Callitroid (callitrisoid) thickening in Callitris

In softwood tracheids, callitroid thickening is the term given to raised bars located above and below individual bordered pit apertures. Although most well developed in the Australasian genus Callitris, callitroid thickening is reported to be absent from six Callitris species and to vary in its frequency and prominence. An SEM study of callitroid thickening was carried out on multiple wood samples of each Callitris species, taken from trees growing in the wild. The aims of the study were to determine the occurrence, frequency, visual prominence and morphology of callitroid thickening in all Callitris species and to determine whether it could be used taxonomically. Callitroid thickening was found in all (19) species of the genus, in contrast to previous reports of its absence in some species. In general, callitroid thickening occurred at higher frequencies and was more prominent in species from dry habitats than in those from wet habitats. The frequency of thickening varied between and within samples and, for example, it was always more frequent on pits in narrow rather than in wide tracheids. Callitroid thickening varied in its morphology, and types consisting of one to four bars, extending completely or only partially across the inner radial wall of the tracheid, were observed. Thickening on tracheid-ray pits within cross fields occurred in all species, and with similar frequency and morphology to thickening of bordered pits. Our findings suggest that frequency of callitroid thickening is useful taxonomically in separating four groups of Callitris species and in assisting in the identification of certain individual species.