LABORATORY WORK 171 



such as can be obtained from the finger. It is more expeditious, 

 but rather more difficult The point at which no further change 

 in volume occurs by centrifuging longer is recognised by the fact 

 that the corpuscles are so closely packed together that the narrow 

 column appears transparent red, instead of being opaque. 



Plasmolysis. Make a preparation of the hairs of Tradescantia 

 virginica, the epidermis of the leaf of T. discolor, or a thin section, 

 made with a razor, of the root of the red beet. The protoplasm 

 of the cells in each case is a bag containing coloured cell sap. If 

 exposed to a solution which has a higher osmotic pressure than the 

 cell contents, provided that the cell membrane is impermeable to 

 the solute, water passes out and the protoplasmic sac shrinks, 

 leaving gaps in places between itself and the cellulose envelope. 

 Solutions of potassium nitrate may be used. As a rule, 0.15 molar 

 (=1.5 per cent.) gives no effect, 0.25 molar (2.5 per cent.) has an 

 obvious effect, 0.2 molar is about isotonic. This is equivalent to 

 an osmotic pressure of five atmospheres 



Tnrgor. Take the stalk of a flower, such as the daffodil or 

 dandelion. It is stiff owing to the tension of the cell walls pro- 

 duced by the difference of osmotic pressure between the cell 

 contents and the very dilute watery solution outside. As the 

 tissue dies it becomes flaccid. The effect can be produced quickly 

 by exposure to ether vapour in a test-tube, and, if the action has 

 not been too great, recovery may be brought about by soaking in 

 tap water. 



The cells may also be killed by putting a little water in the 

 bottom of a test-tube, placing the stalk in the tube and then 

 boiling the water. 



Contractile Vacuole. To see the discharge of this vacuole in 

 the amoeba or other protozoon, add suspension of Indian ink to 

 the liquid in which the organism is. Do not use too much. When 

 the vacuole contracts, the particles are driven away by the current 

 from it. 



Direct Measurements of Osmotic Pressure, Such measurements 

 are not easy, on account of the difficulty of preparing suitable 

 membranes. To see the fact of the production of pressure, parch- 

 ment paper may be used with a solution (5 to 10 per cent.) of an 

 electrolytically dissociated colloid, such as caseinogen or congo-red. 

 The paper may be clamped in an osmometer of the pattern described 

 by Moore and Roaf (Biochem. Journ., vol. 2, p. 34), or the simpler 

 form of Roaf (Quart. Journ. Exper. PhysioL, vol. 3, p. 79). It is 

 possible also to fix the membrane on the edge of a small bell glass 

 open at the top and provided with a flange at the lower edge. 

 The membrane, in this case, must be glued on with gelatin, and 

 parchment paper is not readily wetted with the solution. It should 



