" ROOT-PRESSURE " — WHITE 



491 



mercury column corresponding to the 13.5:1 differential in weight 

 between mercury and water. Figure 1 shows the result of such an 

 experiment using manometers 500 mm high. Contrary to expec- 

 tation, no such observable decrease occurred. Both columns rose at 

 the same rate. The pressure differential apparently had no effect on 

 the secretion rates. The only conclusion to be drawn is that the 

 curves do not represent pressures at all but only volumes. 



Closed manometers in which the pressure would build up rapidly 

 with very little volume change were tried, but such manometers proved 

 hard to clean and did not permit the detection of leaks at the root- 

 manometer juncture. Leveling bulbs were then resorted to. Results 



50 100 



150 200 250 300 

 Hours 



Figure 1.— Curves showing rise of liquid in two similar manometers, one filled with water, the other with 

 mercury. The roots failed to secrete during the first 48 hours after the experiment was set up, presumably 

 until they had recovered from the shock of manipulation. This lag does not always occur. The curves 

 have almost identical slopes in spite of the 13.5:1 differential in weight between mercury and water. 



of an experiment with such a device are represented in figure 2. 

 During the course of the experiment it was thrice necessary to replace 

 or extend the manometer tube, and since the tubes were not all of the 

 same diameter, a different scale had to be used for each. Scales were 

 chosen, such that the slope of the curve for an hour or so before and 

 after each change should be approximately the same. In this experi- 

 ment the water column rose 1,790 mm in 6 days. A leveling bulb was 

 then attached, and an atmosphere of pressure — 760 mm Hg — was 

 applied. Since the column continued to rise at an undiminished 

 rate, the pressure was increased after 20 hours to 2 atm. (1,520 mm 

 Hg). The water still continued to rise. Sufficient mercury to give 

 another atmosphere of pressure would have carried the bulb through 



