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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
Identifiant : ark:/12148/cb34422226r/date
Identifiant : ISSN 00804622
Source : Bibliothèque nationale de France
Relation : http://catalogue.bnf.fr/ark:/12148/cb34422226r
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 : bnf.fr
Date de mise en ligne : 13/01/2009
1908 (VOL199).
Page
(Screen 393 / 473)
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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
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.
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