W. A. Davis and G. C. Sawier 361 



Estimation of Starch and "Soluble Starch." 



The dried potato leaf obtained after completely extracting the 

 sugars and other substances soluble in 80 per cent, alcohol was found 

 to contain large quantities of a substance readily soluble in water and 

 possessing a high positive rotation. This made it necessary to modify 

 the method of estimating starch which we have employed (Davis and 

 Daish [1914]) by first completely extracting this substance with water 

 from the portion of material used in the analysis. At certain times of 

 the day (4 p.m. to 8 p.m.) the aqueous extract so obtained contained 

 a substance which resembled soluble starch or dextrin in yielding a 

 mixture of maltose and dextrose on treatment with taka-diastase. 

 In all cases the reducing power (if any) and rotatory power of the 

 aqueous extract were determined after diluting to a known volume 

 ("250 cc); an ahcjuot portion (1-50 cc.) was then treated with taka- 

 diastase, and, after the conversion, with basic lead acetate (which 

 generally produced a copious precipitate owing to the presence of tannins, 

 etc.), being then diluted to a known volume (200 cc). The reducing 

 and rotatory powers of the solution were determined and from the change 

 in these brought about by the taka-diastase the "soluble starch'' (or 

 dextrin) was calculated. In most cases the "soluble starch" was nil, but 

 between 4 p.m. and 8 p.m. considerable quantities could be detected. 

 Even in these cases, however, the amount of soluble starch found in 

 this way did not account for more than 2-5 to 50 per cent, of the rotation 

 observed in the aqueous extract ; in all cases, too, the basic lead acetate 

 added after the conversion produced a heavy, gelatinous precipitate, 

 pointing to the presence of tannins, gums, etc. The aqvieous extract 

 before conversion invariably had a slight cupric reducing power (50 cc. 

 of the 250 cc. gave 0-01 to 0-02 grm. CuO) which may perhaps have been 

 due to unextracted sugars ; but as in the experiments with mangold 

 leaves, the extraction of sugars was always complete, it is probable 

 that the reduction was due to a substance of the tannin class. For 

 purposes of comparison we give in Table I the actual values calculated 

 for the rotation (a)„ in a 200 mm. tube of the aqueous extract of the 

 leaf material corresponding with 100 grms. of the total vacuum-dried 

 matter of the leaf (including the sugars and alcohol soluble substances). 

 The value is also given for the "soluble starch"' ([a]^= 202^) that this 

 would correspond with, calculated as a percentage on the total vacuum- 

 dried matter. Thus a comparison can be made of the true soluble 



