I'm not quite getting how potatoes, onions and garlic are seeds? They're roots aren't they?

I have been reading a book about the science of food and how plants fight back against being eaten. This is much of the reason we have to cook certain foods. Plants have evolved some pretty elaborate defense systems to keep from being eaten. Then again, other plants seem to count on being eaten to spread (well... their fruit, like berries).

There's a warning in there about mushrooms. Some component of them that seems to be a problem and one should limit their intake of mushrooms. I'll have to reread that part of the book.