Pollen wall stratification trends in Ulex (Genisteae, papilionoideae: Leguminosae) in the Iberian Peninsula

Abstract

Pollen grains of Ulex are isopolar, 3-zonocolporate with an ill-defined endoaperture, and tectateperforate to reticulate. The pollen grains have distinct internal folds due to the thickening of the endexine along the colpi. The general morphology does not vary substantially from one taxon to another, but the pollen wall structure is very variable. Three types have been recognized based on the stratification: Type I: a compact structure with an ill-developed infratectum mainly composed of granules; Type II: a transitional infratectal layer composed by granules and columellae; and finally, Type III: with a well differentiated columellar infratectum.

These structural types correlate to a high degree with the different ploidy levels found in Ulex. Most of the diploid taxa have Type I, most of the tetraploids have Type II, and all of the hexaploids have Type III.