Chapter VIII — 155 — Uptake and Movement 



tenance of continuity under conditions of greatly reduced pressure, and pre- 

 vent, by the action of surface tension, the penetration of undissolved gas 

 through the wet cellulose walls. Theoretical values for the cohesion force of 

 water are very great (cf. Table 2, Chapter II, internal pressure). 



Harkens (1926) calculated the force of cohesion from the surface ten- 

 sion of water in the following manner. The surface tension of water at 

 20° C. = 72.8 dynes per cm. Assuming that a water column one square 

 centimeter in cross section is ruptured to produce two new surfaces, each of 

 one square centimeter, the energy required is equal to twice the free surface 

 energy or 145.6 ergs per cm-. The distance over which molecular attractive 

 forces are effective approximates molecular dimensions, and force diminishes 

 as a power of the distance. Assuming that the summation of molecular at- 

 tractive forces acts as a constant force through a distance of 10"^ cm., then, 

 since force = ^^^^ — , the force required to rupture the water column would 



distance 



be ^ = 1.456 X 101 dynes or 1.48 X 10^ grams per square cm. This 

 is equivalent to about 14,000 atmospheres. The force of cohesion as cal- 

 culated by van der Waals' equation is 11,000 atmospheres; by vaporiza- 

 tion studies it is 10,500 atmospheres (Shull, 1924) ; from internal pressure 

 calculations it is approximately 17,000 atmospheres. All of these represent 

 ideal values ; practically the conditions necessary for obtaining such co- 

 hesions are impossible of attainment. 



Experimental values for the cohesive force of plant sap obtained by 

 measurement of the diffusion pressure deficit of water in the cells of fern 

 sporangia are about 316 atmospheres, sufficient to support a static column 

 around 10,000 feet in height, and entirely adequate to account for movement 

 through xylem ducts to the top of the tallest tree (Ursprung, 1915 ; Ren- 

 NER, 1915). 



Dixon (1914) extracted sap from Ilex aqiiifolium and tested its tensile 

 strength by sealing it in a capillary tube, warming until the sap just filled 

 the tube and then cooling until the column was ruptured. From the thermal 

 expansion of glass and water he was able to calculate the tension at the 

 temperature of rupture ; values determined were between 133 and 207 atmos- 

 pheres. 



In view of the fact that cellulose walls imbibe water and are thoroughly 

 wet, the forces of adhesion must exceed those of cohesion. Many centers 

 of hydration occur along the cellulose chains making up the walls. Conse- 

 quently, in the immediate spheres of influence of these coordinating centers 

 water must be held with such force that its molecules are oriented, the quasi- 

 lattice structure assuming a closer packed and more stable condition than in 

 the body of the liquid. At the same time, due to the rigidity of the walls, 

 and the excess of transpiration over uptake, water in the centers of the 

 lumina of conducting elements must be at times in a rarified or tensile condi- 

 tion. This would tend to expand the centers of abnormal coordination 

 (holes) reducing viscosity and facilitating flow. 



Liquid Continuity in the Xylem: — The cohesion mechanism, to 

 function, depends upon continuous water columns. Evidence for and against 

 such a condition is cited by Copeland (1902), Dixon (1914, 1924, 1938a), 

 Maximov (1929a), Meyer (1938), and Strugger (1943). 



When a gas bubble is present in a conducting element of the xylem, 

 that element is eliminated, for the time, as a conductor of liquid water under 

 tension. Many objections have been raised against the cohesion theorv 



