||"With typical plant cell sizes in the range of d=10–100µm, diffusion and advection of sugars are equally
effective over these length scales when the Peclet number
Pe=vd/D=1 [primary source], where v is the flow speed and
D is the diffusion coefficient (D=0.5?10^-9m^2/s
for sucrose [CRC Handbook of Chemistry and Physics, edited by
W. M. Haynes (CRC Press, Boca Raton, FL, 2012), 93rd
ed.]). [Researchers] therefore expect v~D/d=5–50µm/s to provide a lower estimate of the minimum
flow speed umin."