Retrieve metadata of all species/otus otus (operational taxonomic units) included in the nexml
Examples
if (FALSE) {
data(bird.orders)
birds <- add_trees(bird.orders)
birds <- taxize_nexml(birds, "NCBI")
RNeXML:::get_taxa_meta(birds)
RNeXML:::get_taxa_meta(birds, 'rel')
RNeXML:::get_taxa_meta(birds, 'id')
RNeXML:::get_taxa_meta(birds, 'xsi:type')
}