How to realize dynamic expand tree?

I want to create a dynamic expand tree,like follows:
－Root
　－－＋Directory1
　－－＋Directory2

When click Directory1,this page will read contents of Directory1 from database and show them,the result is follows:
－Root
　－－－Directory1
　　　－－＋Directory1－1
　　　－－－File1
　　　－－－File2
　－－＋Directory2

How to realize above function? Is there have open source code for it?

Assuming that you want to look at directories and files on a server, thinks like this can either be solved using pure server side scripting, or a combination of client side / remote scripting and server side functions.

To find information about remote scripting, Google for "ajax xmlhttprequest" or "remote scripting". PHP often is the server side language of choice for its availability and large number of resources and script examples, Google for "php scripts" to find your way to a number of these resources.

Being a novice myself on this subject and having read a good book on it recently, believe me when I tell you that this is no simple, cut-n-paste stuff; you'll have to really dive into it to understand these technologies and to make them work for what you want to accomplish.