Wood anatomy of the endemic woody Asteraceae of St Helena I: phyletic and ecological aspects

Quantitative and qualitative data are given for samples of mature wood of a il eight species of woody Asteraceae, representing three tribes, of St Hele na I. The quantitative features of all except one species are clearly mesom orphic, corresponding to their mesic central ridge habitats. Commidendrum r ugosum has more xeromorphic wood features and occurs in dry lowland sites. Commidendrum species are alike in their small vessel pits and abundant axia l parenchyma. Melanodendrum agrees with Commidendrum in having fibre dimorp hism and homogeneous type II rays. The short fibres in both genera are stor ied and transitional to axial parenchyma. Elongate crystals occur in ray ce lls of only two species of Commidendrum, suggesting that they are closely r elated. Wood of Commidendrum and Melanodendrum is similar to that of the sh rubby genus Felicia, thought closely related to Commidendrum on molecular b ases. Commidendrum and Melanodendrum have probably increased in woodiness o n St Helena, but are derived from shrubby ancestors like today's species of Felicia. Petrobium wood is paedomorphic and indistinguishable from that of Bidens, from which Petrobium is likely derived. The two senecionid species (Senecio leucadendron = Pladaroxylon leucadendron; and Senecio redivivus = Lachanodes arborea, formerly Lachanodes prenanthiflora) also show paedomor phic wood. Wood of the various St Helena Asteraceae is consonant with relat ionship to African or South American ancestors that reached St Helena via l ong distance dispersal. Derivation from genera of Pacific islands or Austro malesian regions is considered less likely. However, DNA evidence is needed to clarify origins, times of colonization on St Helena and divergence from closest relatives, and the nature of evolutionary patterns. (C) 2001 The L innean Society of London.