Display value AND apply templates

I've been looking at this for too long and can no longer think clearly. Take 
this example:

<myxml>
    <para>The notation for water is
        <chem>H
            <sub>2</sub>
        0
        </chem>
    </para>
</myxml>

I'm trying to transform to XHTML. The desired output for the above would be:

<p>The notation for water is <span class="chem">H<sub>2</sub>0</span></p>

The problem I have is that if I use value-of "." I can get the value of node, 
but no further templates are applied. If I use apply-templates "*" I get the 
sub-templates but no value. 

For example:

<xsl:template match="chem">
    <span class="chem"><xsl:apply-template select="*" /></span>
</xsl:template>

<xsl:template match="sub">
    <sub><xsl:value-of select="." /></sub>
</xsl:template>

This gets the <sub> but loses the value of the <chem>.

As I say, I've lost all clarity of mind on this, so I'm expecting it to be 
obvious! Any and all suggestions most welcome.

Thanks for your time.

Drew McLellan 
             

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