The Origin and Identity of the Calyculus in Loranthaceae:Inferred From the Floral Organogenesis of Loranthus tanakae Franch.&Sav.

The flowers of the hemiparasitic family Loranthaceae are always subtended by a rimmed structure known as the calyculus.The origin and identity of the calyculus have been disputed for more than a century.Various hypotheses have been proposed,for example,an outgrowth of the axis,a reduced calyx,and a bracteolar(prophyllar)origin,but controversies remain.To obtain a plausible explanation of the origin of the calyculus,we investigated the flowers of Loranthus tanakae using scanning electron microscopy and light microscopy to observe the entire developmental process of the floral parts.Our results show that bracts are not present in L.tanakae.The calyculus,which lacks serving vascular bundles,initiates as a semicircular primordium and then develops into a circular structure by an adnation at both sides.The flower primordium usually cleaves into six petals from its centre along a whorled pattern in two series with three petals each,before or after the calyculus closed.Isomerous stamen primordia probably follow the same initiation pattern as petals do.Several carpels of different sizes initiate simultaneously as a united primordium.We support the hypotheses that the calyculus is of bract or bracteole origin due to its independent initiation from the inflorescence rachis,its similar morphology and positioning as the bract or bracteole,and that having no developmental relationship with the petals.We suggest keeping the usage of the term“calyculus”.Loranthus flowers should be considered monochlamydeous with three whorls of floral parts,namely petal,androecium,and gynoecium.