tree fragments in variable?!

Hello XSLTies,
when I use something like this:
<xsl:variable name="header">
  <tr>
  <th>Element</th>
  <th>Description</th>
  </tr>
</xsl:variable>
Is there a way to address certain nodes with XPath-like "$header/*" for example? When trying this, it seemed that the content is stored like a string in the variable. Though, when I used xsl:value-of the tags had been eliminated.
Thank you and greetings, 
Jan Mendling

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