Morphology Of Flowering Plants - Aestivation

Aestivation refers to the mode of arrangement of sepals or petals in a floral bud, and NCERT lists valvate, twisted, imbricate and vexillary types. In the pea or bean flower the corolla shows vexillary or papilionaceous aestivation, in which the largest petal called the standard or vexillum overlaps two lateral wing petals, and the two anterior petals are fused to form the keel that encloses the stamens and pistil. Valvate aestivation has sepals or petals merely touching at the margins without overlapping, as in Calotropis, so it does not fit the overlapping pea pattern. Twisted aestivation, seen in china rose and cotton, has one margin of each petal overlapping the next in a regular spiral, which is not the standard-and-keel arrangement. Imbricate aestivation, as in Cassia and gulmohur, has overlapping margins but without the definite vexillary order. As stated in NCERT Class 11, Chapter 5 (Morphology of Flowering Plants), the pea corolla shows vexillary aestivation. A structural check confirms that the standard-over-wings-over-keel pattern is unique to vexillary aestivation.