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Factors causing decreasing rate of longitudinal flow of liquids through freshly cut woody stems [microform].
Abstract
Thesis--Yale University. Microfilm.
... This decline, which can begin within the first minutes of measurement (Zimmermann, 1978), has been observed for xylem of several species under a variety of experimental conditions. Explanations include blockage by air coming out of fluid within the sample (Kelso, Gertjejansen & Hossfeld, 1963); narrowing of vessel diameters due to swelling of surrounding tissue (Jeje, 1986); eleetro-osmosis (Buckman, Schmitz & Gortner, 1935); particulate clogging (Krier, 1951); and swelling of intervascular pit tnembranes (Zimmermann, 1978). The second and less obvious problem with conductivity measurements is the potential presence of naturally occurring air-filled tracheids or vessels. ...
Abstract Hydraulic conductivity of the xylem is computed as the quotient of mass flow rate and pressure gradient. Measurements on excised plant stems can be difficult to interpret because of time-dependent reductions in flow rate, and because of variable degrees of embolism. Using Acer saccharum Marsh. stems, we found that certain perfusing solutions including dilute fixatives (e.g. 0.05% formaldehyde) and acids with pH below 3 (e.g. 10 mol m−3 oxalic) prevent long-term decline in conductivity. Xylem embolism can be quantified by expressing the initial conductivity as a percentage of the maximum obtained after flow-impeding air emboli have been removed by repeated high-pressure (175 kPa) flushes. Correlation between microbial contamination and declining conductivity suggests that long-term (> 4h) declines are caused by microbial growth within the vessels. Unpredictable trends in short-term (< 4h) measurements may be caused by movements of air emboli in vessels and/or participate matter.
The different approaches to the use of steady-state gas flow data in the prediction of the steady-state axial permeability of wood to liquids are reviewed. Since these may be shown to have certain theoretical shortcomings, a new predictive method based on a computer analysis is presented. This and previous methods are tested experimentally; apparently, none can be relied upon to predict the liquid permeability accurately. However, it is clear that the true liquid permeability is not measured; the reasons for this are uncertain. Until this difficulty is resolved, the precision of predictive methods cannot be accurately assessed.
Longitudinal air permeability measurements were made on specimens of British Columbia Interior and Coastal types of Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) sapwood and heartwood, kiln dried and solvent dried, which were successively reduced in length from 3.5 to 0.5 cm. Most sapwood, which was quite permeable, was found to obey Darcy's law with respect to length. Heartwood and kiln-dried Interior sapwood specimens were less permeable, and were found to deviate from Darcy's law. A physical model was proposed in which, due to random blockage of tracheids by aspirated pits, the number of conducting tracheids was reduced exponentially with depth of penetration. This was expressed as a linear relationship between the logarithm of permeability and specimen length at lengths exceeding one tracheid. This model conformed with the experimental data. A new mathematical equation was proposed:
\fracdQdt = \fracKA e - bl DPhl \frac{{dQ}}{{dt}} = \frac{{KA e^{ - bl} \Delta P}}{{\eta ^l }}
which differs from the Darcy equation only by the insertion of e-bt
, where e = base of natural logarithms andb is a positive constant determined by experiment. Whereb=0, e-bl
=1, and the equation reduces to the Darcy equation.
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