CELL-CONTENTS AND CELL-WALLS. 73 



cells are faint pink. (3) Treat other sections with copper sulphate 

 and caustic potash these cells become violet. The contents of 

 these cells therefore give somewhat similar reactions to those of 

 proteins ; whereas cells containing only tannin do not react to 

 Millon's and the biuret test more than do ordinary parenchyma 

 cells with their protoplasm lining, which stains a pale pink. 



(g) For comparison with Cherry Laurel, make similar sections of 

 the leaf of Portugal Laurel. Note that in this species there is a 

 corresponding layer of cells around the bundles, and prove by tests 

 that in this case the leaf contains tannin but no emulsin. 



(h) Grind up leaves of Cherry Laurel and of Portugal Laurel 

 and make a watery extract, which will contain emulsin only in the 

 former case. Prove this by adding each extract to some salicin or 

 to some amygdalin containing extract of Bitter Almonds. 



(i) The glucoside phloroglucin (which is used with acid as a test 

 for lignin, see 79) is obtained from the wood of various plants. 

 Make a phloroglucin solution, and note that it gives (1) a violet 

 colour with ferric chloride, (2) a violet colour to a freshly-cut piece 

 of Pine wood dipped into the solution after adding hydrochloric 

 acid. 



(j) Pound up some Horse Chestnut bark with glacial acetic acid 

 to extract the glucoside aesculin, and note that a fine blue fluores- 

 cent colour is given on making the solution alkaline with potash. 



84. Tannins. Under this name are included various 

 substances found chiefly in bark and in pathological gall 

 formations. The best known is tannic acid, which occurs 

 along with the allied gallic acid in " gall nuts " (oak galls). 

 The tannins are probably related to the glucosides, and in 

 some cases are of similar importance in metabolism, yield- 

 ing glucose on being hydrolysed. Like most glucosides, 

 they have an astringent taste, and their most characteristic 

 reaction is the dark blue or green colour which they give 

 with salts of iron. 



(a) For the general reactions of tannin use commercial tannin 

 (tannic acid) dissolved in water. (1) Add a few drops of ferric 

 chloride a deep blue or blue-black colour. (2) Add a few drops 

 of potassium ferricyanide and some ammonia a red or brown 

 colour. (3) Add potassium dichromate a reddish-brown colour. 

 (4) Add some ammonium chloride solution, followed by some am- 

 monium molybdate solution a yellow precipitate. 



(b) The tannins are widely distributed in plants, and their 

 presence is easily recognised on testing cut surfaces with ferric 

 chloride, ferrous sulphate, or potassium dichromate. As material 



