126 ISOTONIC COEFFICIENT. [CH. V 



The observations here suggested are meant as illustra- 

 tions of the very simplest aspect of turgor, chiefly to 

 show that turgor is an osmotic phenomenon, since the 

 condition of the cell is clearly regulated by the relation 

 between the cell sap and the environing fluid. 



(148) Osmotic strength of cell sap in terms of KNO3. 



The method of de Vries^ depends on the fact explained 

 in experiment 163 (Section C) that when a turgescent 

 shoot is bisected longitudinally each half curves outwards, 

 i.e. with the epidermis on the concave side. If the 

 curved portions are put in water the curvature increases 

 greatly : if they are placed in strong NaCl solution (S^o) 

 they uncurl, i.e. become straight again, or they may even 

 become convex on the epidermic side. Therefore an 

 intermediate strength of salt solution must be discoverable 

 which equals the cell sap in osmotic force, and which 

 neither produces increase nor decrease in curvature. 



In summer we use the scape of the dandelion. 

 Taraxacum ; in winter the hypocotyl of Ricinus seedlings. 



The dandelion is split longitudinally into four strips which, 

 on being dipped for a moment into water, curl up into 



^ Pringsheini's Jahrbiicher, xiv. 



