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

The resistance to convective flow was examined in two morphologically disti nct aquatic macrophytes: Typha domingensis and Phragmites australis. Resist ance was found to be primarily determined by the lacunal cross-sectional ar ea so that smaller lacunal dimensions resulted in higher resistances. The r esistance of plant material was predicted with the Poiseuille-Hagen equatio n and compared with the observed resistances; predicted resistances were co nsistently lower. When the observed and predicted values were expressed as a ratio, different plant tissues had different ratios. The highest ratio wa s found in the rhizome of P. australis (5019 +/- 4705), and the lowest rati o in the leaves of T. domingensis (18.68 +/- 8.68). The discrepancy between observed and predicted resistances was investigated by estimation of Reyno ld's number which indicates the potential for flow turbulence. Reynold's nu mber 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 turbul ent flow will only occur within the rhizomes of P. australis. The anatomy a nd resistance to convective flow was quantified along the length of the lea f of ir: domingensis and the stem of P. australis. In both species there we re changes along the length of the aboveground material with the highest re sistance 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. (C) 2000 Elsevier Science B.V. All rights reserved.