Orbicules and the ektexine are homologous sporopollenin concretions inSpermatophyta

Abstract

During microsporogenesis sporopollenin becomes accumulated independently by the only two sporopollenin-producing cell types in the higher plants—the anther tapetum and the microspores—not only within the exine, but also within the orbicules. In Spermatophytes usually only a single orbicule type is found, but sometimes, e.g., inEuphorbia palustris, two different types exist within a single species. Very interestingly, most orbicules are strikingly similar in their structure, sculpture, shape and dimension to the respective ektexine: e.g., smooth sporopollenin globules are found near the totally smooth exines ofEupomatia laurina andPentaphragma sinense respectively, while inDelonix elata andEuphorbia palustris the orbicules often look like some “incomplete” tectum pieces. All these parallelisms can be attributed to the homologous and therefore highly similar genetic information to form sporopollenin in the sporogeneous tissue and the anther tapetum. The orbicules should be seen neither as sporopollenin storage, as sporopollenin transfer vehicle, nor as sporopollenin surplus.