CHAPTER XIII. 



HOW PLANTS GET THEIR CARBON FOOD. 

 CONCLUDED. 



II. STARCH FORMED BY GREEN PLANTS. 



Exercise 28. 



136. To test for the presence of starch in green^ leaves. Take green 

 leaves which have been for several hours in the bright sunlight. Boil them 

 in alcohol, using great care not to set the alcohol on fire. This removes the 

 chlorophyll. If it is desired not to use the alcohol, boil the leaves in water 

 for a short time. Then place them in alcohol, changing the alcohol occa- 

 sionally. The green color is extracted slowly by this process, It may be 

 extracted more rapidly if the preparation is placed in the sunlight. When 

 the leaves are decolorized, place them in a solution of iodine in potassium 

 iodide. In place of this solution, a tincture of iodine purchased at drug- 

 stores answers fairly well. Observe the color of the leaves. This color is 

 due to the presence of starch, the starch becoming dark blue or nearly 

 black when treated with iodine. 



137. Starch is formed only in the green parts of variegated 

 leaves. If we test for starch in variegated leaves like the leaf 

 of a coleus plant, we shall have an interesting demonstration of 

 the fact that the green parts of plants only form starch. We 

 may take a leaf which is partly green and partly white, from a 

 plant which has been standing for some time in bright light. 

 Fig. 58 is from a photograph of such a leaf. We should first 

 boil it in alcohol to remove the green color. Now immerse it 

 in the potassium iodide of iodine solution for a short time. 

 The parts which were formerly green are now dark blue or 

 nearly black, showing the presence of starch in those portions 



74 



