Wood and Stem Anatomy of Misodendraceae: Systematic and Ecological Conclusions

Abstract

The wood anatomy of the monogeneric South American family of epiparasites, Misodendraceae, contains numerous peculiarities. In the four species of the subgenusAngelopogon the cambium is virtually inactive in ray areas. In three of those species, a second ring of fascicular areas (“bundles”) forms in the pith only well after the initiation of the outer ring. Fibers occur in fascicular areas in paired strands (sheathed by crystalliferous parenchyma), as the background tissue, or may be absent, depending on the species; rays consist wholly of fibers or are composed of thin-walled cells, depending on the species. In the subgenusMisodendron (seven species), the cambium is active in fascicular areas and interfascicular areas equally; it contributes fibers to potential ray areas and to fascicular areas. If occurrence of these fibers is a criterion, the subgenusMisodendron may be considered rayless (except forM. gayanum, in which fibers occur only in rays). Growth rings are present in all species. Vessel elements are extremely short and narrow, numerous per mm², and pores are markedly grouped; all these are highly xeromorphic features appropriate to a parasite in an extreme climatic zone. In subgenusAngelopogon, lateral wall pitting of vessels consists of laterally much-widened pits, an appearance falsely resembling helical bands of primary xylem vessels. In subgenusMisodendron, pits on vessels are alternate or scalariformlike. The difference in vessel pitting between the two subgenera may relate to wood parenchymatization. Perforation plates are simple. Pits have wide apertures inM. angulatum andM. recurvum. Pith is sclerenchymatous in subgenusMisodendron but composed of thin-walled cells in which vessel strands occur in subgenusAngelopogon. Because of the numerous specific characteristics which correspond to the taxonomic system for the family, a key to species based on wood anatomy is given. New names must be employed for both subgenera for nomenclatural reasons; thus the combinationM. subgenusAngelopogon (=M. subgenusEumyzodendron) is coined. Wood anatomy of Misodendraceae is similar to that ofPsittacanthus and other Loranthaceae, disregarding the unique features found inMisodendron.