MOVEMENT OF WATER IN PLANTS, 



605 



place in a solution of lithium citrate, and then examined the ashes of successive inter- 

 nodes by the spectroscope. The solution was found to rise from 42 to 46 cm. in one 

 hour. But neither method of calculation is exact or probably of much value. 



The current of water in the woody substance which replaces the loss occasioned 

 in the leaves by transpiration is not caused by osmose, since at the time when the 

 evaporation is strongest and therefore the current in the wood quickest, the cavities 

 of the conducting wood-cells do not contain sap but air, or at the most are only 

 partially filled with fluid. If the rising of the water took place by endosmose from 

 cell to cell, the cells would themselves possess closed cell-walls and be full of sap, the 

 concentration of which would constantly increase from below upwards in the wood. 

 But the conducting cells are at this time not closed, but partially or altogether (as in 

 Goniferie) connected with one another by open bordered pits. In the spring, before 

 strong transpiration sets in, and therefore at a time when the water in the wood is com- 

 paratively at rest, the wood-cells also, it is true, contain sap, flowing in quantities out 

 of their communicating cell-cavities when holes are bored in the trunks (as in the birch, 

 maple, &c.) '. But this sap does not, as is proved by analysis'^, show a concentration 

 increasing from below upwards. The fact also that water rises in cut leafy branches 

 placed with their upper end in water or planted and taking root, and flows there- 

 fore in a direction opposite to the ordinary one in the branch, shows that endosmose 

 depending on a definite distribution of the concentration of the sap cannot be the cause 

 of the current of water. Since vessels and wood-cells communicating with one another 

 through their open pores form narrow cavities which sometimes become wider as they 

 proceed, sometimes narrower, the woody substance may be represented by a bundle of 

 narrow glass tubes alternately bulging and contracting, in which the water which fills 

 them rises by capillary attraction. But how little efficacious a contrivance of this kind 

 would be is seen at once from the width of the capillary tubes, which is much too great 

 to raise water to a height of 100 feet or more. It must also be pointed out that in the 

 summer, when the current of water is strongest, it is principally air and not fluid that 

 is conveyed through the cavities of the cells. 



Since it is evident from what has been said that the movement of the water takes 

 place in the woody substance and not in the cell-cavities filled with water, there remain 

 only two hypotheses; viz. (i) that the movement takes place in the water contained 

 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 th^ 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 both the kinds of motion mentioned proceed along the surface as well as in the 

 substance of the cell-walls (the contents not participating in it) to the surface of the 

 root, where the water contained in the soil is sucked up. 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. 



• The older statements of Unger are referred to in my ' Experimental-Physiologie '; others will 

 be found in Schroder, Jahrb. flir wiss. Bot. vol. VII, p. 266 et seq. 



2 The conduction is however by no means so considerable in the reversed as in the ordinary 

 direction, as Baranetzky found in the laboratory at Wiirzburg; but this may be connected with other 

 peculiarities of the organisation. 



3 This hypothesis follows from the discoveries of Quincke on capillarity and has been commu- 

 nicated to me by him. 



