60 CELL-CONTENTS AND CELL-WALLS. 



cells just within the cork-layer) and mount them in a drop of the 

 acid ; but it is perhaps better to soak the pieces of tuber in the acid 

 for an hour, then wash them with weak alcohol, and keep them in 

 strong alcohol for a few days. Then cut sections near the surface, 

 treat with iodine, and mount in glycerine. Note the small rounded 

 starch-grains (blue), each with a small leucoplast (yellow) attached ; 

 the leucoplasts are usually found near the nucleus of the cell, and 

 in the outer cells of the tuber there may be seen leucoplasts which 

 have not yet formed a starch-grain. 



72. Dextrin. This name is given to a series of soluble 

 carbohydrates, formed in the processes by which starch is 

 converted into reducing sugar, and therefore found in 

 plant tissues where starch has been stored. In the hydro- 

 lysis of starch the intermediate products formed differ 

 considerably, some giving various colours with iodine, 

 while others are not coloured by iodine. 



(a) Examine some commercial dextrin. It is a yellow-brown 

 powder, soluble in either cold or hot water ; the solution is clear. 

 Pour into one test-tube some of the dextrin solution, and into 

 another tube an equal volume of water, then add to each an equal 

 volume of iodine solution. The water is coloured yellow only, 

 but the dextrin solution becomes reddish-brown the colour dis- 

 appears on heating and reappears on cooling. 



(6) To some dextrin solution in a test-tube add alcohol ; the dex- 

 trin is precipitated. 



(c) To some dextrin solution add basic lead acetate solution ; 

 dextrin is not precipitated (cf. starch). 



(d) Dextrin can be obtained, as a sticky mass, by moistening a 

 little starch with hydrochloric acid and heating gently in a dish. 



(e) Make 10 grams of starch into paste with 20 c.c. of water, add 

 30 c.c. of 20 per cent, sulphuric acid, and boil for several minutes. 

 Cool, add alcohol, collect the white precipitate of dextrin, wash it 

 with alcohol, dry it in a watch-glass, and test with iodine. 



(/) Boil some starch in water (about 1 gram starch to 100 c.c. 

 water) ; cool, add a few drops of 20 per cent, sulphuric acid, and 

 again heat the fluid becomes clear, and on adding iodine to a 

 cooled sample of it the blue colour is still given. Continue to boil 

 the solution, and remove from it every five minutes a small sample 

 to which when cool iodine is added. The first samples, containing 

 dextrin, turn violet with the iodine, the later ones reddish-brown, 

 then yellowish, as the conversion of the successive dextrins into 

 sugar proceeds. 



