CH. V] ISOTONIC COEFFICIENT. 129 



found that the cell sap = 0-1 3 KNOg and also=0-20 cane- 



sugar, 



I a of sugar ^r^^^ 



3 20 



.-. I. C. of sugar=l-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 epidermis of parts 

 of the leaf of Tradescantia discolor, Begonia manicata, or 

 Curcuma ruhricaulis. Of these Tradescantia discolor is 

 the most universally available and is the only one of which 

 we have any experience. In Tradescantia the part of the 

 leaf used is the epidermis of the under-surface : to get 

 good results it is necessary to use closely adjacent parts of 

 the epidermis taken from the midrib. De Vries makes 

 parallel incisions 1^ or 2 mm. apart in the epidermis of 

 the midrib : the areas so marked out can then be freed 

 by a surface-cut with a razor. The fragments of the 

 epidermis must remain in the solutions for at least an 



D. A. 9 



