Morphology of Flowering Plants · Lesson 6 of 11
Calyx, Corolla and Aestivation
“Understand sepal and petal forms and compare the four major arrangements of petals in a bud.”
• Describe the role and forms of sepals and petals. • Differentiate free and united members of calyx and corolla. • Define aestivation precisely as arrangement within a bud. • Recognise valvate, twisted, imbricate and vexillary aestivation with examples.
The two outer floral whorls
A typical flower has a calyx, the outermost whorl made of sepals, and a corolla, the next whorl made of petals. Sepals often surround and protect the developing bud. They are commonly green but can vary in colour and texture. Petals are often more conspicuous and may provide signals or landing surfaces for pollinators; their shapes include tubular, bell-shaped, funnel-shaped and wheel-shaped forms. Not every flower relies on visually attractive petals, and sepals may be coloured in some species.
When sepals are separate from one another, the calyx is described as polysepalous. When their bases or more of their lengths are joined, it is gamosepalous. Similarly, polypetalous means free petals, while gamopetalous means united petals. Joining of members within one whorl is called cohesion. This is not the same as adhesion of members from different whorls, such as stamens attached to petals. A botanist can therefore describe a flower as having united petals but stamens free of one another.
The outermost floral whorl, composed of sepals.
The floral whorl inside the calyx, composed of petals.
| Structure | Free members | United members |
|---|---|---|
| Sepals of calyx | Polysepalous | Gamosepalous |
| Petals of corolla | Polypetalous | Gamopetalous |
What is aestivation?
Aestivation describes the way members of the same sepal or petal whorl lie against or overlap one another while the flower is still a bud. It is not the overall pattern of leaves on the stem—that is phyllotaxy. Nor is it the arrangement of flowers on a branch—that is inflorescence. To identify aestivation, imagine looking down on an unopened bud and following the margins of the petals around the whorl.
Adjacent petal or sepal margins just touch without overlapping, as in Calotropis.
Each member overlaps the next in one consistent direction around the whorl, as in china rose, lady's finger and cotton.
In valvate aestivation there is contact but no petal covers the margin of its neighbour. In twisted aestivation the overlap repeats in a spiral-like direction: each petal lies over the next at one margin and under the previous at the other. This repetition is the distinguishing feature. A single observed overlap is not enough to prove a twisted pattern unless the same directional relation continues around the bud.
Members overlap, but not in one uniform successive direction, as in Cassia and gulmohur.
A distinctive unequal overlap in a papilionaceous corolla: the large standard petal overlaps two wings, which overlap the two keel petals, as in pea and bean.
In imbricate aestivation some margins overlap, but the overlap order is not an uninterrupted twisted chain. Vexillary aestivation is more specialised. In a pea-like flower the largest posterior petal is the standard; beneath it lie two lateral wings; and these in turn overlap the two anterior keel petals. Thinking of a layered hierarchy—standard, wings, keel—is clearer than trying to memorise a picture without recognising those three components.
| Aestivation | Margin relationship | Characteristic example |
|---|---|---|
| Valvate | Touch without overlap | Calotropis |
| Twisted | Regular one-direction overlap | China rose |
| Imbricate | Overlapping without single directional sequence | Cassia |
| Vexillary | Standard over wings over keel | Pea, bean |
A reliable method for interpreting flower drawings
First check whether margins overlap at all; if they only meet, choose valvate. If they do overlap, trace whether every margin participates in the same directional sequence; if yes, twisted is likely. If the relations change unpredictably around the whorl, imbricate is more suitable. If one broad standard dominates paired wings and a paired keel in a bilateral flower, identify the vexillary arrangement.
For a real specimen, gently tease apart a bud under magnification before it opens completely. Keep track of which margin lay above and which lay below its neighbour. Once the petals spread during anthesis, their original bud overlap can be difficult to reconstruct. Do not classify aestivation merely from the open flower's colour or from the number of petals.
Polysepalous or gamosepalous describe fusion within the calyx; aestivation describes margin arrangement while in the bud. These are separate traits. A flower can have five united petals and still show a valvate relationship along its free margins.
Quick Quiz
Quiz
What is valvate aestivation?
What is the correct overlap sequence in vexillary aestivation?
What does gamopetalous mean?
Which aestivation type shows overlap without a consistent direction?
Which term describes the arrangement of leaves on a stem rather than petals in a bud?
Practice Problems
- Compare the four common types of aestivation.
- Distinguish gamopetalous and polypetalous corollas.
- Why should aestivation be observed in a bud?
- Name the characteristic petals in a pea-like flower.
Model Answers
1. Valvate margins merely meet; twisted overlap proceeds around one direction; imbricate overlap lacks uniform direction; vexillary shows standard over wings over keel.
2. Gamopetalous corollas have united petals; polypetalous corollas have free petals. Both terms describe cohesion of corolla members.
3. The natural overlap arrangement occurs before full opening. Once petals spread, the original above-below relationship may be obscured.
4. The corolla has one large standard, two lateral wings and two anterior keel petals, arranged in a vexillary overlap pattern.
Key Takeaways
Calyx is made of sepals; corolla is made of petals. Members may be free or united: polysepalous/gamosepalous and polypetalous/gamopetalous. Aestivation concerns neighbouring members of a floral whorl in an unopened bud. Valvate = contact only; twisted = directional overlap; imbricate = irregular overlap; vexillary = standard over wings over keel.