THE CONDUCTION OF WATER. II 



73 



sure corresponding to the tension of water vapour. On the next air-bubble, 

 lying farther down, there will be the pressure of a water column of i mm. in 

 height, and hence it will not be stretched quite so much as the top one and so 

 on, each more deeply placed air-bubble being less stretched than the one next 

 above it. But there is another consideration to be taken into account. In 

 a horizontally placed tube, where the effect of the weight of the water columns 

 is eliminated, each separate segment of the chain opposes the movement in 

 a way not quite understood theoretically, but admitting of practical estimation. 

 ScHWENDENER found it to be approximately the weight of 4 mm. of water. On 

 the whole each small column of water opposes the upward ascent with a force 

 equal to the weight of 5 mm. of water. Under these conditions, and bearing 

 in mind that in the uppermost bubbles the tension is never that of water 

 vapour, but very much higher (one-third to one-fifth of an atmosphere), 

 ScHWENDENER estimated that the suctional power of the leaf was transmitted 

 backwards not quite 5 m. At this distance an air-bubble would exhibit only 

 atmospheric pressure. 



The assumed length of the air-bubbles, viz. i mm., is, as a matter of fact, 

 too great ; but if one imagines them smaller, then their suctional efficacy is all 

 the less. On the other hand, the amount of the opposition to the movement of 

 the separate water columns has perhaps been placed too high, and thus the suction 

 would stretch them out more. On the whole, however, Schwendener's criticism 

 may be accepted as correct ; the suction of the air-bubbles which raises the water 

 can act only a few metres backwards from the top of the tree, perhaps to the 

 base of the branches, but certainly not into the stem. Other forces must, there- 

 fore, be forthcoming to effect the elevation of water in the stem. Again, negative 

 air pressure in the vessels, which can exist independently of transpiration (com- 

 pare p. 76), occurs in the trunk of a transpiring tree, and it has never yet been 

 observed that this pressure increases continuously from the apices of the 

 branches towards the base of the stem. We feel bound to conclude from this that 

 there is something in Schwendener's assumption not in accordance with fact. 



There are other difficulties which stand in the way of our acceptance of 

 the idea of movement of the entire water and air chain. This theory assumes 

 that these vessels are continuous throughout the entire tree, while in reality 

 the vessels have always only a limited length. Furthermore it would be a 

 puzzle to determine where the air goes to when the water is absorbed by 

 the cells at the upper ends of the vessels. It is obvious that it would result in 

 the rapid formation of a cavity filled with air at low pressure, whose 

 removal later would be impossible. Under these conditions it will repay us to 

 keep the other possibility before us, and to inquire whether it may be possible 

 that a movement of the water in the chain takes place, though the air-bubbles 

 remain stationary. Such a hypothesis as this does not appear unjustified, 

 because in the tracheids, e. g. of coniferous wood, the conditions completely 

 forbid a movement of the air-bubbles for any great distance. 



How then can suction, exerted on the upper end of the water column in 

 a long vessel, be transmitted downwards while the air-bubbles in the vessel 

 remain stationary ? Three ways are possible ; either the water flows between 

 the air-bubbles and the wall, or in the wall itself, or, finally, through the 

 neighbouring cells. In this last case, if no cell or vessel in the neighbour- 

 hood is capable of interfering in virtue of its vital activity, we may imagine 

 a system of several vessels lying one behind the other and acting together, re- 

 placed by a sinuous vessel subdivided frequently by transverse walls. This 

 assumption obviously has no advantages over a straight and continuous vessel 

 filled with water : there is nothing to be gained by discussing it further. The 

 difficulties in the way of a movement of water in the wall itself have been 

 already mentioned (p. 48), and these are sufficient to make the second possi- 



