238 THE MOVEMENT OF WATER THROUGH THE PLANT 



Eventually the cohesive force of the water in the cells of the annulus is ex- 

 ceeded, the water ruptures, and the annulus recoils suddenly into approxi- 

 mately its original position, scattering the spores into the atmosphere by its 

 violent rebound (Fig. 64, C). The cohesive force of the water in cells such 

 as these can be measured by allowing the annuli to come to equilibrium with 

 known vapor pressures, and determining the highest vapor pressure at which 

 rupturing of the water occurs. The osmotic pressure corresponding to this 

 vapor pressure (Table 24) is equal to the cohesive force of the water in the 

 annulus cells. Such measurements indicate the cohesive force of the water 

 in these cells to be, at a minimum, between 300 and 350 atm.ospheres. 



A Be 



Fig. 64. Behavior of fern sporangium during drying. 



The cohesive force of water can also be demonstrated by means of an 

 apparatus such as that shown in Fig. 65. The first experiment of this type 

 was performed by Askenasy ( 1897). As evaporation proceeds from the porous 

 clay cup of this apparatus water moves up the vertical glass tube, followed 

 by mercury from the reservoir. If the demonstration is successful the mercury 

 will continue to rise above the level to which it will stand in a barometer. 

 The water is now being pulled up the tube, a phenomenon which is possible 

 only because of its cohesive force. The water in the tube is thus thrown into 

 a state of tension which is transmitted to the mercury column below it, due 

 to the relatively enormous adhesive force between water and mercury. The 

 forces exerting this pull originate at the evaporating surface of the cup, and 

 are due to the adhesive and cohesive forces operative in the maintenance 

 of innumerable microscopic menisci in the pores of the clay cup. Thut ( 1928) 

 succeeded in demonstrating a rise of mercury to a height of 226.6 cm. in 

 such an apparatus. This is approximately three times as high as a column 

 of mercury will be supported by atmospheric pressure acting alone. While 

 the maximum tension developed in the water column in these experiments 

 does not exceed two atmospheres, the demonstration graphically illustrates a 



