Q.Describe the structure of a typical flower under the four floral whorls, and explain the terms symmetry, aestivation, and placentation, referring to the convention used to write a floral formula and draw a floral diagram.

A typical flower is built of four successive whorls arranged on the receptacle at the tip of the pedicel (flower stalk). The outermost whorl, the calyx, is made of sepals, usually green, whose main job is protecting the bud; the next whorl, the corolla, is made of often brightly coloured petals that attract pollinators -- together, the calyx and corolla make up the non-reproductive perianth. Within these lie the two reproductive whorls: the androecium, made of stamens (the male reproductive parts, each typically a filament bearing an anther that produces pollen), and, at the very centre, the gynoecium, made of one or more carpels (the female reproductive parts, each differentiated into an ovary containing ovules, a style, and a stigma that receives pollen).

Flowers are described as actinomorphic (radially symmetrical) when they can be divided into two equal halves along more than one plane passing through the centre, as in mustard or datura, or zygomorphic (bilaterally symmetrical) when only one such plane of division is possible, as in the pea flower or Cassia. Within a whorl, aestivation describes how the members are packed against one another in the unopened bud: valvate (margins just touch, no overlap), twisted/contorted (each member's margin overlaps the next member's in one consistent direction), imbricate (irregular overlap, with one member fully external and one fully internal), and vexillary (the special imbricate-type pattern of pea-flower corollas, where the large posterior 'standard' petal overlaps two lateral 'wing' petals, which in turn overlap two smaller anterior petals fused into a 'keel').

Inside the ovary, placentation describes how the ovules are attached to the placenta: marginal (ovules along the ventral suture of a single folded carpel, as in pea), axile (placenta along a central axis, forming as many chambers as carpels, as in China rose), parietal (ovules on the inner ovary wall, as in mustard), basal (a single ovule at the base of the ovary, as in sunflower), and free-central (ovules on a central axis not connected to the ovary wall by septa, as in Dianthus).

All of this structural information can be captured in two standard shorthand conventions. A floral formula uses symbols for symmetry (⊕ for actinomorphic, % for zygomorphic) and sex (bisexual/unisexual), followed by letters for each whorl -- K for calyx, C for corolla (or P for a perianth not differentiated into calyx and corolla), A for androecium, G for gynoecium -- each followed by the number of members, with fused members shown by enclosing the number in brackets and the ovary's position (superior or inferior) shown by a line above or below the G's number. A floral diagram instead shows the same information pictorially, as a schematic cross-section of the flower with each whorl's parts drawn in their true relative position and number, making both symmetry and aestivation visible at a glance.

Move outward-in through the four whorls first, then explain symmetry, aestivation and placentation each as a separate structural axis of description, and close by explaining how the floral formula and diagram together encode all of it.

• Treating aestivation and placentation as the same concept, when one concerns the perianth in the bud and the other concerns ovule attachment inside the ovary.
• Omitting that calyx + corolla together are called the perianth, and forgetting the alternative single term 'perianth (P)' used when they are not distinguishable.