The object tree is a Java applet, and not everyone can see Java. Older browsers (such as Mosaic and Lynx), people who are blind and search engines are three important groups that cannot use Java applets. Therefore it is a good idea to provide alternative content for them.
Then simply paste the resulting html code anywhere between your
<APPLET CODE="tree.class" ...>
opening tag and your
</APPLET>
closing tag. People who can see Java will see the tree, those who can't will see the html list instead.
If you really want to get fancy, you could use a server-side include to call a script that generates the alternate content on the fly based on the current contents of your data file. This would be ideal for situations where your data file changes frequently (or is itself dynamically generated). To do this, grab the code for the above form, and modify to taste.