STAECH. 5 



starch cannot pass through an animal membrane, it must 

 be changed to a diffusible form before it can be absorbed. 

 This is effected by ferments in the saliva and pancreatic 

 fluid. 



By heating to 160 to 200 starch is converted into 

 dextrin. 1 By boiling a solution with a dilute acid it is 

 changed first into dextrin, then into glucose. Ptyalin 

 changes it first into dextrin, finally to maltose. The dias- 

 tase of malt gives the same products. 



Starch gives an intense-blue color with a solution of 

 iodin. This color disappears on heating the liquid; but 

 if it is not heated too long it becomes blue or purple again 

 when it cools. The color will also be destroyed by the ad- 

 dition of anything which will form a compound with the 

 iodin, such as sodium thiosulphate, silver salts, or the alka- 

 line hydrates. 



1. Starch may be prepared from a potato by grating it upon 

 a tin grater, stirring it up with a little water, and squeezing the 

 water, which contains a large part of the starch, through a piece 

 of unbleached muslin. After repeating this with several portions 

 collect the water in one vessel and allow the starch to settle to the 

 bottom. Pour off the water, add more, and allow to settle again, 

 repeating till the starch appears clean and white. Take what is 

 needed for the experiments and" let the rest dry. Enough starch 

 for the microscope examination can be obtained from the scrap- 

 ings from a potato without washing. The cellulose- fibers will then 

 be seen also. 



2. Examine the starch under the microscope. No- -Y 

 tice the shape of the granules. 



3. Place a drop of very dilute iodin solution upon 

 the slide so that it runs under the cover-glass and notice 



1 Unless otherwise stated, all degrees of temperature will be 

 understood as referring to the centigrade scale. 



