THE FOOD OF THE PLANT 47 



Inference. — Starch is turned blue by iodine solution. 



This is the method that is always employed to detect 

 the presence of starch. This knowledge can now be 

 applied to the case of a green leaf. 



Experiment 28 



Aim. — To test for starch in a green leaf that has 

 been picked in the afternoon of a sunny day. 



Method. — The working out of this experiment is 

 complicated by the fact that the leaf is green. This 

 green colouring matter, or chlorophyll, must therefore 

 first be got rid of, otherwise it will mask the blue 

 iodine reaction. A hquid must be used which will 

 dissolve out the chlorophyll from the tissues of the 

 leaf, but which will leave the starch unaltered. 



It is evident from common observation that chloro- 

 phyll is not soluble in water; vegetables that have 

 actually been boiled in water still retain their green 

 colour. Chlorophyll is, however, soluble in methylated 

 spirit. 



Chlorophyll dissolves out slowly in cold methylated 

 spirit, but it comes out much more quickly on heating. 

 This must be done very carefully, as methylated spirit 

 ignites so readily. 



It is well to boil the leaf in water first. As it boils, 

 bubbles of air are seen to escape from the leaf, especi- 

 ally from the under sm'face. By this preliminary boil- 

 ing the air in the leaf is expelled and its place taken 

 up by water. When the escape of bubbles has almost 

 ceased the leaf should be taken from the water and 

 boiled in methylated spirit. The green colouring 

 matter is then readily dissolved, as the passage of the 

 spirit into the leaf is rendered easy. 



When the leaf is quite colourless it should be washed 

 in water and a little dilute solution of iodine then 

 poured over it. After a few minutes it should be again 

 washed in water. 



Observations. — The leaf becomes a dark blue colour. 



