110 THE WORK OF PLANTS 
warm the water so that the starch will be well wetted. 
Now coolit by moving the end of the tube in cold water, 
or by holding it in running cold water from a tap. 
Add a few drops of the tincture of iodine. The liquid 
immediately appears blue 
because the numerous 
starch grains are coloured 
blue by it. Now hold the 
end of the tube over the 
flame again for a few 
minutes, but do not let it 
get hot. The blue colour 
disappears, because the 
warm water extracts the 
lodine from the starch. 
Cool the tube again and 
the blue colour reappears. 
5 To test the starch in a 
Fig. 145. Corn flour dissolved in water, potatotuber. Cuta potato 
and tincture of iodine added. At left the 
solution is cold, middle one is heated in two, and on the cut 
Slightly, at right it is cooled. 

surface scrape some ot 
the potato into a pulp with a knife. Apply some of 
the tincture of iodine to the potato pulp. It becomes 
blue. The potato, then, is largely made up of starch. 
Place some of the pulp in water in a test tube, and add 
a few drops of the tincture of iodine. Then heat it 
gently to see if it behaves like the corn flour. 
