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is almost constantly of thé kind hidicated» and produees. an apparent temporary fall of osmotip pressure, though not a tangible and measurable one. It is more correct therefore to use the term plasmolysing concentration for results obtained in this manner, and a séries of observations on the ])lasmolytic concentrations for leaf cells of species of Acacia, Eucalyptus, and Grevillen taken from 1-12 mètres height show that the concentration may vary f'rom 2-6 percent.. KNO., in one and the same plant, that tbe variations between leaves at the saine level is at first as great as between leavès at différent levels, and that the size of the cell and thé age of thé leaf appear to influence thé plasmolytic concentration more than any other factors apart from food storage and assimilation.*Hence this promising line of investigation into the prablem of the ascent of sap must be abandoHod as inaccurate and misleading.
In criticising the conclusions based upon the expérimental results irf my former paper, Professor LarmorI suggests that the main part of the ascent of sap may be by a chain of osmotic actions from vessel to vessel, and that the weight, of liquid in each vessel may rest on the base of \he vessel, so that no summation to a high total pressure occurs. That no high total pressure or tension appears at any -point in a = tall tree is certain, but thé explanation given by Professor Larmor does not coincide with the facts. In involves three assumptions :– (l) That the sap increases in concentration upwards (2) that the vessels contain semi-permeable membranes; (3) that the weight of the separate columns can be supported on the perméable bases of tbe vessels without being transmitted through them. The last-nàmed could only be the case if the membranes were impermeable, when ail upwardflow would cease, and any decrease of their permeability simply increases the resistanceto flow, and hence also the pressure required to produce a given rate of flow upwards or downwards. Further, the vessels, being dead elements, contain no semi-permeable membranes. Where imperforate transverse partitions of lignified cellulose do occur they are equally perméable to water and dissolved salts, so that within the vessels themselves no osmotic raising of water is possible, even if the required concentrations existed at the higher levels. In regard to the lâttër^the following data may be given –
Section 5.- – Concentration OF THE SAP IN Vesskls.
The sap obtained from bleeding stems may contain as much as 1-3:5 per cent. of sugar, although usually very much more dilute. According to Hartig.J sugar is présent in quantities approaching the above amounts in4fee sap obtained from l wood in winter time, especially in the case of the Willow and Poplar, but also in that of Conifers. ,?~~
See also.PRINGSHEIM, loe. cit.
t Roy. Soc. Proc. 1905, p. 460. -––-––––~
l Bot. Ztg. 1888, p: 405.