Question

Take a potted plant with variegated leaves, for example, money plants or crotons. Keep the plant in a dark room for three days so that all the starch gets used up. Now keep the plant in sunlight for about six hours. Pluck a leaf from the plant. Mark the green areas in it and trace them on a sheet of paper. Dip the leaf in boiling water for a few minutes. After this, immerse it in a beaker containing alcohol. Carefully place the above beaker in a water bath and heat till the alcohol begins to boil. Question 1, What happens to the color of the leaf? What is the color of the solution? 
Now dip the leaf in a dilute solution of iodine for a few minutes. Take out the leaf and rinse off the iodine solution. Observe the color of the leaf and compare this with the tracing of the leaf done in the beginning. 
Question 2, What can you conclude about the presence of starch in various areas of the leaf? 

Answer 1

Hint: The above steps represent an iodine test. This activity shows chlorophyll is essential for the photosynthesis process.
-The green part of the leaf turns blue-black and the iodine solution will be orange-brown in color.
The presence of starch indicates that photosynthesis occurs only in the green part of the plant and chlorophyll plays an important role in photosynthesis.

Complete answer:
Iodine test steps and the role of each step is given below:
-Step 1: Take a potted plant with variegated leaves – like a money plant or croton. In the experiment, a variegated leaf is used because some part of the leaf is green in color, while some part is non-green. Owing to the presence of chlorophyll, the green part of the leaf is green and can perform photosynthesis. The non-green component has no chlorophyll and thus does not conduct photosynthesis.
-Step 2: Hold the plant for three days in a dark space, so that all the starch is used up.
The plant is then kept in the dark for 2-3 days so that all the food processed is used up in the form of starch.
In the absence of light, there is no photosynthesis and, thus, no more food production takes place.
-Step 3: Now keep the plant for about six hours in sunlight.
The plant is then placed for 6 hours in sunlight to allow for photosynthesis. Carbohydrate formation occurs only in the green parts of the plants, due to photosynthesis.
-Step 4: Pluck out a plant leaf. Label and trace the green areas within it on a sheet of paper.
The starch-containing leaf is plucked and labeled for green areas which will help us compare the results of the iodine test further.
-Step 5: Dip the leaf a few minutes in boiling water. Then immerse it into an alcohol-containing beaker. Place the above beaker carefully in a water-bath and heat until the alcohol starts boiling.
The leaf is treated with alcohol so that after treatment with iodine the leaf loses its green color due to the chlorophyll pigment and takes up the blue-black color.
-Step 6: Now dip the leaf for a few minutes in a dilute iodine solution. Take the leaf out and rinse off the iodine.
-The green parts of the variegated leaf have chlorophyll in it. Only those sections may thus carry out photosynthesis and generate food (starch). Starch reacts with iodine in the presence of iodine and gives it a blue-black color. This development of color indicates the presence of starch in the leaf and thus shows that chlorophyll is essential to the photosynthesis process.

Note: Sucrose is the end product of photosynthesis and the primary sugar transported in the phloem of most plants. While sugar has been suggested to encourage floral transition in many plant species, growth in high concentrations (5 %[weight/volume]) of sucrose has significantly delayed flowering time, leading to an increase in the number of leaves in Arabidopsis.