On the Conductivity of the Xylem in Trees

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Abstract
The xylem of three tree species, the ring porous Fraxinus excelsior L., and the diffuse porous Acer pseudoplatanus L., and Salix atrocinerea Brot., has been examined to determine how closely flow through it obeys the Poiseuille equation. The equation was most closly obeyed in the relationship between flow-rate and pressure difference between the ends of the xylem cylinder, though there were differences between the ring and diffuse porous types. The relationship of flow-rate and specific conductivity to the radius of the xylem cylinder was complex.Fraxinus showed a rise with increase of radius up to a given size (0.54cm), followed by a steep decline. This decline was shown to be caused by a progressive increase in the number of vessels which were unable to conduct water. Small increases in the viscosity of the xylem liquid produced a disproportionately large fall in the rate of flow, and the specific conductivity of the xylem decreased. With viscosities greater than 2.5 centipoise, the specific conductivity remained constant. An effect of alternately raising and lowering the hydrostatic pressure of the xylem water on the specific conductivity of the wood is described. These results are discussed in relation to the structure of the xylem, and particularly with regard to the possibility that the conduction channels in the wood consist of a system of microcapillaries surrounding the vessel cavities.

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