Comparative morphology of epicortical roots in Old and New World Loranthaceae with reference to root types, origin, patterns of longitudinal extension and potential for clonal growth

The large mistletoe family, Loranthaceae, contains 75 genera, three of which are terrestrial root parasites. The remaining 72 genera are aerial parasites. Four basic haustorial system types are found in aerial genera: epicortical roots (ERs), wood roses, clasping unions and bark strands. The focus of this report is on genera in which ERs are present. Presence of ERs is based on our worldwide collection of haustoria and from literature sources. Our collections include 78% of all aerial genera and 72% of genera with ERs. Collections were analyzed using comparative morphological methods. Of the 72 aerial genera 40 (56%) have ERs and 75% of these are Old World. ERs are the most common haustorial type for Loranthaceae on every major landmass except Africa. Three ER types are described, basal, cauline and adventitious. Basal and adventitious ERs occur in both the Old and New World, whereas cauline ERs are exclusively New World. Adventitious ERs form in a few species in response to injury or epiparasitism. Significant differences occur between basal and cauline ERs in the extent and pattern of elongation, frequency of lateral root formation, and production of haustoria and shoots. Three patterns of axis extension of ERs are recognized, the monochasial sympodium, dichasial sympodium and monopodium. Marked differences in patterns of axis elongation occur between the Old and New World genera analyzed. In Old World taxa 94% of lateral roots contributed to a monochasial sympodium, whereas in New World taxa 84% of root extension was monopodial. Two strategies of resource procurement occur in genera with ERs; the “phalanx” strategy is found in species with basal ERs only, the “guerilla” strategy in New World species with cauline ERs. Species with ERs have the potential for clonal growth through fragmentation of stems, ERs, or both, but the extent of clonal growth in nature is unknown. The large number and wide distribution of genera with ERs add support to the hypothesis that the presence of ERs is an ancestral trait for aerial Loranthaceae.