208 PART II. VEGETABLE HISTOLOGY. 



Solutions containing iodine should be excluded from the light to prevent 

 the formation of iodic acid. 



Chloriodide of Zinc Solution. This is prepared as follows : Dissolve 

 zinc in pure hydrochloric acid until saturated, then evaporate the solution in 

 contact with zinc until it has a syrupy consistence, then saturate it with potas- 

 sium iodide, and, lastly, with metallic iodine. Keep the solution in an amber- 

 colored glass-stoppered bottle. Used in the undiluted form, it constitutes one of 

 the best tests for cellulose and lignified tissues, staining the former a purple 

 and the latter a brown color. Suberized or cutinized tissues are also stained 

 by it a yellowish brown. Starch grains are first stained blue and then rapidly 

 swell and disappear. The peculiar form of cellulose found in fungi remains 

 uncolored, both by this reagent and by iodine and sulphuric acid. 



A very dilute solution of chloriodide of zinc is used as a test for tannin, 

 cells containing this substance being colored violet or red by it. 



Potassium Hydrate. This is one of the most useful of reagents. Its 

 value depends, in the main, on its solvent effects upon proteid matters and 

 starch, and on its power to cause cell-wall tissues to swell, but it is also service- 

 able as a test for tannin, for cutinized tissues and for some other purposes. It 

 is best obtained in sticks and kept in tightly stopped bottles until required for 

 use, as upon exposure it takes up carbon dioxide and water from the air, and 

 becomes converted into the carbonate. When a solution is required for use, 

 remove a small piece from the bottle, dip it for a moment in water to remove 

 the film of carbonate and dissolve the remainder in a fresh portion of distilled 

 water. For some purposes a dilute solution is required, for others it must be 

 used strong. As a clearing agent for dissolving cell contents a five per cent, 

 solution is best. A section which has soaked for some time in the reagent is 

 placed on a slide, the potash neutralized with a drop of acetic acid, when it 

 will be found ready for study. For the study of the markings of starch grains 

 a one or two per cent, solution is applied to the edge of the cover glass, and 

 allowed to run under by capillary attraction ; in the meantime the changes in 

 the starch grains should be carefully watched. They first swell slightly and the 

 markings are revealed with great distinctness, but as further swelling takes place 

 they disappear an J the grains themselves soon become disorganized. When a 

 five or six per cent, solution is used the lamination of thick-walled cells is often 

 brought out with much greater distinctness. As a reagent for suberized or 

 cutinized tissues a strong solution should be used. It communicates a yellow 

 color to such tissues, which deepens if the slide is warmed, and if heated so 

 that the solution boils the suberin exudes in the form of yellow drops. A 

 strong solution is also best as a test for tannin. The tannin is recognized by 

 the yellow or yellowish brown color which is produced by this reagent. The 

 ferric chloride test for tannin is, however, to be preferred. Caustic potash 

 solution in dilute form is also useful as a means of distinguishing between 

 protein crystals or crystalloids and crystals of inorganic matters. The former 

 immediately swell and lose their angles, while the latter are unaffected. 



A very good form of this reagent is Russow's Potassium Hydrate. A small 

 quantity of a concentrated solution of potassium hydrate in distilled water is 

 prepared. This is added to ordinary alcohol until a precipitate begins to form. 



