IV.] VARIOUS THEORIES, &c. 69 



tion, because the slightest displacement is sufficient to 

 disturb the equilibrium of the whole continuous 

 column of water. 



Of course the great advantage claimed for the 

 hypothesis was that it did away with the difficulty 

 presented by the height of tall trees. Sachs supposes 

 the molecules of water to be as it were dissolved in 

 the substance of the cell-walls, held by molecular 

 forces in the same way that a particle of salt is 

 commonly supposed to be suspended between mole- 

 cules of water in a solution ; and just as any particle 

 of salt in the ocean, for instance, is free to move in 

 any direction to and fro, or up and down and quite 

 independently of gravitation, so he thought his mole- 

 cules of water could be regarded as infinitely mobile 

 between the particles of the cell-wall. It matters 

 not whether we regard the cell-wall as composed of 

 micelles, or aggregate molecules, or other structural 

 units, in this connection, as the hypothesis simply 

 turns on the freedom of movement in spaces beyond 

 the ken of rough physics. 



When we come to experiments offered in support 

 of this hypothesis, the weak points come out some- 

 what vividly. 



If the stem of a hop, flax, &c., be sharply bent on 

 itself, the cavities of the vessels, &c. are of course com- 



