ABSORPTION OF MATERIALS IN GENERAL 115 



replaced by water. Cells with colored sap are very good for plasmolytic ex- 

 periments, since the coloring matter is retained within the shrinking vacuole, 

 leaving the space between the protoplasm and the cell wall filled with colorless 

 solution. By the use of such cells plasmolysis may be readily detected, 

 even in its incipient stages. DeVries used mature cells with colored sap and 

 determined the concentration of the plasmolyzing solution when the latter was 

 just strong enough to cause separation of the protoplasm from the wall at the 

 corners of the cell (Fig. 68, 3). If no further contraction of the protoplasm 

 occurs it follows that the osmotic pressure within the vacuole just equals that 

 of the external solution. The same experiment was repeated with various 

 substances, and the limiting concentration (i.e., that concentration which is 

 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 

 isosmotic or isotonic. 



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

 the leaves of Tradescantia discolor, and of the petiolaf 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 1 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) 1 per liter of solution. 

 [See note j. p. 113.] A volume-molecular solution (m) of potassium nitrate 

 contains, for example, 1 g.-mol. (101.1 g.) of the salt in a liter of solution, and 

 a tenth-molecular solution (0.1 ' m.) contains 10. n 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 different pres- 

 sures obtained being, relatively, 0.066, o. 100, 0.133, 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 



1 Ostwald, Wilhelm, Lehrhuch der allgemeinen Chemie. 2te Aufl. 2: 212. Leipzig, 1906. [Idem, 

 Outlines of general chemistry. Translated by James Walker. London, 1895.] 



k Much shorter vials are more convenient, about 1 cm. in diameter and 2 cm. high. — Ed. 



