Premise of research. Loranthaceae flowers exhibit exceedingly variable ground plan, size, and pollination syndromes. The homology of floral organs (especially the calyculus and the perianth), the evolutionary shifts from bisexual to unisexual flowers, and the trends in ovary and ovule reduction, primarily studied in Old World taxa, have been controversial. We investigate the development and morphoanatomy of early- and late-diverging Neotropical lineages with broad floral diversity to test organ homology and postulate floral plesiomorphies and apomorphies throughout the family.Methodology. We examined eight species from six genera, all native to the Colombian Andes. Standard LM and SEM methods were used.Pivotal results. All species studied exhibit bisexual flowers, but partial or total stamen sterilization was observed in the small-flowered Passovia pyrifolia and Peristethium archeri. All flowers examined possess an irregular calyx developed from a ring primordium, distinct from the four to seven petals in...