Perforation plate pit membrane remnants and other vessel details of clethraceae: Primitive features in wood of ericales

Wood of 12 species of Clethra was studied with SEM; wood features are illustrated here for eight of these. Pit membrane remnants were observed in perforations of all species, although in some, the remnants occur only in the form of small webs at the lateral ends of perforations. Pit membranes take the form of threads or porose sheets; the former are more common. Extensive pit membrane remnants are uncommon in tropical species and are more characteristic of temperate species, perhaps because of water flow characteristics. As with other dicotyledons, occurrence of pit membrane remnants in perforations is considered a primitive feature in Clethraceae. Pit membrane remnants also occur in families of Ericales placed in primitive positions in the order and close to Clethraceae: Cyrillaceae and Sarraceniaceae. Bars of perforation plates are numerous and are prominently bordered. Grooves interconnecting pit apertures occur in distal portions of vessel elements and are accompanied by pairs of ridges in some species; this feature has not been reported so far either in Clethraceae or in Ericales. Grooves associated with pit apertures are very short or absent in central portions of vessel elements. The significance of groove occurrence on vessel walls is discussed. Presence of pit membrane remnants is clearly correlated with mesic habitats in which moisture availability is constant and may indicate, for the phylad, uninterrupted occupancy of such habitats.