xslt condition for multiple line of text column

i am having one issue in displaying description column data from the list in data view web part.Actually in my list i've a column named as Description type of this column is multiple line of text what i want if length of the text in description column is greater than 20 characters than display only 20 characters otherwise show all characters. I've written the following xslt to do that.

<xsl:choose><xsl:when test="string-length(@Description) &gt; 20"><xsl:value-of select="substring(@Description, 1, 20)" />...</xsl:when><xsl:otherwise><xsl:value-of select="@Description" disable-output-escaping="yes"/>       </xsl:otherwise></xsl:choose>

Same thing is working for the column whose type is single line of text,can anyone tell me how to achieve this for the column whose type is multiple line of text