iS2 PLANT PHYSIOLOGY 



to lower strengths are designated as decinormal, etc., and written, N .1, etc. 

 Others write them in fractional form thus: «/8, «/i6, etc. 



(Normal solutions are especially well discussed in Livingston's " R61e of 

 Diffusion and Osmotic Pressure," cited in the Literature, and in Arrhenius' 

 " Text-book of Electrochemistry," translated by J. McCrea, 1902, 9, 10. 

 The article by Dandeno in Botanical Gazette, 32, 1901, 229, may be noted, 

 and Kahlenberc's reply in the same volume, 437.) 



The student is now in contact with facts and phenomena 

 not only of great interest in themselves, but important for the 

 part they have played in the development of the physical chemis- 

 try of osmosis, and he should make himself acquainted with 

 the classical work of De Vries thereon. He should make sure 

 of an understanding of, and should clearly describe, (a) the use 

 of plasmolysis as a method of determining the comparative 

 osmotic power of different substances; (b) the meaning of De 

 Vries' isotonic (or isosmotic) coefficients, in their relations to the 

 degrees of ionization of the respective substances; (c) the relation 

 of these coefficients to the theoretical pressures of equimolecular 

 solutions. He should also take note, and may profitably repeat, 

 De Vries' other method of determining these coefficients, viz., 

 by the elastic bending of strips of soft tissues. 



Osmotic Quantities. These are given with the most satisfactory 

 fullness and clearness in the important De Vries-Pfeffer table in Pfeffer's 

 "Physiology," 1, 146 (reprinted by MacDougal, 338). Osmotic pressures 

 in the cells of the higher plants range generally from 5 to 11 atmospheres, 

 rising to 21 in some special structures, but in the lower plants the pressures 

 range very much higher, even to 160 atmospheres in some Fungi. We can 

 therefore hardly express any conventional constant for osmotic pressure 

 in plant cells. Cane-sugar gives a pressure of 22.7 atmospheres for a normal 

 solution (viz., a 34.2 solution), and the same pressure is yielded by normal 

 solutions of all substances which are not ionized, with higher pressures for 

 those which are. Morse has actually measured over 34 atmospheres for 

 cane-sugar. But much weaker strengths prevail in plants. The pressures 

 given by the three substances most concerned in osmotic pressures in the 

 plant, viz., cane-sugar, grape-sugar, and potassium nitrate, are, respectively, 

 .69, 1.25, and 3.50 atmospheres for a 1% solution, in the proportion to one 

 another therefore of 1, 2, 5. 



Literature of Osmosis. "Osmosis, especially with respect to 

 osmotic pressure, has attracted great attention in recent years from 

 students of physical chemistry, with a correspondingly copious litera- 

 ture. The foundation works are two botanical investigations, the 



