CELL-CONTENTS AND CELL-WALLS. 61 



(g) Make a watery extract of Pea flour, or of pulverised Peas, let 

 the turbid liquid stand for an hour, and filter it. Pour some of the 

 filtrate into a watch-glass and place in it a crystal of iodine ; the 

 liquid gradually turns brown. For comparison pour into two other 

 watch-glasses, each containing an iodine crystal, (1) a little water 

 the iodine only turns the water yellow ; (2) some of the dextrin you 

 have prepared from starch note the brown colour produced in the 

 dextrin. A few drops of iodine solution may be used in each case 

 instead of iodine crystals. 



(h) Pour some of the filtered Pea extract into a test-tube, add 

 some Fehling's solution, and boil ; no reduction occurs, since no 

 reducing sugar is present in dry Peas. 



(i) To another portion of the Pea extract in a test-tube add a 

 little sulphuric acid and boil for a few minutes, then add some 

 Fehling and boil again ; the red copper precipitate appears, be- 

 cause the dextrin has been converted by the action of the acid into 

 a reducing sugar. 



73. Digestion of Starch. Starch is converted into 

 sugar by hydrolysis, which may be brought about either 

 by (a) simply boiling starch in water or in mineral acids, 

 or (6) by the action of diastase enzymes. Of the latter 

 several varieties occur in both plants and animals ; in 

 mammals, for instance, the digestion of sugar is effected 

 by the ptyalin of saliva and the amylopsin of pancreatic 

 juice. 



(a) Heat some starch in water, and put a little of the 

 cooled paste on the tongue. After a time the sweet taste 

 shows that part of the starch has been converted into 

 sugar by the diastase (ptyalin) of the saliva. 



(6) For experiments on the digestion of starch use 

 either saliva, or (better) malt extract or commercial 

 diastase. To obtain saliva for the experiments induce 

 secretion by rinsing the mouth with water and then chew- 

 ing a bit of rubber. Collect the saliva in a test-tube, and 

 dilute with about five times its volume of water; if it is 

 very turbid or frothy filter it. Make starch paste by 

 rubbing up 10 grams of starch with 30 c.c. of cold water, 

 adding 200 c.c. of boiling water, and cooling the thin 

 mucilage formed in this way. 



