ABSORPTION 153 



first by Pfeffer, who made the first actual measurements of osmotic 

 pressures, with results contained in his " Osmotische Untersuchungen," 

 Leipzig, 1877, and summarized in his "Plant Physiology," Volume 1. 

 The second paper is De Vries' "Eine Methode zur Analyse der 

 Turgorkraft," in Jahrbiicher fur wissenschaftliche Botanik, 14, 1884, 

 427, in which he gave a large number of pressures determined by 

 comparison with definite standards, which were plant cells, and he 

 showed a relation between osmotic pressure and equimolecular solu- 

 tions. The later physical studies upon osmotic pressure, which have 

 largely turned upon the correspondence between osmotic pressures 

 and gas-pressures, are all admirably summarized in a work well-nigh 

 ideal for. the use of the student in this course, Livingston's "R61e of 

 Diffusion and Osmotic Pressure in Plants," in the University of Chi- 

 cago Decennial Publications, 1903. There is also a very important 

 chapter by van't Hoff in his " Physical Chemistry in the Service • 

 of the Sciences," in the same series, 1903, which sets forth very clearly 

 his theory that osmotic pressures and gas-pressures are essentially 

 identical in kind. This is consistent with the later measurements 

 of pressures by Morse and others, who used Pfeffer's method im- 

 proved in details, with results described in the American Chemical 

 Journal, 34, 1905, 1, and later. Other students have found some 

 support for a surface-tension theory of osmotic pressure (Nature, 72, 

 1905, 541). More recently the whole subject has been restudied by 

 Kahlenberg (Journal of Physical Chemistry, 10, 1906, 141, and 

 Philosophical Magazine, 9, 1906, 214) to a conclusion strongly against 

 the gas-pressure theory and in favor of the older explanation of an 

 affinity between solute and solvent. But Kahlenberg's work has 

 been criticized and must be confirmed (Nature, 74, 1906; see Index). 



The phenomena of plasmolysis already studied imply that 

 it is the turgor of the osmotically tense cells which keeps the 

 tissues firm and stiff, a matter of such importance as to deserve 

 experimental study. 



What effect is produced upon soft tissues by neutralizing the 

 osmotic pressure inside the cells? 



This end may readily be effected by immersing the tissue in a 

 •solution stronger than that inside the cells. 



Experiment. Select a soft-textured leaf, one lacking any water- 

 proofing bloom or wax, such as Coleus, Heliotrope, or Primula, and 

 immerse it in a normal, or, for extremely rapid action, saturated, 

 solution of salt or sugar, and observe and interpret the effect soon 

 produced ; then place it for a time in pure water. 



Demonstration Methods. The above method, when a saturated 

 solution is used, yields a collapse of the leaf within 15 or 20 minutes, and 



