146 



PLANT PHYSIOLOGY 



Demonstration Methods. The form F of figure 38 is very effective, 

 as the rise due to osmosis, molasses being used, progresses fast enough to yield 

 a visible result within an hour. And the cup, when cleaned, may be dried 

 and preserved attached to its ring, ready for future 

 use. Far more effective, however, especially for 

 lecture-table demonstration, is the arrangement 

 originally described by MacDougal (Journal of 

 Applied Microscopy, 1, 1898, 56) and shown modi- 

 fied in figure 39. A piece of parchment-paper 

 tubing (see section following), some 15 cm. I r ng, 

 is soaked a few minutes in water, then pleated and 

 tied tightly together with waxed thread at one end, 

 while the other end is tied tightly over a good 

 stopper (preferably rubber) pierced by two holes. 

 k] Molasses is poured through one of the holes until 

 the cup is filled; then into one hole is inserted a 

 ' stout tube of about 50 cm. length and 1 mm. bore 

 (a smaller bore works badly because of the viscosity 

 of the molasses), which is all the better if white- 

 backed and of the magnifying form (pear-shaped 

 in section). Into the other hole is inserted a 

 thistle-tube or funnel with an interposed stop- 

 cock. The cup is then supported upright in a 

 vessel of water, which, for quickest results, should 

 be lukewarm. Enough water is now poured into 

 the funnel to raise the molasses into the tube, and 

 the stop-cock is closed, when within a few minutes 

 the liquid will begin to rise in the tube at a rate of 

 several millimeters per minute, so that its progress 

 can be seen by the class from a distance. As it 

 nears the top the stop-cock is to be opened, when 

 the liquid will drop back to the starting-point. 

 Moreover, by removing the tube and funnel, the 

 filled cup with stopper may be kept stored in a 

 slender bottle of molasses, and is always ready 

 for immediate use upon simply rinsing it with 

 water. This simple arrangement really leaves little 

 to be desired as an effective demonstration of 

 osmosis. It could also be used to demonstrate 

 pressure by sealing the tube at the top with seal- 

 FiG.39.— Demonstration ing-wax, and calculating the pressure by Boyle's 

 osmoscope; Xi- Law > though, owing to escape of the solution, 



Explanation in text. onl y tne existence of pressure and no approach to 

 its true amount can thus be shown. A method of 

 demonstrating pressure by use of a thistle-tube is described by E. A. Bogue 

 in Science, 13, 1901, 791. 



Membranes. Of these a number are available from both animal and 



