SULCUS AND OPERCULUM STRUCTURE IN THE POLLEN GRAINS OF IRIDACEAE SUBFAMILY IXIOIDEAE

Ixioideae, the largest and most derived of the four subfamilies of Iridaceae, is distinct in the family in having specialized pollen grains with tectate-perforate and scabrate sculpturing. This is in contrast to the prevailing tectatereticulate exine in the rest of the family and its putative allies. In addition, most species have monosulcate grains with a 2-banded operculum, a feature that is probably synapomorphic for the subfamily. Exceptions among taxa with monosulcate grains having either a 1-banded or disorganized operculum are seen as most probably derived. Important exceptions to this pattern are Micranthus, which has zonasulcate grains with reticulate exine; one of seven species of Thereianthus, T. racemosus, which also has a reticulate exine except on the operculum; Zygotritonia, which has 3-aperturate inoperculate grains; and Savannosiphon, in which the grains have an irregular distribution of the exine and may be described as inaperturate but functionally omniaperturate. The origin of the operculum and the significance of the variation within Ixioideae is considered in relation to the systematics of Ixioideae.