182 The Living Plant 



easy of experiment and the answer plain; 

 the cup becomes stretched or even pushed 

 from the tube, or sometimes (and always if 

 provided with a semipermeable membrane) 

 it bursts. This shows that osmotic ab- 

 sorption, if confined, develops osmotic 

 pressure. Of course the pressures have 

 been measured exactly, chiefly by aid of an 

 instrument invented by the great botanist 

 Pfeffer, and shown by the accompanying 

 picture (figure 62). When its porous cup, 

 lined with a semipermeable membrane, is 

 filled with a solution of sugar like that in 

 root hairs, and then is immersed in pure 

 water, the gauge actually exhibits a pres- 

 sure equal to that of three or four atmos- 



CQ> i I! pheres, or fifty to sixty pounds to the square 



I J) PJI] inch. Nor is this all, for when very strong 



solutions are used, which require, of course, 



FIG 62 Pfcffcr's ceil as an instrument of enormously greater 

 strength, pressures of surprisingly high 



to M his size). magnitude have been registered, even up to 



A semipermeable mem- 



brane is formed ail over twenty-four atmospheres, or 360 pounds to 



the inner face of the , , , i i_ T. 



porcelain cup, which is the square inch, a much higher pressure 



Bright ollhe^ur"! lndeed than 6Ver 1S US6d ln the Steam b ilerS 



The cup, and ail the re- o f even the swiftest express locomotives; 



mainder of the appara- 



tus, is then filled with while recently even higher ones have been 

 which 8 ," Absorbing water measured. Nor are such pressures of merely 

 when the cup is im- aca d e mic interest to the botanist, because 



mersed, presses the mer- 



cury up against the air others higher yet, above one hundred at- 



in the gauge to a height j . x j 



which balances, and mospheres, have been found to exist under 



The^maining meThant Special Conditions in plant Cells. 



cai features are con- jj ere f o n ows another paragraph which the 



nected with filling and F J 



sealing the cup. reader may skip if such be his inclination, 



