Define the following term:
Aestivation

(N/A) Aestivation is defined as the mode of arrangement of sepals or petals in a floral bud with respect to the other members of the same whorl.
The main types of aestivation observed in plants are:
$1$. Valvate: When sepals or petals in a whorl just touch one another at the margin,without overlapping.
$2$. Twisted: If one margin of the appendage overlaps that of the next one and so on.
$3$. Imbricate: If the margins of sepals or petals overlap one another but not in any particular direction.
$4$. Vexillary: In pea and bean flowers,there are five petals; the largest (standard) overlaps the two lateral petals (wings) which in turn overlap the two smallest anterior petals (keel).