String output of XML documents

Hi,

I wish to manage XML documents using the w3c DOM standard interface. I supposed I could find in this interface a method enabling to format the XML tree content to a string value. This support seems to exist, but I found it in the DocumentEx implementation offered in the projectX package com.sun.xml, which is not standard w3c recommendation.
Is there anything I missed about the w3c DOM interface that would enable to do that, or do I have to generate my own string from the XMl tree ? Why would there be no standard support for getting XML string representations (when this seems to be defined for HTML) ?
Many thanks to anyone who has an answer to these questions

Arnaud