MAGNITUDE OF THE COHESIVE FORCE OF WATER 237 



own weight. Water, however, encounters a certain amount of resistance in 

 moving through the conducting tissues. The greater the velocity of the cur- 

 rent of water, the greater the resistance which it will encounter. Dixon has 

 shown that at the velocity corresponding to usual rates of transpiration this 

 resistance is approximately equal to the pressure required to support water 

 for the height of the plant. He used the wood of the yew (Taxus baccata) 

 for his determinations, in which higher values for resistance would be expected 

 than in non-conifers in which most water traverses vessels rather than tracheids. 

 For high rates of conduction Huber (1924) estimates the resistance to be sev- 

 eral times as great as this. Accepting Dixon's estimates as an average value, 

 the required cohesive force of water must be doubled, giving a value of 24 

 atmos. To this must be added the resistance normally encountered by water 

 in crossing the tissues of the root and the mesophyll cells of the leaves which 

 is equivalent to only a few atmospheres. The estimated minimum cohesive 

 force required to lift water to the tops of the tallest tree is therefore about 

 30 atmos. Even if we assume a resistance three times as great as that found 

 by Dixon, the value of the necessary cohesive force becomes only about 50 

 atmos. 



Values experimentally determined for the cohesive force of water range 

 up to 350 atmos. Dixon obtained values as high as 207 atmos, for sap centri- 

 fuged from branches of holly {Ilex aquifolimn) . The cohesive force of water 

 is therefore much in excess cf the minimum required for the lifting of water 

 to the tops of even the tallest trees. Although the estimates given in the 

 preceding paragraph represent the minimum cohesive force actually required 

 for the movement of water to the top of tall trees, many botanists believe 

 that when an internal water deficit exists in plants, tensions as great as lOO 

 atmos. or even more may develop in the water columns of at least some species 

 of plants. 



One of the most ingenious methods of studying the cohesion of water 

 employs the sporangia of the ferns (Fig. 64, A) for this purpose (Urspning, 

 1 915). Around the edge of a fern sporangium occurs a ring of dead, thick- 

 walled cells, known as the annulus. As the sporangium matures water is 

 gradually lost from the cells of the annulus by evaporation. Because of the 

 powerful adhesion between the cell walls and water, the resulting shrinkage 

 in the volume of water in each of these cells results in the thin outer wall of 

 each cell being drawn inward, while the ends of the thicker lateral walls 

 are pulled toward each other. This results in tearing open the weak side of 

 the sporangium, and eventually, in a complete inversion in the position of 

 the annulus, exposing the spores on its outer surface (Fig. 64, B) . The 

 annulus is now in a condition similar to that of a highly strained spring. 



