l82 



The Living Plant 



Fig. 62.— Pfeffcr's cell, as 

 pictured by himself in his 

 own book (but reduced 

 to ?^4 his size). 



A semipermeable mem- 

 brane is formed all over 

 the inner face of the 

 porcelain cup, which is 

 shown, in half, at the 

 lower right of the figure. 

 The cup, and all the re- 

 mainder of the appara- 

 tus, is then filled with 

 the sugar solution, 

 which, absorbing water 

 when the cup is im- 

 mersed, presses the mer- 

 cury up against the air 

 in the gauge to a height 

 which balances, and 

 measures, the pressure. 

 The remaining mechani- 

 cal features are con- 

 nected with filling and 

 sealing the cup. 



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- 

 pheres, or fifty to sixty pounds to the square 

 inch. Nor is this all, for when very strong 

 solutions are used, which require, of course, 

 an instrument of enormously greater 

 strength, pressures of surprisingly high 

 magnitude have been registered, even up to 

 twenty-four atmospheres, or 360 pounds to 

 the square inch, — a much higher pressure 

 indeed than ever is used in the steam boilers 

 of even the swiftest express locomotives; 

 while recentl}^ even higher ones have been 

 measured. Nor are such pressures of merely 

 academic interest to the botanist, because 

 others higher yet, above one hundred at- 

 mospheres, have been found to exist under 

 special conditions in plant cells. 



Here follows another paragraph which the 

 reader may skip if such be his inclination. 



