MICROCHEMICAL TESTS 671 



with iodine. In the presence of KI or of hydriodic acid the 

 colour is more violet-brown. The use of dilute solutions is always 

 best, as the grains become almost black in concentrated solutions ; 

 further, the lamellte show in dilute solution. Prepare an aqueous 

 solution immediately before using by adding a few drops of an 

 alcoholic solution to a few cubic centimetres of distilled water. 

 Stain starch, in tissues, with 2 per cent, aqueous cotton red and 

 counterstain the plastid with methylen blue. 



Free starch grains are best stained with crystal (or gentian) 

 violet (Nem£c, Ber. Deut. Bot. Ges., xxiv, 1906, p. 528) ; after 

 staining pour off the dye and wash with saturated aqueous picric 

 acid. Dry and mount in balsam. Sections can be dehydrated 

 and cleared in xylol. Safranin and thionin are also good stains. 

 See also Kraemer, Science, Ivii, 1923, p. 175 ; Dodge, J. Appl. 

 Micros., i, 1898, p. 99. 



In polarised light, starch grains which are doubly refractive 

 give a characteristic black cross, the centre of which corresponds 

 with the hilum. See Sponsler {Amer. J. Bot., ix, 1922, p. 471) 

 for method of X-ray study of starch grains. 



Glycogen (see also § 667) occurs in solution in fungal cells. It 

 gives with iodine solution (0-1 grm. iodine, 0-3 grm. KI, 45 c.c. 

 water) a red-brown colour which disappears temporarily on 

 warming. It can be precipitated in the cell and, in fact, may be 

 troublesome in fungi. Treat the material {e.g. fungal hyphas) with 

 absolute alcohol or Carnoy for twelve hours, and then with 10 per 

 cent, tannic acid for twelve hours. Wash quickly and stain in 

 ferric chloride solution ; glycogen stains black. 



For Best's carmine stain, see § 670. See also Mayer, Zeits. Wiss. 

 Mikr. mikr. Tech., xxvi, 1910, p. 513. 



1350. Tannins occur in solution in vacuoles and give a blue- 

 black or green colour with neutral solutions of iron salts. Use 

 an ether solution of anhydrous ferric chloride (Moeller, Ber. 

 Deut. Bot. Ges., 1888, p. 66) or concentrated aqueous solution of 

 ferrous sulphate (LoEW and Bokorny, Bot. Centrbl., xxxix, 

 1889, p. 369). 



They can be precipitated and stained in the living state with 

 aqueous methylen blue (1 : 500,000) (Pfeffer). Fix the stain with 

 saturated aqueous picric acid for a few hours, rinse, dehydrate, 

 clear in xylol and mount. 



They reduce Fehling's solution, are precipitated by basic lead 

 acetate and other metallic salts, and give a brownish precipitate 

 with strong aqueous potassium bichromate or 1 per cent, chromic 

 acid. A red-brown to brown colour is given by a sparing amount 

 of a dilute ammoniacal solution of potassium ferricyanidc ; excess 

 of the reagent destroys the colour. Aqueous iodine-potassium 

 iodide solution, mixed with a little 10 per cent, ammonia produces 

 a brilliant red colour. 



