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 Sun-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 iri the cell-walls of the wood (Ueb. die Porosilat des Holzes, Arb. d. 

 bot. Inst, in Wiirzburg, II. 2, 1879).] 



2 See Nageli u. Schwendener, 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, i. 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 Ilohnel, in Mittheil. d, k. k. Landwirth-ch. Laborat, in Wien, 1877.)] 



