Flow in xylem
Abstract
Solutions of either ³²P or ⁴²K were drawn through segments of green and dormant poplar by applying a vacuum pressure at the upper end. Uptake was shown to consist of two fractions, the first passing through the stem via the vessels, and the second passing into the other xylem components where it was retained. Specific retention in green material was significantly higher (2½% level) than in dormant although the green material had a lower original specific weight (10% level).
Mechanical reduction of xylem cross-sectional area exposed to solutions did not reduce flow in proportion to area, the flow being greater than expected. Using certain assumptions, resistance to flow of a fibre tracheid was estimated at about one order greater than resistance to flow of a vessel. An estimate of the percentage of vessels available for conduction was made and-gave a maximum figure of about 30%, with a possible lower limit of 5-10%.
When vacuum was applied to an already transpiring stem segment an increase in uptake occurred, and on releasing the vacuum the transpirational uptake was found to remain greater by a factor of 15. Dormant segments taken from a hedge were found to take up water without application of any vacuum and this was attributed to the action of surface tension forces. The effect was associated with low rainfall over the preceding two months.
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Conductivity of the material ranged from 1.4 to 7.8 (mean 3.6) cm²/hour cm-H₂O, under an applied vacuum pressure.
Using half stem immersion with a vacuum pressure, lateral velocities of phosphate ions were obtained which were up to 30% of the longitudinal velocity.
Activity-time charts obtained from Geiger-Muller counters placed along the stem were interpreted as showing reversible flow into and/or through walls of the vessels. Using a simplified mathematical treatment a lateral velocity through the vessel wall of 3.4 x 10⁻³ cm/min was obtained for the H₂PO₄⁻ ion and 7 x 10⁻³ cm/min for the K⁺ ion. Both ions could be substantially flushed from the stem with water and no evidence against mass flow was found.
A critical analysis is made of the literature concerned with flow in xylem, attention being paid to theories of sap ascent, particularly the cohesion theory.
