Any extra food a plant makes through photosynthesis is stored in its tissue as starch. Testing a leaf for starch is one way to test whether it has been performing photosynthesis. (Adult supervision required.)
What You Need:
Test for starch in plants:
- Put one of the plants in the dark for 24 hours; leave the other one on a windowsill.
- After 24 hours, put some ethyl alcohol in a beaker and place that in a saucepan full of water. Heat the pan until the ethyl alcohol begins to boil. Remove from the heat.
- Use tweezers to dip each of the leaves in the hot water for 60 seconds. Then place them in the beaker of ethyl alcohol for two minutes or until they turn almost white. Set them each in a shallow dish.
- Cover the leaves with some iodine solution and watch.
The hot water kills the leaf and the alcohol breaks down the chlorophyll, taking the green color out of the leaf. When you put iodine on the leaves, one of them will turn blue-black and the other will be a reddish-brown. Iodine is an indicator that turns blue-black in the presence of starch. The leaf that was in the light turns blue-black, which demonstrates that the leaf has been performing photosynthesis and producing starch.
Try the test again with a variegated leaf (one with both green and white) that has been in the sunlight. A leaf needs chlorophyll to perform photosynthesis — based on that information, where on the variegated leaf do you think you would find starch?