152 PHYSIOLOGY OF NUTRITION. 



aqueous solution of the ash, by adding to it Silver nitrate solution. 

 The presence of glucose is to be determined by means of Fehling's 

 solution. Cane-sugar may be detected by the method given in 

 the Third Section. As a test for Oxalic acid we employ Calcium 

 chloride, and if (first filtering if a precipitate has been produced) 

 we add to the Calcium chloride-containing fluid excess of alcohol, 

 Malic salts separate out if present. 



The acid reaction of most plant juices indicates that the bases 

 present are not sufficient to neutralise the whole of the organic 

 acids. For quantitative researches on the composition of plant 

 juices, the student is referred to the directions on p. 570 of the 

 valuable treatise of H. de Vries, cited below. 



Often plant juices (e.g. that from the leaf-stalk of Heracleum 

 Spondylium) are very rich in glucose, and in such cases this sub- 

 stance is of the utmost importance in determining the osmotic 

 energy of the cell-sap, and therefore also in determining the 

 osmotic pressure of the cells. In other cases, e.g. leaves of Sola- 

 num tuberosum, the quantity of glucose is far less than that of 

 other bodies. 



It is now very important to observe that the osmotic energy of 

 equal quantities of the different substances present in the cell-sap 

 of plants is by no means the same. On the contrary, one substance 

 is able to attract water with great intensity, while another can do 

 so only in a more limited degree. H. de Vries has determined 

 numbers which express the relative attraction for water of one 

 molecule of a body in dilute solution. These numbers he terms 

 the isotonic coefficients of the different substances. As a starting- 

 point for the whole of his researches, H. de Vries selected the force 

 of attraction for water of Potassium nitrate, The isotonic coeffi- 

 cient of one molecule of this compound has been taken as three, 

 so as to render it possible to work with whole numbers. 



Putting aside theoretical considerations, we proceed at once to ex- 

 periments, which will serve to indicate the nature of the reasoning 

 and the method of H. de Vries. We prepare four solutions of Potas- 

 sium nitrate in water. The first has O'l, the second 0'12, the third 

 O13, the fourth 0*15 molecules of the salt, expressed in grams, per 

 litre (molecular weight of Potassium nitrate, KN0 3 , = 101). We 

 further prepare four aqueous solutions of cane-sugar, of which 

 the first contains 0*15, the second 0'2, the third 0'22, and the 

 fourth 0'25 gram molecules of cane-sugar per litre (molecular 

 weight of cane-sugar, C^H^On^ 342). We pour 15 c.c. of 



