20 



PHYSIOLOGY. 



investigated, have the power of taking up water under press- 

 ure. 



40. Cell-sap a solution of certain substances. From these experiments we 

 are led to believe that certain substances reside in the cell-sap of plants, which 

 behave very much like the salt solution when separated from water by the 

 protoplasmic membrane. Let us attempt to interpret these phenomena by 

 recourse to diffusion experiments, where an animal membrane separates two 

 liquids of different concentration. 



41. An artificial cell to illustrate turgor. Fill a small wide-mouthed 

 vial with a very strong sugar solution. Over the mouth tie firmly a piece 

 of bladder membrane. Be certain that as the membrane is tied over the 

 open end of the vial, the sugar solution fills it in order to keep out air- 



FIG. 28. Puncturing 

 a make-believe cell 

 after it has been 

 lying in water. 



FIG. 29. Same as Fig. 28 

 after needle is removed. 



bubbles. Sink the vial in a vessel of fresh water and leave it there for twenty- 

 four hours. Remove the vial and note that the membrane is arched out- 

 ward. Thrust a sharp needle through the membrane when it is arched 

 outward, and quickly pull it out. The liquid spurts out because of the 

 inside pressure. 



42. Diffusion through an animal membrane. For this experiment we 

 may use a thistle tube, across the larger end of which should be stretched and 

 tied tightly a piece of a bladder membrane. A strong sugar solution (three 

 parts sugar to one part water) is now placed in the tube so that the bulb is 



