ABSORPTION OF MATERIALS IN GENERAL 107 



just strong enough to cause incipient plasmolysis) was determined for each. 

 In this way concentrations of various substances were found that produced the 

 same osmotic pressure with the same membrane. Such solutions are termed 

 isostnoHc or isotonic. 



The colored epidermal cells of the leaf sheath of Curcuma rubricaulis, of 

 the leaves of Tradescantia discolor, and of the petiolar scales of Begonia manicata, 

 are all very well suited to such experiments as that just described. Twelve 

 preparations may be made for each experiment, six being placed in various 

 concentrations of the substance to be studied, and the other six in corresponding 

 concentrations of potassium nitrate. All preparations must be taken from the 

 same region of the leaf or other plant organ. To accomplish this, a narrow 

 rectangle is marked on the leaf, and divided longitudinally into halves and 

 transversely into six divisions, the area of each of the resulting sections being 

 about I sq. mm. Each piece of epidermis is removed with a razor and placed 

 in a glass cylinder (about 10 cm. tall and 2 cm. in diameter*) containing the 

 solution to be tested. The cylinders are loosely stoppered to prevent evapora- 

 tion, and the preparations are left in the solutions about two hours. 



Volume-molecular solutions were employed, containing the molecular weight 

 of the solute in grams (called a gram-molecule or a mol.) ', per liter of solution. 

 [See note j, p. 105. 1 A volume- molecular solution (w) of potassium nitrate 

 contains, for example, i g.-mol. (loi.i g.) of the salt in a liter of solution, and 

 a tenth-molecular solution (o.i m.) contains 10. 11 g. of the salt per liter. In 

 physiological studies it is generally more convenient to calculate solution con- 

 centrations as gram-molecules per liter than to consider them in terms of 

 percentage. 



DeVries compared the osmotic pressures developed by equimolecular solu- 

 tions of various substances, and found that the substances tested fell into four 

 groups according to the amount of pressure developed, the four difierent pres- 

 sures obtained being, relatively, 0.066, o.ioo, 0.13^ and 0.166. The second 

 group represents the pressure caused by potassium nitrate. These numbers 

 are approximately in the proportion of 2 : 3 : 4 : 5 , so that if the pressure produced 

 by a volume-molecular solution of potassium nitrate be considered as 3, then 

 the pressure developed by a volume-molecular solution of any other substance 

 not in the same group is 2, 4, or 5, according to the group in which the given 

 substance belongs. On this account deVries adopted as his unit of osmotic 

 pressure one-third of the pressure produced by a volume-molecular solution of 

 potassium nitrate, so that a volume-molecular solution of this salt, or of any 

 other salt belonging to the same group, always produced a pressure of 3, and the 

 three other groups of substances gave pressures of 2, 4 and 5, respectively. The 

 numbers 2, 3, 4 and 5 were termed isosmotic coefficients; they represent the 

 relative osmotic pressures developed by equimolecular solutions of the various 

 substances. 



> Ostwald, WUhelm, Lehrbuch der allgemeinen Chemie. 2te Aufl. 2: 212. Leipzig, 1906. [Idem, 

 Outlines of general chemistry. Translated by James Walker. London. iSgS-l 



* Much shorter vials are more convenient, about i cm. in diameter and 2 cm. high.^ — Ed. 



