Wood anatomy of the endemic woody Asteraceae of St Helena I: phyletic and ecological aspects
2001
CARLQUIST, SHERWIN
Quantitative and qualitative data are given for samples of mature wood of all eight species of woody Asteraceae, representing three tribes, of St Helena I. The quantitative features of all except one species are clearly mesomorphic, corresponding to their mesic central ridge habitats. Commidendrum rugosum has more xeromorphic wood features and occurs in dry lowland sites. Commidendrum species are alike in their small vessel pits and abundant axial parenchyma. Melanodendrum agrees with Commidendrum in having fibre dimorphism and homogeneous type II rays. The short fibres in both genera are storied and transitional to axial parenchyma. Elongate crystals occur in ray cells of only two species of Commidendrum, suggesting that they are closely related. Wood of Commidendrum and Melanodendrum is similar to that of the shrubby genus Felicia, thought closely related to Commidendrum on molecular bases. Commidendrum and Melanodendrum have probably increased in woodiness on 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 prenanthiflord) also show paedomorphic wood. Wood of the various St Helena Asteraceae is consonant with relationship to African or South American ancestors that reached St Helena via long distance dispersal. Derivation from genera of Pacific islands or Austromalesian 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.
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