366 



ANISTJAL REPORT SMITHSONIAN INSTITUTION, 1940 



the rate and amount of excretion of liquid from the base of the 

 root can be measured. It was found in the first experiment that 

 the rate of secretion into the manometer was rather large and, sur- 

 prisingly, that it was the same whether water or mercury was used 

 in the manometer. Figure 4 shows the results of an experiment 

 which was continued for a period of 12 days. It will be seen that 

 the secretion continued undiminished even against a pressure of at 

 least two atmospheres. It was next attempted to determine how 

 great a pi-essure could be developed by the roots. For this purpose 

 compressed air was applied to the manometer. (See fig. 5.) Since 



.V;parjtu} rmvei 

 jjim 



o500 



FiQDRB 4. — Curve showing the effect of Imposing pressures up to 3 atmospheres against 

 the secretion pressure developed by a single excised tomato root. Courtesy of Dr. P. R. 

 White (Amer. Journ. Bot., March 1938). 



the pressure gage employed could not be used at pressures above 

 100 pounds per square inch, the experiment had to be terminated 

 after six atmospheres had been applied; even this pressure did not 

 bring about any observable retardation of the liquid excretion. It 

 must be concluded from this finding that even six atmospheres is 

 so small in comparison with the secretion pressure actually devel- 

 oped by the roots as to be quite insignificant. But even six atmos- 

 pheres is sufficient to sustain a column of water 200 feet high, far 

 in excess of the requirement of any ordinary tomato plant. From 



