Not show [+] [-] Icon when load Tree with ajax

[dungla2011]

I have simple Tree content:

<li id="x-3">
    <div>Item 3</div>
    <ul>
        <li id="x-30"> <div>Item 30</div> </li>
        <li id="x-31"> <div>Item 31</div></li>
    </ul>
</li>
<li id="x-4"><div>Item 4</div></li>

It work normal if plain html But when I load that content from ajax, It do not show [+] [-], at Item 3, like this img:

I don't understand that my mistake some where, or this is default of this tree?

Thank you much!

This image is loading tree content with ajax:

[dungla2011]

I found that is ok: if load remote content before call Tree function It works well now: