418 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1910. 



pressure, is transmitted equally in all directions. It is not sustained, 

 consequently, by a single point, but affects the whole internal wetted 

 surface of the containing vessel. In another particular the stressed 

 liquid differs greatly from the stressed solid. It is much more un- 

 stable. A small flaw (i. e., a bubble) in the tensile liquid rapidly 

 spreads and almost instantaneousl}^ severs the whole column ; it mat- 

 ters not how large the cross section of the unbroken part may be, a 

 comparatively feeble tension will tear it across. In the solid — a metal 

 wire, for example — on the other hand, if the cross section of the un- 

 broken part is sufficient, a small discontinuity in its substance is im- 

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

 This difference in the behavior 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 exi^eriments 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 endeavor 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 trachecc are constantly filled with water and they inclose 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 effective in raising wat«r in plants is the 



1 H. H. Dixon, Physics of the Transpiration Current, loc. cit. 



