Conditions and Principles of Absorption 81 



the progressive plasmolysis and wilting of the cells of the plant 

 through the root-system, but the osmotic strength of the cell- 

 sap may be more accurately studied through the next two 

 experiments. 



Osmotic pressure of cell-sap; observation upon tissues. From 

 the stock solutions used in the preceding experiment prepare in 

 stender dishes or Syracuse watch glasses dilutions which shall 

 contain the following strengths, .10, .12, .15,. 18, and .20 molec- 

 ular (M.). Split the apical portions of several flower stalks 

 of the dandelion (or other scape which has been found suitable) 

 each into four approximately equal parts. These strips will 

 curve outward, the epidermis being within or on the concave 

 side. Dip strips momentarily into water in which spirals will 

 be formed, then cut into distinct rings. Place one or two of 

 the rings in each of the above solutions and also in distilled 

 water. Follow and note the changes which occur. Further 

 curling of the strips indicates absorption of water, that is, the 

 solution is too weak; no change in the curvature indicates a 

 solution equal in osmotic strength to the cell-sap (isosmotic 

 with the cell-sap) ; and elongation or reverse curvature indicates 

 loss of water and plasmolysis. Intermediate dilutions may also 

 be made, and the threshold of plasmolysis more accurately 

 determined. 



Osmotic pressure of cell-sap; direct observation upon plas- 

 molysis. The osmotic pressure of the cell-sap may be deter- 

 mined fairly accurately by direct observation upon the 

 plasmolysis of the cell, employing as the plasmolytic agents 

 substances which penetrate the cell only very slowly. The 

 substances employed above, also other neutral salts, cane-sugar, 

 etc., may be used. Cells with protoplasts the limits of which 

 may be easily seen are best for preliminary study, especially 

 algae, such as Spirogyra, Pithyophora, etc. All precautions as 

 to the cleanliness of vessels, also purity of the reagents and 

 distilled water, should be observed. 



From the stock solutions of the monovalent salts previously 

 used prepare for a preliminary test a small quantity of a .2 M. 

 solution. Mount in a drop of this solution one or two filaments 

 o 



