66 



Plant Physiology 



but is impermeable, or very slowly permeable, to the sub- 

 stance in solution. Semipermeable membranes are of 



the most diverse sorts. A piece of 

 pig's bladder, a commercial article 

 readily obtainable, is a very satis- 

 factory membrane. This, after be- 

 ing soaked in water, is tied tightly 

 over the bell end of a thistle-tube 

 (Fig. 18) with waxed thread. The 

 solution to be tested, preferably a 

 known strength of some solute, say 

 20 per cent sugar, is carefully 

 poured into the stem of the thistle- 

 tube (easily accomplished with a 

 guide-wire, or without when the 

 tube is clean and wet) until the 

 bell is filled with the sirup. The 

 tube is then lowered into a vessel 

 FIG. 18. Diffusion shell o f wa t e r until both liquids, which 



and thistle-tube for . 



demonstration osmo- should have come to room tern- 

 scopes, perature, are at the same level, when 

 the tube is clamped to a support. It is well to add about 

 1 per cent of formalin to both liquids as a preservative. 

 If the experimental conditions are properly carried out, 

 the liquid in the thistle-tube will rise perceptibly in a few 

 hours, and an experiment to be continued a day or more 

 will require at the outset an extension of tubing. There 

 is, therefore, a major flow of water through the membrane 

 to the strong solution ; thus there is manifest a pressure 

 which, if it could be completely measured by this column 

 of water in the apparatus, would be the osmotic pressure 



