Hi dimple
I've found number 30 via onelook.com. Just in case you haven't used it before: try typing the relevant number of question marks and ':tropical tree' into their reverse dictionary section. It gives you a number of alternative answers and it doesn't take long to find which one of them is the tree being asked for.
I'm not ignoring your suggestion, sleuth. Thanks for the hint about this question but I'd tried typing variations on your suggestion into Google but hadn't found anything. Yes, the twin brothers, both playwrights, do have the same last name and dimple's on the right lines.