Date of Completion
Gregory Anderson, Pamela Diggle
Field of Study
Ecology and Evolutionary Biology
Master of Science
The nectaries of Pelargonium are unusual in the Geraniaceae in that the flowers only have one deep dorsal tube as opposed to five shallow canals. The genus is also known for having great length variation in nectar tubes, variation that has been suggested to play an important role in the diversification of this genus. Interpretations of Pelargonium nectaries contradict each other as some describe the nectar tube as an adnate sepal spur while others describe it as an hypanthial tube as a result of an elongate receptacle. In addition, the developmental basis of differing nectar tube lengths is unknown.
SEM analysis of P. ionidiflorum and P. odoratissimum dissections revealed that displacement of the dorsal antesepalous stamen by the flanking antepetalous stamens anticipates the space where the nectar tube will develop before the tube begins to form. Soon after all floral organ primordia have been initiated, the concavity of the nectar tube of Pelargonium initiates and then elongates through intercalary growth of the receptacle. Early stages of development up to a bud length of 4.0 mm are similar in both species. Functional Data Analysis of longitudinal growth of buds 0.25 mm to maturity showed that the tubes of P. ionidiflorum become longer through both higher rates of elongation and a longer duration of elongation than those of P. odoratissimum. Nectar tube elongation also continues for nearly a week after anthesis.
Evidence suggests that the nectar tube of Pelargonium is not evolutionarily derived from a sepal spur that becomes fused to the pedicel. No involvement of the spur is observed at any stage in the development of the nectar tube in Pelargonium. Furthermore, the development of the spur begins late in floral ontogeny in contrast with the initiation of the nectar tube, which begins early in development of Pelargonium flowers. This contrasts with the pattern in species with free nectary perianth spurs, such as Aquilegia olympica or Impatiens columbaria. Finally, sections along the length of mature P. ionidiflorum tubes revealed a lack of vascular pattern that would support an interpretation of a spur fused to the pedicel. Pelargonium is more accurately classified as having a deep receptacular nectary.
Tsai, Timothy, "The Receptacular Nectar Tubes of Pelargonium (Geraniaceae): A Study of Development, Length Variation, and Histology" (2016). Master's Theses. 950.