This question has been answered by BobRay. See the first response.
[[UltimateParent?
&topLevel=`1`
&id=`21`
]]
<div class="sisea-result">
<h3>[[+idx]]. <a href="[[+link:is=``:then=`[[~[[+id]]]]`:else=`[[+link]]`]]" title="[[+longtitle]]">[[+pagetitle]]</a></h3>
<div class="extract"><p>[[+extract]]</p></div>
<p>Parent Pagetitle: [[!ParentPagetitle? &docId=`[[+id]]` ]]</p>
</div>
/* ParentPagetitle snippet */
$docId = $modx->getOption('docId', $scriptProperties, '');
$output = 'Not Found';
if (!empty($docId)) {
$uParent = $modx->runSnippet('UltimateParent', array('topLevel' =>1, 'id' => $docId));
$parent = $modx->getObject('modResource', (int) $uParent);
$output = $parent->get('pagetitle');
}
return $output;