Lacunal anatomy and resistance to convective flow in Typha domingensis and Phragmites australis

Abstract

The resistance to convective flow was examined in two morphologically distinct aquatic macrophytes: Typha domingensis and Phragmites australis. Resistance was found to be primarily determined by the lacunal cross-sectional area so that smaller lacunal dimensions resulted in higher resistances. The resistance of plant material was predicted with the Poiseuille-Hagen equation and compared with the observed resistances; predicted resistances were consistently lower. When the observed and predicted values were expressed as a ratio, different plant tissues had different ratios. The highest ratio was found in the rhizome of P. australis (5019+/-4705), and the lowest ratio in the leaves of T. domingensis (18.68+/-8.68). The discrepancy between observed and predicted resistances was investigated by estimation of Reynold's number which indicates the potential for flow turbulence. Reynold's number was highest in the rhizome of P. australis (2359+/-2884) and lowest in the leaves of T. domingensis (1.4+/-0.9). As turbulent flow will only occur at values of the Reynold number >2000, these data suggest that turbulent flow will only occur within the rhizomes of P. australis. The anatomy and resistance to convective flow was quantified along the length of the leaf of T. domingensis and the stem of P. australis. In both species there were changes along the length of the aboveground material with the highest resistance being found in the distal regions of the leaf or stem. The changes in resistance were in concert with the changes in lacunal cross-sectional area.