Taxonomic significance of sporoderm structure in pollen of Euphorbiaceae: Tribes Plukenetieae and Euphorbieae

Abstract

A pollen sporoderm study of tribe Plukenetieae - subtribe Tragiinae and tribe Euphorbieae was carried out. Most of the species of subtribe Tragiinae examined have a reduced exine, with a perforate tectum, short and irregular columellae, and a thin, undulated or absent foot layer. The endexine is homogeneous, fissured or spongy. The intine is well developed, thicker than the exine, and usually three-layered. In the apertural region both ectexine and endexine are present but are fragmented and disorganised. Only in the clavate-baculate pollen of Tragia volubilis is the exine well-developed and the intine thinner than the exine. The thicker intine is correlated with a thin and weakly developed exine that may provide support, compensating for the severe reduction of the foot layer and the poorly developed, fissured and spongy endexine. The pollen morphology of tribe Euphorbieae is concordant with the common pollen type in the genus Euphorbia . Nevertheless the pollen grains of this tribe have a particular apertural system, that includes modifications to the ectexine, endexine and intine at different aperture levels. The most important observation was the unusual intine with two thickenings that run along each side of the aperture. In general, pollen of the Euphorbiaceae has a thick and well-developed intine. This intine may be uniformly thickened around the grain or may form a lens-shaped structure (oncus) beneath the apertures. Intine thickenings such as those described for tribe Euphorbieae are unknown elsewhere in the family. The more specialised apertural structure, described for tribe Euphorbieae, supports the circumscription of this tribe.