Q.What is meant by 'aestivation' in a flower?

Aestivation is the specific way sepals or petals overlap (or fail to overlap) one another while still folded up inside the flower bud, and it is one of the more diagnostic characters used in plant identification and family classification. In valvate aestivation, the margins of adjacent sepals or petals simply touch each other without any overlap, as in the calyx of members of Malvaceae, Calotropis and Annona. In twisted (contorted) aestivation, one margin of each petal or sepal overlaps the next member all the way around, as in the petals of China rose. In imbricate aestivation, sepals or petals overlap irregularly - one member is entirely external, one is entirely internal, and each of the remaining three has one margin external and the other internal - as in Cassia and Delonix; imbricate aestivation has three named variants: ascendingly imbricate, quincuncial and vexillary. Quincuncial aestivation has two members wholly external, two wholly internal, and the fifth with one margin of each (guava; the calyx of Ipomoea and Catharanthus). Vexillary aestivation, typical of Fabaceae (pea, bean), has a large posterior standard petal with both margins overlapping the two lateral wing petals, while the wing petals in turn each have one margin overlapping the two smaller anterior keel petals.