Modelling the hydrodynamic resistance of bordered pits

Previous studies of the hydrodynamics of plant stems have shown that resistance to flow through bordered pits on the side walls of tracheids makes up a significant proportion of their total resistance, and that this proportion increases with tracheid diameter. This suggests a possible reason why tracheids with a diameter above around 100 μm have failed to evolve. This possibility has been investigated by obtaining an estimate for the resistance of a single pit, and incorporating it into analytical models of tracheid resistance and wood resistivity. The hydrodynamic resistance of the bordered pits of Tsuga canadensis was investigated using large-scale physical models. The importance of individual components of the pit were investigated by comparing the resistance of models with different pore sizes in their pit membrane, and with or without the torus and border. The estimate for the resistance of a real bordered pit was 1.70 × 1015 Pa s m-3. Resistance of pits varied with morphology as might be predicted; the resistance was inversely proportional to the pore size to the power of 0.715; removing the torus reduced resistance by 28%, while removal of the torus and border together reduced it by 72%. It was estimated that in a 'typical tracheid' pit resistance should account for 29% of the total. Incorporating the results into the model for the resistivity of wood showed that resistivity should fall as tracheid diameter increases. However, to minimize resistance wider tracheids would also need to be proportionally much longer. It is suggested that the diameter of tracheids in conifers is limited by upper limits to cell length or cell volume. This limitation is avoided by angiosperms because they can digest away the ends of their cells to produce long, wide vessels composed of many short cells.