CONDUCTiniLITF OF WOOD FOR WATER 



243 



ourselves directly by cutting off single pieces. This wood, when once dried up, is 

 no longer in a condition to convey water from the roots to the buds ; and so the 

 plants perish. 



We thus see that the wood owes its significance as the organ for conducting 

 water to a series of most remarkable properties, which are found in no other natural 

 body ; and after what has been said, it would be simply childish still to attempt, as 

 has been done previously, to derive the properties of the wood and the mechanics 

 of the ascending water current from observations on capillary and porous bodies, 

 such as gypsum, or from the endosmotic processes in an apparatus made with 

 animal bladder, or even from the properties of parenchymatous tissues. The wood 

 is a body sui generis, and especially adapted by nature for the purpose of conveying 

 water (and even water laden with nutritive substances) from the roots up into the 

 transpiring organs of assimilating plants. The utterly incorrect views of the 

 mechanics of the ascending water-current which were held previously to my re- 

 searches 'On the porosity of wood,' 1879, were especially characterised by the 

 difficulties arising from the fact that water ascends in the wood up to heights of more 

 than TOO m. into the leaves of tall trees, because it was always assumed here 

 that it was a matter of capillary tubes. It is true water can also ascend in capillary 

 tubes, but to a considerable height only when they are extremely narrow. 



Here, however, appears the difficulty perceived, but not overcome, by Nageli 

 and Schwendener, that the capillary movement in very narrow cavities is so 

 exceedingly slow that it no longer suffices for the requirements of transpiration. 

 It was simply an entirely false principle from which observers formerly proceeded ; 

 since it depends not upon a phenomenon of capillarity, but upon imbibition and 

 swelling, where, as I have shown in the preceding lecture, quite other molecular 

 relations and forces are put in requisition. The force wiih which water is held 

 fast in bodies which imbibe it is so enormously great, that it is quite immaterial 

 whether the mass of wood saturated with water extends 10 m. or 100 m. into the 

 air above the absorbing roots ; just as in the case of the salt solution of sea-water it 

 is immaterial whether the dissolved salt molecules are suspended 100 m. or 1000 m. 

 above the sea-bottom. The one point of special importance to be considered here 

 is the facile mobility of the water thus held fast by the cell-walls. This, however, 

 is to be understood on observing that the wood cell-walls are saturated with 

 water from their origin, and that every displacement of a molecule of water in 

 them causes a movement of other molecules of water ; whence it will be immaterial 

 to the forces of imbibition whether the water molecule A or B enters into the 

 sphere of attraction of a given molecule of wood. 



The main result of all these considerations is this, that the ascending cur- 

 rent of water depends upon the motion of the relatively small number of water 

 molecules which are contained between the molecules, or ' micellae,' of the wood cell- 

 walls. This much is established, that this movement can only occur when the 

 wood cell-walls at the upper end of this system lose a portion of their water molecules. 

 By this loss its state of saturation with water becomes disturbed, and the equilibrium 

 altered : the parts of the wood cell-walls which have become poorer in water will 

 tend to restore the equilibrium by attracting water from the nearest wood-cells, 

 which, in their turn and for the same reason, take it up again from parts of the 



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