87 



A. Transpiration, the vis a fronte giving the upward 



pull at the leaves, and causing a demand for the 

 water absorbed by the roots to supply the loss which 

 it occasions. 



B. Variations of temperature which, by causing expan- 



sions and contractions of the air within the vessels 

 affect the relative distances between the micellae 

 composing their walls, and thus produce local ex- 

 pansions and contractions, followed by currents of 

 water. 



C. The action of the wind, which causes the swaying of 



the branches and leaves. It is well known that if 

 you bend a tube, such as a vessel , the calibre or size 

 of the bent part is smaller than when the tube was 

 straight, and therefore a bent tube will hold less 

 than a straight tube ; but not only does bending 

 affect the calibre, it affects also the solid sub- 

 stance, by diminishing the distance between the 

 component micellae at particular points, and in- 

 creasing them at others, for while one part is con- 

 tracted another part is expanded. The result is that 

 water is forced out where the distances are dimin- 

 ished, and since it cannot be forced downwards 

 against all the forces acting concurrently which tend 

 to bring it upwards, it passes upwards, for in this di- 

 rection it has only its own weight to oppose it. The 

 effect of the bending is thus to produce a local ten- 

 sion, and when this is removed there will be an up- 

 m ward current to supply the space which is left empty. 

 In addition to this, in the parts where the distance 

 between the micella? has increased, more water is 

 needed, and so there is an upward current to those 

 parts also. So, then, every alternate bending, first 

 to one side and then to another, produces a corres- 

 ponding upward movement of the water, and the 

 branches act as so many pumps, so to speak, doing 

 their work slowly but surely, not by their cavities 



