COLOR REACTIONS. 171 



and also to different grains of the same starch, and even of parts of the same grain, have 

 long been known. Boiled starch and soluble starch with ^'ery rare exceptions (p. 53) alwaj's 

 j^elds a blue reaction, but starch-grains, and the grain-sacs after boiling starch, may give a 

 blue, violet, or reddish reaction, depending upon the source, or upon the amount of iodine 

 used and other conditions. It is quite common, after boiling, for the capsular part of the 

 gelatinized grain to give a violet reaction, while the inner part becomes an indigo blue. A 

 weak iodine-iodide of potassium solution is usuallj^ employed, preferably a 1 to 2 per cent 

 Lugol's solution. The differences in the behavior of starch, erythrodextrin, and aclu'oodex- 

 trin with iodine has rendered tliis agent of great value in the recognition and differentia- 

 lion of these suljstances. The relatively greater intensity of the blue reaction over the red 

 with a mixture of starch and erythrodextrin may cause the color reactions of the latter to 

 be masked, unless the iodine be used cautiously. 



IODINE VAPOR. 



The use of iodine vapor to distinguish different kinds of starch was adopted by Dubosc 

 (Chem. Zeit., 1904, xxviii, 1149). Iodine crystals are placed on a watch-glass which is 

 put on a glass plate, on which also is the starch, and the whole is covered with a bell-jar 

 and set aside for 24 hoiu's. Corn starch was colored a blackish-violet, wheat starch a 

 bluish-gray, sago starch a brownish-gray, and potato starch a yellowish-gray, the intensity 

 of the yellow being proportional to the amount of foreign matter. 



lODlNE-tHLORAL HVDRATK. 



A modification of the iodine reagent as ordinarily employed is prepared by saturating 

 a saturated solution of cMoral hydrate \vith iodine. The chloral hydrate usually causes 

 swelling and bursting of the starch-grains, which is coupled with the iodine reaction. Green 

 (The Soluble Ferments and Fermentation, Cambridge, 1901, 59) made use of this reagent to 

 trace certain changes in the reserve starch in germinating pollen-grains during the develop- 

 ment of the pollen-tube. Green wTites that the plant whose pollen gave the most satisfac- 

 tory results was Lilium pardalimmi. The ripe pollen-grains when treated with a solution 

 of iodine-chloral hydrate, are rendered transparent and the starch-grains are stained blue. 

 Mixed with them here and there were a few grains staining Uke erythrodextrin. As the 

 tube was put out from the grain these granules were gradually carried over into the pro- 

 truding portion, and they flowed slowly down the tube as it extended. When the tube 

 was as long as twice the diameter of the grain, if the iodine-chloral hydrate solution was 

 added, the grains were found to be somewhat cUfferent in color, becoming shghtly purple 

 with iodine. With longer tubes, the grains, still traveUng forward, showed this change 

 more and more markedly, particularly near the tip of the tube. When a tube which had 

 attained a length of 20 to 30 times the diameter of the grain was treated in the same way, 

 the general effect of the iodine was very different. There were but few blue granules, wliich 

 were in the part nearest the pollen-grain. The greater part of the length of the tube was 

 studded with purple grains, and towards the tip they became nearly red. The starch was 

 evidently in the process of digestion under the action of diastase, which other experiments 

 had shown the same pollen to contain. The starch-grains did not change their shape nor 

 show signs of corrosion even when seen under high magnification. The amount of chloral 

 hydrate was not sufficient to cause the swelling that is observed with strong solutions. 



IODINE-LACTIC ACID. 



By the addition of iodine to hot sjTupy lactic acid, Lagerheim (Svensk. Farm. Tidskr., 

 1901; Jour. Soc. Chem. Ind., 1901, xx, 1245) prepared a reagent wliich, like the iodine- 

 chloral hydrate combination, renders the plant tissues transparent and at the same time 

 colors the starch, by which means he proposes not only to determine the presence of starch 

 in situ, but also to differentiate between exhausted and non-exhausted tea-leaves. 



