Max count?

I have some xml which looks like :-

<datanode>
	<column id="1">
		<line>text</line>
		<line>text</line>
	</column>
	<column id="2">
		<line>text</line>
		<line>text</line>
		<line>text</line>
	</column>
	<column id="3">
		<line>text</line>
	</column>
</datanode>

I need to retrieve the maximum number of line nodes contained by a
column node within the datanode node. So for the example xml it would be
3, as column 2 has three line nodes.

Can anyone give me any pointers?

Cheers,

Dan