40 Henry H. Dixon. 



example — on the other hand, if the cross section of the unbroken 

 part is sufficient, a small discontinuity in its substance is immaterial, 

 and the stress may be successfully resisted, by the intact part. This 

 diiference in the behaviour of the two forms of matter when sub- 

 mitted to a stretching- force is to be referred to the fact that the 

 particles of a liquid are perfectly mobile and are free to move round 

 each other without being- opposed by any sensible internal forces, 

 whereas in solids there is a great opposition to the relative motion of 

 the parts. To this property solids owe their rigidity. In fact, in 

 tension experiments the liquid becomes capable of sustaining ^) and 

 transmitting tensile stresses only when it is adhering completely to a 

 rigid, envelope which confers on the liquid a pseudo-rigidity. The 

 state of tension then persists because the stretching forces act solely 

 against the cohesive properties of the liquid (i. e. in an endeavour to 

 separate the water molecules from one another — a separation which 

 a liquid is able to withstand as well as a solid). If however the 

 liquid is free to change its 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 

 as described above, show that the adhesion between their walls and 

 water is 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 tracheae, 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 is eftective in raising water in plants is the 

 pressure exerted by the atmosphere. The amounts of water forced 

 up by root-pressure are insignificant 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 m. When allowance is 

 made for the resistance opposed by the conducting tracts to the 



^) H. H. Dixon, Physics of the Transpiration Current, loc. cit. 



