc. It further categorizes into two groups:
- Marcescent: When the persistent calyx acquires shrivelled appearance and shows no growth after fertilization, then it is termed as the marcescent calyx. Example; Guava.
- Accrescent: In this type, a persistent calyx simultaneously grows in size with the increasing size of the fruit even after fertilization, and termed as the accrescent calyx. Example; Physalis.
Depending upon the colour of sepals
Sepaloid: In this type, the sepals are green in colour.
Petaloid: In this type, the sepals are non-green or colourful like petals. Example: Mirabilis etc.
Depending upon the calyx-modification
The structure of calyx modifies into various appendages and categorizes into the following types:
Pappus: Here, the sepals transform into feather-like or hairy structures. Example; Sunflower, Sonchus etc.
Spurred: The sepals transform into a beak-like structure. Example; Delphinium etc.
Petaloid: In this kind, the sepals sometimes become enlarged and appears brightly coloured like petals. Example; Mussaendra etc.
Hooded: Here, one of the sepals extends and transforms into a hood-like structure over the flower. Example; Aconitum etc.
Spinous: In this kind, the sepals modify into spines. Example; Trapa etc.
Therefore, the calyx is a group of sepals that functions as an outermost whorl that harbours the flower from its budding phase to the blooming or flowering stage against harsh environmental conditions, desiccation, and sometimes modify to perform special tasks.