iv COHESION THEORY OF ASCENT OF SAP 89 



shape, not adhering to any rigid envelope, the smallest 

 forces, whether of compression or of tension, spend them- 

 selves in leading to a readjustment of form, to which the 

 liquid owing to its mobility readily submits, and no stress 

 is produced. On the other hand, if a pull is exerted on 

 a liquid which thoroughly wets and adheres to the internal 

 surface of a rigid vessel, and, if there are no bubbles or 

 discontinuities in the liquid, a state of tension inevitably 

 supervenes. 



We have seen that the evaporation taking place from 

 the outer surfaces of the mesophyll cells is continually 

 abstracting water from the tracheae of the leaf. It is a 

 matter of common observation that these tracheae are 

 constantly filled with water, and they enclose no bubbles. 

 Experiments on pieces of the conducting tracts of plants to 

 be described later, show that the adhesion between their 

 walls and water is at least as great as, and probably much 

 greater than, the adhesion between glass and water. Hence, 

 if water is given off from the cells more rapidly than lifting 

 forces raise it in the trachea), the water in the latter 

 must inevitably fall into a state of tension. 



Apart from root-pressure, investigation has shown that 

 the only force from below which could be effective in raising 

 water in plants is the pressure exerted by the atmosphere. 

 The amounts of water forced up by root-pressure are in- 

 significant compared with the losses due to transpiration. 

 Atmospheric pressure can supply the evaporating cells at 

 most only up to a level of about 10*3 metres. When allow- 

 ance is made for the resistance opposed by the conducting 

 tracts to the motion of water in them, we must conclude, that 

 the supply of water raised by these two forces to a height 

 of 10 metres above the roots, must be exceedingly small. 

 It follows that the water in the tracheae above this level is 

 at all times in tension, and, in times of vigorous trans- 

 piration, whenever the loss cannot be made good by the 

 lifting pressure of the atmosphere, the water in the tracheae 



