494 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 8 



day before, so at 9 a. m. the pressure was increased to 3 atm., at 11 

 a. m. to 4 atm., at 2 p. m. to 5 atm., and at 4 p. m. to 6 atm. At 7 

 o'clock that evening the column was still rising, and by 9 the next 

 morning had risen an additional 110 mm, although it had been sub- 

 jected to a pressure of 90 pounds per square inch over that period. 

 As already stated, the gage read only to 100 lbs. Another atmos- 



Figure 3.— Curves showing the rates of secretion of two similar tomato roots over a period of 3^4 days, one 

 with uniform (atmospheric) pressure, the other against imposed pressures up to 90 lbs./sq. in. 



phere would have given a pressure of 105 lbs., so the attempt to reach 

 a pressure sufficient to stop secretion had to be abandoned. The pres- 

 sure was, therefore, removed and the remainder of the curve to the 

 top of the manometer plotted. A control curve, obtained without 

 imposed pressure, is plotted above the experimental one, for com- 

 parison. 



