On the economy and safety of hollow non-septate peduncles

Abstract

We examined the allometry and mechanical properties of the peduncles of Hieracium pilosella and other species in the Asteraceae (H. aurantiacum. Taraxicum officinale, Tragopogon pratensi) to evaluate the hypothesis that tapered, tubular, and non-septate peduncles optimize the trade-offs among stem biomass allocation, elevating flowers and thus their wind-dispersed fruits, and the requirement for a factor of safety against mechanical failure. This hypothesis was evaluated by comparing peduncle morphometry (e.g., biomass M s, floral biomass Mf, and length L) and mechanical properties (e.g., bending rigidity EI) for populations growing in windy and wind-sheltered sites as well as transplants between sites. Regardless of ambient wind speeds, Ms ∝ L4/3 and EI ∝ L1 1/4 ∝ Ms5/3, whereas Mf/M s ∝ L-1.15, i.e., peduncles disproportionately increase in their biomass as they increase in length, but mechanically support a disproportionately smaller floral biomass relative to their biomass. Calculations show that the tall peduncles from wind-sheltered sites have a larger fruit dispersal range and a lower factor of safety than the shorter peduncles produced in open sites. These and other observations are interpreted to indicate that tubular peduncles enhance relative fitness in terms of propagule dispersal (but not propagule number per stem) while maintaining a sufficient factor of safety against mechanical failure.