682 
MOLECULAR FORCES IN THE PLANT. 
in the lignified cell-walls (or in other words imbibed by them) ; and (2) that it is caused 
by a very thin stratum of water which overspreads the inner surface of the wood-cells 
and vessels^. In both cases it must be assumed that the transpiration in the tissue of 
the leaves causes the upper parts of the wood to contain less water, and therefore 
to draw up the water from the parts which lie lower. The woody bundles of the 
roots are surrounded by succulent parenchyma, from which they remove the water ; 
and these again absorb it from the soil by endosmose. It may however be imagined 
that this movement in the substance of the cell-walls (the contents not participating in 
it) extends as far as the surface of the parenchyma of the root, where the water con- 
tained in the soil is absorbed. The question whether the attraction of the cell-walls 
for water, — putting aside the question whether it moves in their substance or only on 
their surface,— is sufficiently powerful to sustain the weight of a column of water of 
the height of 100 or even 300 feet or more attained by some trees, may be answered 
without hesitation in the affirmative, since we have to do here with molecular forces 
in relation to which, the action of gravity altogether disappears. But it is another 
question whether the rapidity of the molecular movements of water of this nature is 
sufficient to cover the requirements of the foliage of a tree which amounts on a hot day 
to hundreds of pounds^. 
The hypothesis finally that the water necessary to supply the loss by transpiration 
is forced up into the stem as far as the leaves by root-pressure must be abandoned, 
since this could only operate in the cavities of the wood ; and these are always empty 
in energetically transpiring plants. In the case of tall trees the pressure would also not 
be sufficient ; and if I at one time assumed that this might be a cooperative cause at 
least in shrubs and annual plants, I must retract this after my observations made in 
the year 1870; since these show that the root-stock of such plants as the 8un-flower, 
Gourd, &c., is even subject to a negative .pressure when they are transpiring strongly ; 
i.e. does not press water up, but greedily sucks it in at a cut surface above the ground 
[vide infra) ^. 
The insufficiency of all attempts hitherto made to explain the transpiration-current 
in the wood is especially noticeable from the fact that it is only under certain in- 
ternal conditions which cannot be more accurately ascertained that wood is capable 
of conducting water with the force and rapidity required by the transpiration from 
the leaves. Woody but air-dry branches with a lower cut surface placed in water 
are never able to raise up as much water as is necessary to compensate the evapo- 
ration even from an upper cut surface ; while the same branch in a fresh state 
conducts water fast enough to replace the much greater amount of transpiration from 
the numerous leaves. A change is thus caused in wood simply by drying up which 
deprives it of the power of conducting water rapidly. The natural alteration which 
takes place in wood, by which it is transformed as it increases in age into ' duramen ' — 
the cell-walls becoming harder and of a deeper colour — also deprives it of this power. 
If a tree is deprived not only of the bark but also of the 'alburnum' (the light-coloured 
younger wood on the outside), in an annular zone, the foliage of the tree, according to 
the statement of different writers, dries up, because the water is not conducted suffi- 
ciently rapidly through the duramen. 
^ This hypothesis follows from the discoveries of Quincke on capillarity, and has been commu- 
nicated to me by him. [In consequence of subsequent researches, Sachs is now of opinion that the 
transpiration -water travels only iji the cell-walls of the wood (Ueb. die Porosität des Holzes, Arb. d. 
bot. Inst, in Würzburg, II. 2, 1879).] 
^ See Nageli u. Sch wendener, Das Mikroskop, p. 365 et seq. 
3 [This negative pressure is due to the fact that, in consequence of active transpiration, the air 
which is contained in the vessels of the wood is more rarified, /. e. is at a lower pressure, than that of 
the atmosphere. Consequently, when the stem of an actively transpiring plant is cut through under 
water or mercury, the liquid is violently injected into the cavities of the vessels by the atmospheric 
pressure. (See von Höhnel, in Miltheil. d. k. k. Landv>iith cli. Laborat. in Wien, 1877.)] 
