A Reanalysis of Particle Motion in Sieve Tubes of Heracleum

From an analysis of the Brownian motion of particles in sieveelements of Heracleum mantegazzianum and Heracleum sphondylium,Barclay and Johnson have suggested that the in situ viscosityof sieve tube sap is four to six times higher than has previouslybeen assumed. In particular, they obtained a value for the sapviscosity of about 10^–2 Pa s, which compares with a valueof 2 x 10^–3 Pa s for a 20 per cent (w/v) sucrose solution.The present paper describes a reanalysis of their data. It isargued that Barclay & Johnson underestimated the Brownianmotion of sieve element particles and so overestimated the sapviscosity. An exact correction was not possible, but it is concludedthat the in situ viscosity of Heracleum sieve tube sap mustbe less than 3 x 10^–3 Pa s, which corresponds to a sucroseconcentration of less than 29 per cent. Hence it may not beunreasonable to suppose that the viscosity of sieve tube sapis determined primarily by the concentration of sucrose, ashas been assumed in theoretical analyses of the Munch hypothesis.It is also concluded that the sieve tubes studied by Barclayand Johnson were not functional, in the sense that they didnot exhibit an axial bulk flow of sap. Heracleum, sieve tubes, Brownian motion, viscosity, Munch hypothesis