46 SOME RECENT RESEARCHES IN PLANT PHYSIOLOGY 



ESTIMATION OF STARCH. 



For the estimation of starch they make use of powdered 

 taka-diastase (1914), for this converts it into maltose and 

 glucose, without leaving any dextrin. By a combination 

 of reduction and polarimetric methods it is possible to 

 obtain the data for equations of the type employed to 

 calculate the proportions of each present in a mixture of 

 glucose and fructose. After the substitution of the maltose 

 constants for those of fructose, the separate amounts may 

 be determined, and the starch originally present may be 

 found from these. 



The objection to 'Sullivan's method is that though 

 basic lead acetate (when carefully prepared, I venture to 

 add) does not itself precipitate dextrin, yet when the latter 

 is present in solutions in which a precipitate is produced, 

 as in the purification of those obtained from diastase con- 

 versions, a portion of it is carried down with the precipi- 

 tate. The Sachsse method, as modified, and adopted as 

 official in the United States of America, is unreliable, as 

 actual destruction of glucose occurs during the prolonged 

 treatment with dilute hydrochloric acid. Furthermore, 

 such a procedure is especially objectionable in the case of 

 plant material, for pentoses arise during it from the pen- 

 tosans of the cell walls. 



Davis and Daish also point out the necessity for special 

 care in sampling the material, and for the removal of sub- 

 stances soluble in water, such as gums, which are optically 

 active and introduce errors into the polarimetric readings. 



THE ESTIMATION OF REDUCING SUGARS BY THE METHODS 



OF BENEDICT AND KENDALL. 



For the volumetric determination of reducing sugars, 

 the citrate solution proposed by Benedict (1908) has been 



