330 



PLANT PHYSIOLOGY 



thin adsorbed layer of air, which prevents the perfect adhesion, 

 of water and probably induces premature rupture of the water 

 column. 



That the cohesive force of a liquid allows it to rise to a height 

 exceeding that permitted by barometric pressure was proved by 

 Askenasy's well-known experiment (1895). 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 mth it to exclude gases. The lower end of 

 the tube was dipped into mercury. Care was taken 

 that no air bubbles were left anywhere (Fig. 106). 

 The mercury in the tube began to rise after a short 

 time, owing to the evaporation of water from the 

 surface of the plaster-of-Paris block drawing upon 

 the supply in the glass tube. If all the air was 

 removed from the apparatus and the pores of the 

 plaster block were sufficiently small to prevent the 

 entry of air from outside, then the mercury rose 

 finally to a height somewhat exceeding that caused 

 by barometric pressure, as a result of the cohesion of 

 SuItSn ^pr^ the water molecules and their adhesion to those of 

 duced by mercury. This experiment clearly demonstrates the 

 IJVLZvivom I'ole played by cohesion and the effect of water 

 plaster of evaporation from the surface of a moist porous body. 



^^ter) ^^^^^^ '^^^ ^^^^^ ^^ adhesion demonstrated by the concavity 

 of the meniscuses in the capillaries of the plaster- 

 of-Paris block is the active force in this case, the energy 

 for hfting the water and mercury coming from heat energy 

 of the environment. The greater the resistance to a pull, the 

 more concave will be the water meniscuses and the stronger the 

 tension on them. Transpiration pull evidently is based on this 

 mechanism of capillary tension. The cell walls, from the surface 

 of which evaporation takes place, are made up of cellulose in a 

 colloidal state. The wall pores may be regarded as microcapil- 

 laries with their micromeniscuses of water. When the resistance 

 to water flow is increased, these meniscuses are drawn into the 

 tiny capillaries; they increase in concavity; and as a consequence, 

 suction tension is augmented. Therefore, they draw water from 

 the cell, inducing in it a suction tension of corresponding magni- 



