Heteroaperturate pollen grains: unique features and structures

Abstract Pollen grains with one kind of apertures are widespread in Angiosperms, whereas those with two or more kinds of apertures (heteroapertures) are limited in distribution and exhibit diverse range of structures. Heteroaperturate pollen grains of all the families covering the range of aperture morphology variations are investigated. A total of 13 families are revealed to have one or more species with heteroaperturate pollen grains. These constitute about 4% of Eudicot families. Diverse terms are encountered in these families to describe the nature of the simple apertures in these pollen grains. Pseudocolpi, subsidiary colpi, inter colpar concavities, transverse grooves, apertural areas and meridional bands are among the terms which are considered in the present study as true apertures as their characters comply with the definition of apertures. The present study revealed an interesting diversity in aperture pattern, and 6 distinct sub-types are identified. A bracket key to these sub-types is presented and the basic distinctive features are described. Pollen grains with two kinds of apertures constitute four sub-types, whereas those with three kinds comprise sub-types V and VI, which are quite unique in possessing equatorial and polar apertures. Diverse structures in simple apertures playing a vital role in the process of pollen hydrodynamics are outlined, and the unique features of heteroaperturate pollen grains are defined and discussed.