Santalales

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Santalales

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Discussion

The basically Euro-centered view of all mistletoes contained in a single family, LORANTHACEAE, proved to be unacceptable by the second half of the 20th Century. All specialists since the 1960's have been in agreement that two major natural taxa exist, viz., LORANTHACEAE s.s. and VISCACEAE (Barlow 1964, Kuijt 1968). The major distinction between the two families is the presence of a calyculus (usually interpreted as a reduced calyx) in LORANTHACEAE only, the flower thus being dichlamydeous in contrast to the monochlamydeous Viscacean flower with a single floral whorl. In addition, there are embryological, chromosomal, and other distinctions (Kuijt 1969).
It appears that LORANTHACEAE s.s. are of southern origin while VISCACEAE probably originated in the Northern Hemisphere (Barlow & Wiens 1971, Wiens & Barlow 1971). The great majority of LORANTHACEAE (nearly all the Old World genera) are predominantly bird-pollinated and have large, colorful flowers, but no VISCACEAE appear to be.
Awareness of the exsistence of other groups of Santalalean arboreal parasites has further complicated the taxonomic conception of "mistletoe". In southeastern Asia, several groups of SANTALACEAE parasitize branches of woody hosts; especially striking is the squamate genus Phacellaria which, at first sight, might well be mistaken for an Arceuthobium of VISCACEAE. The extraordinary MISODENDRACEAE of southern S America must similarly be thought of as being mistletoe-like in habit. Thus the conception of "mistletoe" within SANTALALES has assumed a more ecological rather than precise taxonomic cachet, signifying plants which parasitized branches of woody hosts. Ironically, even this circumscription cannot adequately cover LORANTHACEAE s.s., as its most primitive trio of genera (Atkinsonia, Nuytsia, and Gaiadendron) are terrestrial root-parasites, only the latter having the ability to "ascend" a host tree, where it parasitizes fellow epiphytes, probably exclusively (Kuijt 1963).
The above resumé does not exhaust contemporary controversy. A distinctive group of 3 genera (Antidaphne, Eubrachion, and Lepidoceras) which has traditionally been placed in VISCACEAE (or LORANTHACEAE subfam. VISCOIDEAE) were treated as a separate family, EREMOLEPIDACEAE (Kuijt, 1988). There is general agreement today that this is a natural assemblage, but molecular information has been promoted to indicate that EREMOLEPIDACEAE — and even VISCACEAE! — are better placed in SANTALACEAE, these ideas already having been incorporated in the third edition of Maas & Westra's "Neotropical plant families". Since such an alignment to me appears to be neither justified nor useful, the treatment which follows continues to recognize three families of mistletoes in the Guianas.
A. Barlow, B.A. & D. Wiens., The cytogeography of the Loranthaceous mistletoes in Taxon 20. 1971, B. Barlow, B.A., Classification of the Loranthaceae and Viscaceae in Proc. Linn. Soc. New South Wales 89. 1964, C. Kuijt, J. - in The biology of parasitic flowering plants. 1969, D. Kuijt, J., Monograph of the Eremolepidaceae in Syst. Bot. Monogr. 18. 1988, E. Kuijt, J., Mutual affinities of Santalalean families in Brittonia 20. 1968, F. Kuijt, J., On the ecology and parasitism of the Costa Rican tree mistletoe, Gaiadendron punctatum (Ruiz & Pavon) G. Don. in Canad. J. Bot. 41. 1963, G. Maas, P.J.M. & L.Y.T. Westra. - in Neotropical plant families. 2005, H. Wiens, D. & B.A. Barlow., The cytogeography and relationships of the viscaceous and eremolepidaceous mistletoes in Taxon 20. 1971