Carey N. Sydney
Department of Biology, Carleton College, Northfield, MN, USA, 55057
Abstract. The characterizing features of the genus Costus are its unique helical phyllotaxy and their inflorescence relationship with ants, which provide protection from the herbivorous Euxesta larvae. Their phyllotaxy is spiromonostichous and deviates from the Fibonacci series and Hofmeister's Rule. The interaction of the ants and inflorescence and possible benefits of spiromonostichy was investigated for the species Costus malortienaus. There were more Euxesta eggs on inflorescences with ants removed than with ants present, but difference was not significant. It was hypothesized that the phyllotaxy maximized the plant's total leaf exposure to sunlight, thereby increasing the fitness of the plant. The prediction that C. malortienaus could change the tightness of its spiral in response to light intensity proved to be false; however, this does not disprove the leaf-exposure hypothesis. The direction of spirals (clockwise versus counter-clockwise) was random, but within a clone it was always the same. This suggests that the direction of spiraling in C. malortienaus is genetically controlled. Without an obvious advantage to either direction, there is a direction variance in the population that cannot be explained.