WATER 127 



liquid water held in the pores of the soil, in 

 the form of surface films representing the 

 curved surface seen in capillary tubes, and 

 therefore tending to cause the water to move 

 upwards, as well as in all other directions, 

 until uniformity of tension is established, is 

 of vastly higher importance to plant growth 

 than hygroscopic moisture. It not only serves 

 normally as the vehicle of all plant food ab- 

 sorbed during the growth of the usual crops, 

 but also, as a rule, to sustain the enormous 

 evaporation by which the plant maintains, 

 during the heat of the day, a temperature 

 sufficiently low to permit of the proper op- 

 eration of the processes of assimilation and 

 building of cell tissue." * 



The rise of water in capillary systems re- 

 sembling soil, under the action of surface 

 tension, may be as much as ten feet. In soil 

 itself the highest rise under the usual circum- 

 stances is unquestionably as much as four 

 or five feet; but if the surface tension of 

 water were like that of most liquids it could 

 be, under similar conditions, but two or three 

 feet. There seems to be little doubt that the 

 rise of fluids in tall plants is also in large 

 part due to the action of surface tension, and 

 accordingly it must be much favored by the 



1 Hilgard, "Soils." New York, 1907, p. 201. 



