Pitting of Transfusion Tracheids in Cupressaceae

Abstract

The morphology and development of the circular bordered pits on the transfusion tracheids in the leaves of Callitris muelleri are described from light microscopy of semithin sections and SEM studies of cut surfaces. Two parallel bars of thickening develop on the pit border either side of the aperture, and trabecula-like outgrowths develop into the cell lumen from the centre of each bar. These outgrowths are often contorted and may anastomose with the wall or each other. Development is arrested at the barred stage in occasional pits, and both 'barred' and 'trabeculate' pits may occur in the one cell. 'Trabeculate-pitting' occurs in all species of Callitris and Actinostrobus, and was also observed in species of Cupressus and Juniperus. Previous reports of outgrowths on pits in several species of the last two genera suggest that the trabeculate pit is characteristic of both. 'Barred pits' occur in Widdringtonia cupressoides and Catocedrus decurrens. Unadorned circular bordered pits are characteristic of Diselma archeri, and have previously been reported in a range of genera in the family. These three types of pit are considered to represent an evolutionary transformation series in which the last type is plesiomorphic. The occurrence of the apomorphic conditions in both northern and southern hemi- sphere genera does not fit well with the present taxonomy of the family. The probable adaptive significance of the trabeculate pit is discussed.