number of 14).
gametophyte are striking, but structures apparently homologous with the mamelon do occur in both Santalaceae and Olacaceae. Kuijt (1968, 1969) supposes that what may be envisaged as the retention of juvenile features in the ovary may be correlated with the independent transformation of the root-system into a haustorium necessary for a parasitic mode of existence.
The two families appear to have separate geographic origins and a different cytological history (Barlow 1983a, 1990). The more primitive genera of Loranthaceae occur in South America, New Zealand and Australia. A primary base number of x=12 occurs in the relictual genera, with divergence to x=8 in the remaining New World genera and x=9 in most of the Old World genera, with one apparently derived group reverting to x=12. In Viscaceae the basic number of x=14 is consistent through the family, though some genera, notably Notothixos and Viscum, have reduced numbers, down to x=10 in some species. The family seems to have its origins in south-east Asia, from where it has dispersed mainly in the tropics and the northern hemisphere.
Nickrent and Soltis (1995) have recently reported comparative molecular analyses of the chloroplast gene rbcL and the ribosomal gene 18S rDNA. They included representatives from the seven genera of Viscaceae, Antidaphne and Eubrachion from Eremolepidaceae, Santalum and Osyris from Santalaceae, Misodendrum from Misodendraceae, Gaiodendron from the Loranthaceae, Opilia from the Opiliaceae and Schoepfia from the Olacaceae. The rbcL analysis showed three clades (1) Loranthaceae, Misodendraceae, Olacaceae and Opiliaceae, (2) Eremolepidaceae and Santalaceae (the genera intermixed), and, linked to the latter, (3) Viscaceae. The 18S rDNA sequences similarly linked Loranthaceae with Opiliaceae and Olacaceae, and gave strong support for Viscaceae linked to Santalaceae plus Eremolepidaceae, but Misodendraceae was linked with Santalaceae rather than with Loranthaceae. Nickrent and co-workers have found higher than average substitution rates in rRNA and rDNA in parasitic plants generally, which makes them useful tools for discrimination at the family level in Santalales and even generic level in Viscaceae (Nickrent and Franchina 1990; Nickrent and Soltis 1995; Nickrent 1996).
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