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Arjun has a croton plant whose leaves are green in the middle and white at the edges.
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Arjun has a croton plant whose leaves are green in the middle and white at the edges. He keeps it in a dark cupboard for three days, then in bright sun for six hours. He boils one leaf in alcohol in a water bath and puts iodine on it.
- Which part of the leaf turns blue-black, and why?
- Why did he keep the plant in the dark first?
- Why was the leaf boiled in alcohol?
Need a reminder? The key ideaShow
Photosynthesis: carbon dioxide and water, with sunlight and chlorophyll, give glucose and oxygen; the glucose is stored as starch. Carbon dioxide enters through stomata, most of them on the lower surface of the leaf. Iodine turns starch blue-black.
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. The colours. Only the green middle turns blue-black. The white edges stay pale brown. Starch is made only where there is chlorophyll.
Step 2. The dark. In the dark, the plant uses up the starch that it had stored. Any starch found later was made in the six hours of sun.
Step 3. The alcohol. Alcohol removes the green chlorophyll, so the colour of the iodine test can be seen clearly.
Answer: The green part turns blue-black, because only it has chlorophyll to make starch; the dark removed old starch; the alcohol removed the chlorophyll.
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