XML descendants showing up blank?

I wonder if any of you have a suggestion for me here...

var file = new File( /path/to/my/file.xml );

file.open('r');

var fileString = file.read();

var xml = new XML( fileString ); // everything's fine up to this point...

xml.descendants(); // returns the full contents of xml, so I know xml has content.

xml.descendants( 'Row' ); // returns nothing, even though there are definitely <Row> elements in the document.

Am I not using the 'descendants' method properly? Does anybody see something wrong in my syntax?

Thanks