CALCIUM PHOSPHATE TANNIN. 71 



starch grains, turning blue with iodine. The tissue succeeding to 

 this inner portion is formed of radially elongated, polygonal cells, 

 which diminish in length at the periphery of the gall-apple, and 

 iinally end under the small-celled outermost layer, the epidermis, 

 the cells of which are strongly thickened externally. This entire 

 tissue, surrounding the inner sheath, shows no enclosures of 

 definite form ; it is traversed by vascular bundles which are 

 visible here and there. If, however, we lay a freshly prepared 

 section in a drop of watery solution of chloride or sulphate of 

 iron, \ve see that it colours dark-blue throughout. This colora- 

 tion is, moreover, communicated to the surrounding fluid, and 

 gives us, therefore, the iron-blue reaction for tannin. If the 

 action is observed under the microscope, by allowing iron solution 

 to run into a dried section laid under a cover-glass, we see that 

 first a fine dark-blue precipitate is formed, which, however, is 

 soon again dissolved in the reagent, so that now a blue fluid fills 

 the cells. The weakest tannin reaction is given by the starch- 

 containing cells of the inner sheath. For comparison, let us now 

 lay a second section in a watery solution, about 10 per cent., of 

 bichromate of potash, and we see a dense, flocculent, red-brown 

 precipitate, which is permanent, formed in the tannin-containing 

 cells. Or large pieces of tissue can be laid in potassium bi- 

 chromate for at least twenty-four hours, and after washing 

 them, sections prepared. The precipitate can then be accurately 

 localised. Boiling potassium bichromate hastens the process. 

 Lastly, let us place a section in a concentrated solution of molyb- 

 date of ammonia in concentrated ammonium chloride, and an 

 abundant reddish-brown precipitate appears in the cells. This 

 reaction will decide in doubtful cases, because those preceding 

 can also proceed from other reducing bodies. Sections of 

 dried gall-apples also give the above reactions, though less 

 beautifully. 



More roundabout, but specially certain, is the reaction with 

 acetate of copper. The material to be investigated is, while 

 living, cut up into small pieces, and placed in a saturated solution 

 (7 per cent.) of acetate of copper, and allowed to lie therein for 

 eight or ten days, or longer. Sections then taken are laid upon a 

 slide in a drop of 0*5 per cent, solution of sulphate of iron. In 

 this they remain for only a few minutes, as, after longer action, 

 the walls commence to go brown. After the sections have been 

 washed in water, and placed in a watch-glass of alcohol, in 



