I’m sure you’ve noticed that some leaves still have green areas after they fall. It doesn’t seem to make sense since the tree is done photosynthesizing, so why are sections of the leaves still doing that job? It’s almost always because of an insect. It’s still alive and has hijacked the photosynthesizing process, sometimes with the help of bacteria. The phenomenon is called “green islands.” Use a good microscope or loupe and you might be able to see the critter. More about this curiosity at the In Defense of Plants blog.