42 FUNDAMENTALS OF SUBMIC ROSCOPIC MORPHOLOGY I 



to rend a plane, the cohesive forces have to be overcome along a line 

 only, whereas in the case of a rod the force has to be applied to a plane. 

 Hence Figs. 40a and b are graphic representations of the definition 

 of surface tension (force/cm) and cohesive tension, or pressure (force/ 

 cm^) respectively. 



To understand this better, let us compare the surface tension and 

 the cohesive tension of water. For water at 15° C, a amounts to 

 7.30 mg/mm, which in absolute units is 71.6 dynes/cm. In order to 



Fig- 39 



Fig. 39. Measurement of the surface tension of a lamella (from Lecher, 

 1919) - Fig. 40. a) Dimension of surface tension (force/cm); b) di- 

 mension of cohesive tension (force/cm-). 



measure the cohesive pressure or inner pressure (Freundlich and 

 LiNDAu, 1932), one must tear apart planes of water in which the mole- 

 cules cannot change position with respect to each other, for example 

 a film of water between two hydrophilic pistons. Such experiments, 

 however, do not produce reliable evidence. The cohesive tension must 

 also be overcome when water is torn from the cell wall in a desiccating 

 cell. According to the osmotic measurements of Renner (191 5) and 

 Ursprung (191 5) with fern annulus cells, thiscohesive tension amounts 

 to 300 to 350 atm. From the heat of vaporization of water, however, 

 the much larger value of about 10^ atm. is derived (Lecher, p. 60). In 

 absolute units this corresponds to an order of magnitude of 10^" 

 dynes/cm^. Since the surface layer of water has a thickness of at least 

 3 A (the diameterof the water molecule is 2.78 A), about 1/3-10^ of such 

 layers is needed to account for the cohesive tension. Multiplying the 

 surface tension of 71.6 dynes/cm of a monolayer by 1/3 • 10^, we obtain 



