A new paper in Annals of Botany uses macroscopic and microscopic analysis to investigate how flowers are formed in the three different genera of the neotropical plants Galipeinae, and considers the evolutionary implications of the results.
A possible reproductive advantage of having a floral tube is the restriction of the access to the floral reward for nectar robbers; in addition, the accumulation of nectar in the bottom of the tube may reduce nectar evaporation and dilution by rain. The flowers of the Galipeinae offer nectar as a reward to pollinators and are pollinated by nectar-seeking insects with a long proboscis and/or long-beaked birds such as butterflies or hummingbirds.
Fusion within and between whorls of floral organs in Galipeinae (Rutaceae): structural features and evolutionary implications. (2013) Annals of Botany 111 (5): 821-837. doi: 10.1093/aob/mct039
Most genera of the neotropical Galipeinae (tribe Galipeeae, Rutoideae) exhibit several forms and degrees of fusion between the floral organs, including the union of petals into an apparently sympetalous corolla, the joining of the stamens among themselves and to the corolla, and the partial to complete connation of carpels. Though these and others floral traits are currently used in the circumscription of species in Galipeinae, few studies have shown in detail in which way (postgenital or congenital) and to what extent these fusions occur. To elucidate these anatomical conditions, a structural study of the flowers of the Galipeinae species was carried out. Flowers of six species from three genera of Galipeinae were studied in their morphology, anatomy and development with stereomicroscopy, light microscopy and scanning electron microscopy (SEM). The floral tube is formed by synorganization of stamens with petals in all species, and exhibits three main patterns: (1) Conchocarpus heterophyllus and C. minutiflorus have a floral tube formed by marginal coherence/adherence of petals and filaments due to interwining trichomes (postgenital connection); (2) Erythrochiton brasiliensis has a tube formed by congenital fusion of petals and filaments; and (3) Galipea jasminiflora and Conchocarpus macrophyllus have a tube formed distally with the first pattern, and proximally with the second pattern. Although floral tubes seem to be homologous within Galipeinae, this is not true at the level of the family: the floral tube of Correa (from an only distantly related clade of the family) is formed by postgenital union of the petals representing a convergent structure. The gynoecium of the studied species of Galipeinae shows a great variability in the extent of fusion of carpel flanks. Even though different structures for the mature gynoecium were found in each genus, all genera show postgenitally fused carpel apices, which is related to the formation of a compitum, as described earlier for other members of Rutaceae. The degree and diversity of fusions of floral organs in Galipeinae is unique within the order Sapindales. A study of the amount of diversification of Galipeinae in South America and comparison with other clades of Rutaceae would be of interest.