Getting parent element in XSLT

I have an XML like this:

<PurchaseOrder>
    <ID>1</ID>
    <PurchaseOrderLine>
        <DATA>100<DATA>
    </PurchaseOrderLine>
    <PurchaseOrderLine>
        <DATA>200<DATA>
    </PurchaseOrderLine>
    <PurchaseOrderLine>
        <DATA>300<DATA>
    </PurchaseOrderLine>
</PurchaseOrder>
<PurchaseOrder>
    <ID>2</ID>
    <PurchaseOrderLine>
        <DATA>100<DATA>
    </PurchaseOrderLine>
    <PurchaseOrderLine>
        <DATA>200<DATA>
    </PurchaseOrderLine>
    <PurchaseOrderLine>
        <DATA>300<DATA>
    </PurchaseOrderLine>
</PurchaseOrder>
<PurchaseOrder>
    <ID>3</ID>
    <PurchaseOrderLine>
        <DATA>100<DATA>
    </PurchaseOrderLine>
    <PurchaseOrderLine>
        <DATA>200<DATA>
    </PurchaseOrderLine>
    <PurchaseOrderLine>
        <DATA>300<DATA>
    </PurchaseOrderLine>
</PurchaseOrder>

and XSL:

<xsl:template match="PurchaseOrder">
    <xsl:apply-templates select="PurchaseOrderLine"/>
</xsl:template>

<xsl:template match="PurchaseOrderLine">
    <!-- I want to get the PurchaseOrder\ID here for the current PurchaseOrder -->
</xsl:template>

How can I get current parent element value (PurchaseOrder\ID) in PurchaseOrderLine?