Re: What does belladonna do in the plant that makes it?

In your question you state that belladonna comes from a plant, in fact, belladonna is the plant. Its real name is Atropa belladonna but it is commonly called Deadly Nightshade.

The chemical in the plant that causes the dilation of the pupil of the eye is called atropine, and as you said, plants can't use it to dilate their eyes. In fact, they don't use it for anything. Atropine is found in the plant as a defence mechanism. If you were to eat the berries of the deadly nightshade, the pupils of your eyes would dilate, but that would be the least of your troubles. As described in the Stedman's Shorter Medical Dictionary, the symptoms of atropine poisoning as as follows:

Atropine---
Symptoms: Strange indescribable feelings with giddiness, yawning, staggering or falling on attempting to walk; dryness of mouth and throat, sense as of suffocation, swallowing difficult, voice husky; face at first pale later suffused with a scarlatiniform rash which extends to the body; pupils widely dilated; pulse, at first bounding and rapid, later becomes irregular and faint.

So what all this does if you eat the plant, if it doesn't kill you, is make you never want to eat the plant again. The plant uses atropine to warn off predators that might want to eat the plant or its seeds.

Many plants produce chemicals that serve no other function but to discourage predators from eating them. The chemicals range in function from tasting disgusting to being downright poisonous. Thats why its usually not a good idea to eat unknown plants or berries.

Well, I hope that answered your question.

David Barker