Co-evolutionary matches between lengths of floral nectar tubes and pollinator proboscises is associated with speciation in several lineages, including Pelargonium (Geraniaceae), a genus renowned for morphological variation.
Tsai et al. investigate how differences in nectar tube lengths develop in two closely-related species. The Pelargonium nectar tube is unique among plants and arises from a localized invagination in the receptacle. Most subsequent elongation occurs in the tissue surrounding the tube late in development, just before flower opening. The authors propose that separation in time between initiation of the nectar tube and the phase of greatest elongation facilitated evolutionary diversification.
Tsai, T., Diggle, P. K., Frye, H. A., & Jones, C. S. (2017). Contrasting lengths of Pelargonium floral nectar tubes result from late differences in rate and duration of growth. Annals of Botany, 121(3), 549–560. https://doi.org/10.1093/aob/mcx171