Pentaphragma: A Unique Wood and its Significance

Abstract

Qualitative and quantitative data are given for wood anatomy of three species of Pentaphragma (Pentaphragmataceae); the woods of the three species are very similar. Pentaphragma is rayless, but eventually develops rays in at least one of the species studied. This is interpreted as related to secondary woodiness or upright habit within a predominantly herbaceous phylad. The vessel elements of Pentaphragma have features universally interpreted as primitive in dicotyledons: scalariform perforation plates with numerous bars; pit membrane remnants in perforations; scalariform lateral wall pitting; the genus also has fiber-tracheids with prominently bordered pits. These character states accord with the basal position in Campanulales accorded Pentaphragmataceae by Cosner et al. (1992), and suggests that order may have begun with more numerous primitive features than generally recognized. The presence of occasional scalariform perforation plates, often aberrant, in secondary xylem of families of Asterales sensu lato - Campanulaceae, Pentaphragmataceae, Valerianaceae, and even Asteraceae (e.g., certain Lactuceae) - can be attributed to paedomorphosis, extending these plates into secondary xylem from primary xylem. Raylessness in Pentaphragma can be described in terms of secondary woodiness or paedomorphosis. The fact that fiber-tracheids are shorter than vessel elements in Pentaphragma is believed related to raylessness also, because some fiber-tracheids are produced from 'potential' ray areas.