The mechanism of flow of gases through coniferous wood

The mechanism of flow of gases through coniferous wood has been examined and found to follow the viscous/slip regime. According to the general theory the specific flow K of a gas in this regime is a linear function of its mean pressure $\overline{p}$. For coniferous wood, however, we have found that K is a quadratic function of $\overline{p}$ approximating to a linear one at high enough values of $\overline{p}$. It is shown that this is because K is the sum of two linear functions of $\overline{p}$, k$_{1}$ and k$_{2}$ such that 1/K = 1/k$_{1}$ + 1/k$_{2}$ where k$_{1}$ is believed to be the flow through the tracheids alone and k$_{2}$ the flow through the bordered pits. It is shown that the permeability constant for viscous flow K$_{\nu}$ calculated from gas flow is applicable to liquids so that liquid flow can be predicted from gas flow data. With some species the observed flow rate of a liquid differs greatly from the predicted value. Evidence has been obtained that this is because the torus and margo fibrils of the bordered pit are readily displaced by the surface tension and momentum forces developed on them by a liquid causing radical and erratic changes in permeability. Approximate values for the 'diameter' of the smaller flow path have been calculated from the ratio of the viscous to the slip component of flow of k$_{2}$. These were found to be about 1.4 to 1.7 $\mu $m. This is the same order of size as the distances between the torus and the interior of the border of the pit and indicates that it is the geometry of this part of the structure, rather than that of the margo, that controls flow. These results provide strong support for the modern theory of pit structure based on electron microscope photographs.