Explanation:

(a) Aestivation is the arrangement of petals and sepals with respect to one another in a floral bud before it opens. The given figure shows different types of aestivation. Figures $A,B,C$ and $D$ show valvate, twisted, imbricate and vexillary aestivation. In valvate aestivation, sepals or petals or tepals just touch one another without overlapping, e.g. Calotropis. In twisted aestivation, one margin of each petal overlaps the margin of an adjacent petal and the other margin is overlapped by margin of another adjacent petal, e.g. China rose, cotton. If the margin of sepals or petals overlap one another but not in any particular direction as in Cassia and Gulmohar, the aestivation is called imbricate. Vexillary is the characteristics aestivation of corolla of pea when posterior petal is outermost.