Via Wilfried Houjebek's delicious, an intriguing project he calls "geranium as camera"

many natural things are sensitive to light. Long ago people noticed the effect of light on green plants, or how it made coloured fabrics fade. It is the effect of light on plants that makes Roman Photography possible. (...) if you drop a small amount of a solution of iodine on it [chlorophyll], the starch turns black. So that's it really. All you have to do is get a plant to produce lots of starch in the right place, then stain the starch with iodine. Don't let a few details prevent you starting straight away.

The webpage proposes a curious way to make a picture with plants, try to follow it.