The Food-making of Plants 6/ 



A tincture of iodine may be obtained or the crystals dissolved 

 in water. Scrape a small portion of potato and place it in a 

 tube of water. Add a few drops of iodine. The liquid at 

 once turns blue. Place some maizena or laundry starch in 

 slightly warm w^ater. Allow it to cool and add iodine. The 

 same blue colour appears. The particles of starch are coloured 

 blue by iodine. 



Treat a castor-oil bean or a piece of onion in the same 

 manner. No blue appears, because the food in these plants is 

 not stored as starch. 



Starch formed in Green Leaves.— In the afternoon of 

 a bright day, place a few green leaves in a strong solution of 

 chloral hydrate, which will dissolve out the green colour. 

 Leave them overnight or until the green has dissolved. Boil- 

 ing hastens the process. Place them in a porcelain or other 

 dish with a white bottom, and pour over them a solution of 

 iodine. The starch in the leaf will become a dark blue. 



Perform the same experiment with variegated leaves of 

 Coleus. Thin leaves should be used, as the colour is dissolved 

 more readily. 



In the afternoon cover a leaf with tinfoil. Leave it all 

 next day. The following morning boil and blanch as before ; 

 stain with iodine; no blue colour appears. The starch has 

 been used, and no more formed in darkness. 



Corks may be fastened over a portion of a leaf; two pieces 

 of cork on opposite sides matching above and below. When 

 stained the part covered will remain white. 



Much of the food stored in seeds is starch. Remove a 

 mealie seedling from the soil. Cut the seed in two. Cut off 

 a portion of the stem about half an inch long just above the 

 seed. Cut this piece in two lengthwise. Place in a test-tube 

 containing an inch or two of water. Gradually add iodine. 

 Portions of the seed will show blue where some starch is still 

 left. The stem does not stain blue. The food cannot pass 

 to the growing parts as starch ; it has been changed to sugar. 

 The bundles through which the sap is passing up into the 

 leaves are stained a yellowish brown. In the North American 

 maple, the sap is so filled with sugar when it is passing up into 



