A flower in Angiosperms is a highly modified shoot. The shoot apical meristem (SAM) changes to floral meristem upon floral induction.
During floral induction, the internodes, which are longer in vegetative part, do not elongate further. This causes the shortening and condensation of the floral axis. The apex produces different kinds of floral appendages laterally at the successive nodes instead of leaves. These forms the floral whorls (like outermost calyx, corolla, androecium and innermost gynoecium). They are placed on the flat thalamus.