BIOLOGICAL STUDIES ON MISTLETOE

Abstract

The mistletoes ( Loranthaceae sensu stricto and Viscaceae s.s. ) are common in tropical and subtropical regions. They are an interesting group of hemiparasitic flowering plants, the Malesian species of which have not been well studied. The objectives of this study were: 1. Inventory of mistletoe-species which occur in Singapore, 2. Determination of the taxonomic significance of leaf anatomical data in Malesian taxa. From a survey of herbarium specimens and field collections, six to eight species of mistletoe were found to be extinct in Singapore. Species that still flourish in Singapore are: endrophthoe pentandra, Macrosolen cochinchinensis, M. retusus, Scurrula ferruginea, viscum articulatum and V . ovalifolium. S. parasitica is a new record for Singapore. Few studies have been done on the leaf anatomy of mistletoes and an attempt was made to determine whether this source of taxonomic evidence is consistent with the current classification systems used, primarily of Danser (1931). Samples of mature leaves from at least one species in each Malesian genus were made into clearings or sectioned and examined using light microscopy. Twenty-one out of 25 Malesian genera were studied; the remainder were omitted for lack of material. For the species investigated, the paracytic stomatal configuration was observed on both leaf surfaces in most species. Some hypostomatic species were also obtained. The epidermis was found to be uniseriate and the mesophyll isolateral. Venation varied from reticulate to ramified with completely or incompletely looped marginal veins. Expanded terminal tracheids as well as silica deposits were observed. Also, numerous crystals were seen in the mesophyll cells. Four basic sclereid types were observed:- the brachysclereid, astrosclereid, filiform sclereid and fibre sclereid, with a range of intermediate forms. Fibre sclereids were only observed in Helixanthera cylindrica. The form of sclereids, as well as the type of mesophyll crystals present, indicate some taxonomic differences between taxa of the Loranthaceae and the Viscaceae. Difference among taxa within each family is also indicated. The ability to separate species in the subtribe Loranthinae from those of the subtribe Elytranthinae by the presence of crystals in the sclereid generally supports the phylogenetic classification of Danser (1931), which was based mainly on floral morphology.