Definition of Aestivation: Aestivation is the mode of arrangement of the accessory members i.e. sepals and petals or perianth with respect to one another in a flower bud. There are five types of aestivation, such as :
(a) Valvate aestivation — In this type, either the margins of sepals or petals simply touch each other but they do not overlap; valvate sepals are found in Malvaceae. Examples – Hibiscus rosa-sinensis; valvate petals are also found in Calotropis sp. and plants of the family Mimosaceae.
(b) Contorted or Twisted aestivation — In this type, the sepals or the petals are arranged in such a way that one margin overlaps the margin of the next one and the other margin is overlapped by the margin of another, e.g. Hibiscus rosa-sinensis, Gossypium sp. of Malvaceae, species of Nerium sp., Thevetia sp., etc. of Apocynaceae.
(c) Imbricate aestivation — In imbricate, one of the petals or sepals overlaps the margins of the two adjoining members and the two margins of another member are overlapped, while the remaining three behave as twisted fashion. Examples – species of Brassica (Cruciferae), Capparis (Capparidaceae), Delonix, Cassia, Caesalpinia (Caesalpinaceae), etc.
(d) Quincuncial — In this, the margins of sepals or petals (usually in an imbricate aestivation) are arranged in such a way that out of the five members two are outer, two are inner and the odd fifth posterior being overlapped along its one margin, the other margin overlapping i.e. one remain external like twisted. Examples – calyx of most plants of Asclepiadaceae (Calotropis sp.), Myrtaceae (Psidium guajava), etc.
(e) Vexillary aestivation — It is no doubt a type of imbricate aestivation, here out of the five petals the odd fifth posterior (known as standard or vexillum) one is the largest and the outermost, it overlaps the two lateral petals (known as wings or alae) and the lateral petals again in turn partly overlap the two smallest and innermost petals known as keel. Examples – corolla of all flower of plants belonging to the family Papilionaceae.
But according to Michael Hickey and Clive King, 1985 aestivations are of eight types as follows:
1) Open — This type of aestivation is found in polysepalous calyx, polypetalous corolla and polyphyllous perianth. Here sepals, petals or perianth in a whorl do not touch one another at the margin but remain free. Open type is found in sepals of Magnolia champaca (Magnoliaceae), perianth of Ricinus communis (Euphorbiaceae), etc.
2) Valvate — In this type, floral leaves (sepals, petals etc.) are united with each other and just touch one another at the margin. Examples – Hibiscus rosa-sinensis, Calotropis sp.
3) Contorted or Twisted — as mentioned earlier in point no (b).
4) Convolute — This type of aestivation is noted in case of only gamopetalous corolla. Here petals are arranged spirally in bud condition. Example – Ipomoea purpurea, Evolvulus nummularis, etc. of Convolvulaceae.
5) Quincuncial — same as described in item No. (d).
6) Imbricate — same as described in item no. (c).
7) Descending imbricate or Vexillary — same as described in item no. (e).
8) Ascending imbricate — This type of aestivation is found only in corolla members. Out of the 5 petals, the odd 5th petal is posterior, small and innermost, other 4 petals are almost equal. Examples – Cassia sophera, Bauhinia purpurea of S. F. Caesalpinioideae of Family Leguminosae.
Aestivation is very important from taxonomic point of view i.e. it helps in identifying families and genera.