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                  Titre : Philosophical transactions of the Royal society of London. Series B, Biological sciences

                  Titre : Philosophical transactions of the Royal society of London. B

                  Auteur : Royal society (GB)

                  Éditeur : printed by Harrison and sons (London)

                  Éditeur : The Royal society (London)

                  Date d'édition : 1888-1990

                  Type : texte,publication en série imprimée

                  Langue : Anglais

                  Format : application/pdf

                  Droits : domaine public

                  Identifiant : ark:/12148/cb34422226r/date

                  Identifiant : ISSN 00804622

                  Source : Bibliothèque nationale de France

                  Relation :

                  Description : Variante(s) de titre : Philosophical transactions of the Royal society of London

                  Description : Variante(s) de titre : Philosophical transactions of the Royal society of London

                  Description : Périodicité : Irrégulier

                  Description : Etat de collection : 1888 (vol. 178)-1939

                  Provenance :

                  Date de mise en ligne : 15/10/2007

                  First issue for the year 1908 Previous issue 1908 (VOL199). Next issue Last issue for the year 1908
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                  356 DR. A. J. EWART ON THE ASCENT OP WATER IN TREES.

                  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 concen-
                  tration 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

                  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.

                  Source: / Bibliothèque nationale de France

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