XVI International Botanical Congess

The widely accepted model of haustorial evolution in aerial Loranthaceae views epicortical roots as primitive and bark strands as advanced. Solitary unions--wood roses and clasping unions--are regarded as intermediate in specialization. Comparative morphological methods have been used to test this model in Old World species. Our results indicate that a more complex model of haustorial evolution is needed. We found that primitive aerial forms have a small number of robust epicortical roots. In some taxa bark strands may occur concurrently with epicortical roots, as can wood-roses. Some genera with epicortical roots, such as Lepidaria and Ileostylus, form strong primary attachments and few or no secondary attachments. In most species epicortical roots favor basipetal branch growth, have sympodial-like elongation, and form root-born shoots in conjunction with secondary haustoria.The morphological and phylogenetic implications of these findings are illustrated and discussed.