108 BRIGGS: THE LIVING PLANT AS A PHYSICAL SYSTEM 



tion from cells communicating with the stomatal cavities of the 

 leaf. The column is then, so to speak, hung from these leaf cells, 

 and is supported by the adhesion of the water to the walls of these 

 cells. We may consider the outer surfaces of these cells to be 

 saturated with water, which retreats into the wall tissue in such 

 a way as to form a great number of capillary surfaces of high cur- 

 vature, this curvature in fact being such as would be necessary 

 to support a capillary column of the observed height. If 

 through the evaporation of water from the leaves the curva- 

 ture tends to increase, the force exerted by these curved surfaces 

 will increase proportionally, and additional water will be drawn 

 from the soil, tending to restore the equilibrium. 



It will at once be evident that this conception postulates 

 that water must have great cohesion, sufficient in fact to with- 

 stand the weight of a column 300 feet or more in height. Since 

 this is an essential condition of the theory, we may profitably 

 consider the cohesion of water in more detail. 



Berthelot 11 in 1850 showed that a column of water under 

 suitable conditions can withstand a very great tensile stress. 

 A strong capillary tube sealed at one end and drawn to a fine 

 point at the other was filled with water at a temperature of 30°. 

 The water was then cooled to 18° and the tube allowed to draw 

 in air, after which the tube was sealed. The tube was now heated 

 to 30° and the contained air was forced into solution, so that the 

 water occupied the entire volume of the tube. On again cooling 

 the tube to 18° it was found that the liquid continued to occupy 

 the entire volume of the tube. To produce a compression equal 

 to this observed dilatation would require a pressure of about 

 50 atmospheres, from which Berthelot concluded that a water 

 column is capable, under suitable conditions, of withstanding a 

 tensile stress of 50 atmospheres. Dixon, using Berthelot's 

 method, has concluded that the cohesive force of water per unit 

 cross-section amounts to at least 150 atmospheres. The water 

 used in his experiments was saturated with air and also con- 

 tained pieces of the conducting tracts of plants. The measure- 



11 Berthelot, M. Sur quelques phenomenes de dilatation fore ee des liquides. 

 Ann. Chem. et de Phys., 30: 250. 1850. 



