188 TEXTBOOK OF PLANT PHYSIOLOGY 



Water suction by the cells, on the other hand, proceeds quite 

 differently. Here the motive power is the difference between the 

 osmotic suction of the cell sap and the counterpressure of the 

 elastically distended cell walls, which may attain a magnitude of 

 several atmospheres (Art. 42). This force permits the cells to suck 

 water from the adjoining vessel. The pull is then transmitted to 

 the whole water column in the stem, since the water absorbed by 

 the cell immediately draws the rest adjoining it. The air or atmos- 

 pheric pressure plays no part whatever in the whole process. 



The force that makes the water molecules follow each other is 

 called cohesion. It is the same force that holds an iron bar to- 

 gether, permitting it to be lifted by one end. The force of cohesion 

 in solid bodies may be determined in a comparatively simple 

 manner owing to the low mobility of their molecules and their 

 constant external form. When the tensile strength of a bar of 

 metal is tested, the strain at which it breaks is considered as being 

 equal to the force of cohesion. Thus a steel bar, for instance, 

 breaks at a strain of about 80 kg. per square millimeter, which 

 approximately corresponds to the tension of 8,000 atmospheres. 



Since liquids have molecules in a more mobile state, tests of 

 the strength of their cohesion are subject to greater difficulties 

 and errors. The determination has to be conducted in a tube with 

 two very closely fitting pistons. It has been proven by means of 

 proper equipment that the cohesion force of water molecules and 

 their adhesion to the surface of the pistons is very high indeed. 

 Only by applying to the pistons a pulling force of many atmos- 

 pheres can the water layer between the pistons be ruptured. Still 

 it is difficult to establish whether the water column itself has been 

 ruptured or whether it has been only torn away from the surface of 

 the piston or the tube. The latter seems more probable, since on 

 the surface of glass or metal there is always a thin adsorbed air 

 layer, which prevents the perfect adhesion of water and probably 

 induces premature tearing. 



That the cohesion power of a liquid permits it to rise to a height 

 exceeding that induced by barometric pressure was proven by 

 Askenasy's (1895) well-known experiment. He took a long glass 

 tube to whose upper end a funnel containing hardened plaster of 

 Paris was sealed. The tube was filled with boiled water and the 

 plaster block filling the funnel was soaked with it. The lower end 

 of the tube was dipped in mercury. Care was taken that no air 



