CH. V] ISOTONIC eOEFFICIENT. 117 



obtained (combined with experiment 148) it is possible to 

 calculate the isotonieT35efflcienr(I. C.) of cane sugar, i.e. the 

 attraction for water of a molecule of cane sugar expressed 

 in terms of the attraction of a molecule of KNO3 for 

 water. For the sake of convenience the value of this last 

 quantity is taken as 3 instead of 1. We have then the 

 following calculation. Assuming that we have found that 

 the cell sap = 0"13 KNO3 and also = 0'20 cane sugar, 



I. C. of sugar 1-3 „ ,._ 

 3-^ = 2^ = ^'^^' 



. • . I. C. of sugar = 1'95 



or in round numbers = 2. 



In this way, using the plant as an index, it is possible 

 to ascertain the osmotic intensity of solutions of a number 

 of substances in relation to a living protoplasmic mem- 

 brane. 



(150) Microscopic method. 



The principle of de Vries' second method is simple : 

 small portions of tissue are put in a graduated series of 

 salt solutions and the equivalence between one of them 

 and the cell sap is estimated by the degree of plasmolysis 

 observed microscopically. The tissue must contain 

 coloured cell sap so that plasmolysis may be readily 

 observed. De Vries recommends as material the epi- 

 dermis of parts of the leaf of Tradescantia discolor, 

 Begonia manicata, or Curcuma rubricaulis. Of these 

 Tradescantia discolor is the most universally available 

 and is the only one of which we have any experience. 

 In 'Trffdescantia the part of the leaf used is the 



