18 



THE RELATION OF DESERT PLANTS TO 



and the height of the moist column of water was noted from time to 

 time. At 4 h 20 m p.m. the height of this column had attained to that of 

 the water outside the tube and from this time on the rise of the liquid 

 through the soil was due entirely to the capillary power of the latter. 

 The level of the external water was kept approximately at the same 

 height by addition of water as needed, evaporation from the free water 

 surface being avoided by covering this with oil. After September 3 

 several readings on this apparatus were very kindly made by Dr. W. A. 

 Cannon. 



The results of these determinations are given in Table I. The first 

 column gives the times of observation, the second the observed height 

 of the column of moist soil above the water level outside the tube, and 

 the third gives the average rate per hour of the rise of the liquid 

 during the period just ending. 



TABLE I. Rise of Water in Air-dry Soil. 



A curve of these results is given at A, figure 2 (p. 15) . Time incre- 

 ments are plotted on the horizontal axis in days and rates per day in 

 centimeters on the vertical axis. The curve shows graphically the 

 decrease in rate of upward advance of the moist soil column as it rises 

 above the water level. 



From these data it is to be observed that during the first three days 

 the water has risen in this soil a distance of 50 cm., and that it had 

 risen a meter in 81 days. At the end of the last-named period its rate 

 of advance was about one-tenth millimeter per hour. 



