xsl:for-each does not loop inner elements

XML

    <authors>
        <author url="http://dl.acm.org/">Chung-Hwan Lim</author>
        <author url="http://dl.acm.org/">Seog Park</author>
        <author url="http://dl.acm.org/">Sang H. Son</author>
    </authors>

XSLT

<div class="authors">
    <xsl:for-each select="authors">
        <a style="vertical-align:middle">
            <xsl:attribute name="href">
                <xsl:value-of select="@url"/>
            </xsl:attribute>
            <xsl:value-of select="."/>
        </a>
    </xsl:for-each>
</div>

O/P

   <div class="authors"><a style="vertical-align:middle" href="">
                Chung-Hwan Lim
                Seog Park
                Sang H. Son
            </a></div>

The problem is I'm expecting three <a> tags but I'm only getting one. Why is that? why doesn't the `xsl:for-each' loop inner elements? How can I fix this?