I've read the script and still doesn't get the joke about bsp trees. After all, they DO grow underground, and that's what makes them special. I know what I'm talking about, because a friend of mine, who happens to be a cave explorer, showed me a photo of a beautiful piece of underground vegetation, and when I asked him what it was, he told me it was a bsp tree. Well, sorry Mnemonic, but this joke of yours really misfired.
EDIT: NOW I get it. It's Helga's ignorance of the fact that bsp trees grow under the ground that is meant to be funny. How foolish of me...