I got this question from Susan Sims:
What is that gummy, sticky film between the paper and the fruit on a tomatillo?
...and why is it there?
Eagerly awaiting a reply,
My Sciency Answer:
It is all part of a scheme to make sure bugs don't eat the fruit, but birds do. Birds eating fruit is a good thing because then they fly around and poop the seeds out everywhere. But the birds can't eat it if the bugs get to it first. Tomatillos keep the insects away with a two-part defense system: Bad tasting chemicals, backed up by the sticky stuff Susan asked about.
The bad-tasting chemicals are called withanolides. The mature tomatillo makes lots of these in the little paper jacket (technically, the calyx) covering the fruit, but little of it in the fruit itself. Birds can pull off the gross husk and eat the yummy fruit, but insects just start eating, go "ick!" and fly away.
Which seems like a great system. So why does the tomatillo also make a layer of sticky? Because very small insects can slip in under the papery husk through an opening at the end. Very clever, these tiny little evil insects think themselves to be, but, before they can start to eat, they find themselves caught in the sticky layer and die. Bwahahaha!
So, to recap: Tomatillo protect their fruit from insects first with a foul-tasting papery covering, and then, just in case that defense is breached, they also make themselves into flypaper with sticky stuff, to keep their fruits pristine and perfect for hungry, seed pooping birds.
My explanation is primarily based on Chemical defence by withanolides during fruit development in Physalis peruviana by Baumann and Meier, so you can check out all the gory details there if you are so inclined.
Have a question? Send me and e-mail (engeizuki at gmail dot com) and I'll provide a sciency answer!