EDIT

  define('CDM_SUB_TREE_FILTER_UUID', 'cdm_sub_tree_filter_uuid');

  $form['sub_tree_filter'] = array(

    '#type' => 'fieldset',

    '#title' => t('Subtree filter'),

    '#collapsible' => FALSE,

    '#collapsed' => TRUE,

    '#description' => t('The Data Portal can be restricted to the taxa which belong to a specific classification subtree. Other taxa will not be shown or accessible otherwise.'),

  );

  $sub_tree_filter_uuid_value = variable_get(CDM_SUB_TREE_FILTER_UUID, FALSE);

  $taxon_node_info = '';

  if($sub_tree_filter_uuid_value){

    $taxon_node_dto = cdm_ws_get(CDM_WS_TAXONNODE, array($sub_tree_filter_uuid_value));

    if($taxon_node_dto){

      $taxon_node_info = " Current taxon node: <strong>" . cdm_tagged_text_to_markup($taxon_node_dto->taggedTitle) . " [" . $taxon_node_dto->treeIndex . "]</strong>";

    }

  }

  $form['sub_tree_filter'][CDM_SUB_TREE_FILTER_UUID] = array(

    '#type' => 'textfield',

    '#title' => t('Taxon Node UUID') . ':',

    '#default_value' => variable_get(CDM_SUB_TREE_FILTER_UUID, FALSE),

    '#description' => "The uuid of the TaxonNode entity to be used as filter." . $taxon_node_info

  );