In other schools geraniums are kept when testing for starch. As we didn't have any geraniums, today we did a straight test using bind weed leaves and results were as expected when iodine was added. Leaves break down easily. But if you want to do a comparison between a plant that's been left in the dark for a couple of days then the geranium is the way to go. Bind weed (convovulous) is easier to water during the holidays though.