TRANSLOCATION OF WATER 



189 



bubbles were left anywhere (Fig. 84). The mercury in the tube 

 began to rise after a short time, owing to the evaporation of water 

 from the surface of the plaster and its 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 the water molecules and their adhesion to 

 those of mercury. 



This experiment illustrates the role played by cohesion, and the 

 influence of water evaporation from the surface of a moist porous 

 body. The force of adhesion demonstrated by 

 the concavity of the menisci in the capillaries of 

 the plaster of Paris is the active force in this 

 case. The greater the resistance to a pull, the 

 more concave will be the water menisci 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. They have microcapillaries 

 with their micromenisci of water. When the 

 resistance to water flow is increased, these menisci 

 are drawn into the tiny capillaries, they increase 



, ,. • Fig. 84. — Suction 



in concavity and as a consequence suction tension proc j ucec i by evap . 

 is augmented. Therefore, they draw water from oration of water 



., n • j ■ --j. A- x • c from plaster of 



the cell, inducing in it a suction tension ot corre- j> aT i s (after Walter). 

 sponding strength and osmotic in nature. This 

 pressure in its turn induces the suction of water by the cell from 

 the vessels of the plant. 



To approach the conditions found in a plant, Ursprung used, 

 instead of a glass tube, the stem of a liana. He obtained a rise of 

 mercury of twice the barometric height, since the walls of the ves- 

 sels in a plant stem are not only moistened but also imbibed with 

 water. 



These experiments still do not give a true conception of the real 

 magnitude of cohesion force that retains the water molecules near 

 one another and prevents the rupture of the water column in the 

 vessels of a plant. The cohesion power of water cannot be ex- 

 pressed by a few atmospheres. The simultaneous experiments of 



