128 ISOTONIC COEFFICIENT. [CH. V 



to equivalents, and in the case of KNO 3 (which forms the 

 standard) may contain 0'05, 010, Oil, 012, 013, 014 

 gram-molecules per liter ; stronger solutions may however 

 be needed. After a quarter of an hour the result may 

 be noted : if the material consist of dandelion rings the 

 result is obvious on inspection ; with Ricinus the seg- 

 ments must be compared with the sketches. 



Fig. 24 gives tracings of pieces of split Ricinus 

 hypocotyl before and after immersion. 



The upper row of tracings gives the form of the pieces 

 before being placed in the solutions, the lower row shows 

 the change of form produced by the immersion. The 

 numbers 010, 012, etc. give the strength of the KN0 3 

 solution in which each was placed. 



It will be seen that the first two have increased in 

 curvature, while the last two have uncurled and the middle 

 piece (i.e. that in the 013 solution) remains unchanged. 

 Therefore 0'13 expresses the osmotic pressure of the cell 

 sap in terms of KN0 3 . 



(149) Isotonic coefficient. 



The same experiment must be made with cane-sugar, 

 using solutions 016, 018, 0'20, 0'22, 0'24. From the results 

 obtained (combined with those of experiment 148) it is 

 possible to calculate the isotonic coefficient (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 

 KNO 3 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 



