Nomenclature and Typology

An aperture is a region of the pollen wall that differs significantly from the rest of the wall in its morphology and/or anatomy, and is presumed to function usually as the site of germination and to play a role in harmomegathy. Pollen grains lacking an aperture are called inaperturate. The aperture definition fits both angiosperm and gymnosperm pollen, but in gymnosperms the type of aperture usually differs from that in angiosperms, since often a leptoma is present. Note: unless stated otherwise, the following sections deal with angiosperm aperture constructs only.

The polarity of the pollen determines the aperture terminology. A circular aperture is called a porus if situated equatorially or globally; if situated distally it is called an ulcus. An elongated aperture is called a colpus if situated equatorially or globally; if situated distally it is called a sulcus. A combination of porus and colpus is termed a colporus; colpori are situated only equatorially or globally. Colpi and colpori (colpi and pori) may be present simultaneously in some taxa; this condition is called heteroaperturate. A circular or elliptic aperture with indistinct margins is a poroid.

The number of equatorial apertures (pori, colpi, colpori) is indicated by the prefixes di- or tri-. However, tetra-, penta- or

Pollen grain polarity dicots

Bellis perennis Asteraceae polar view equatorial view

Pollen grain polarity dicots

Bellis perennis Asteraceae polar view equatorial view

Poiien grain poiarity monocots

Allium paradoxum Alliaceae proximal polar view distal polar view equatorial view equatorial view

Poiien grain poiarity monocots

Allium paradoxum Alliaceae proximal polar view distal polar view equatorial view equatorial view

hexa- are sometimes used. (Writing numbers instead of prefixes is in common use, e.g., 4-porate or tetraporate, 6-colpate or hexa-colpate. "Pollen Terminology. An illustrated Handbook" prefers the use of prefixes.) Any pollen grain with more than three apertures at the equator is also called stephanoaper-turate (stephanoporate, stephanocolpate, stephanocolporate). Pollen grains with globally distributed apertures are called pantoaperturate.

The polarity gives rise to the polar and the equatorial view. In dicots there is usually one polar and one equatorial view. In monocots, due to the mostly distal aperture, there are four views: a proximal polar, a distal polar, and two different equatorial views.

Proximal germination is unknown in seed plants and is restricted to spores, which germinate at the tetrad mark, the so-called laesura (extensive overview: TRYON and LUGARDON 1991).

Pre-(prae-)pollen (microspores of certain extinct seed plants) is characterized by proximal and distal apertures, and by presumed proximal germination, producing motile spermatozoids.

Pre-pollen polar view

Pre-pollen polar view

Apertures are normally covered by an exinous layer, the aperture membrane. Aperture membranes can be ornamented, e.g., covered with various exine elements, or can be smooth. In contrast, an operculum is a thick, coherent exine shield and covers the aperture like a lid.

In general, aperture membranes are infolded in dry pollen state; after acetolysis the aperture membrane may be lost.

Tetrad mark in spores

Polypodium sp. Polypodiaceae, fossil monolete tetrad mark polar view

Sphagnum sp. Sphagnaceae, fossil trilete tetrad mark polar view indet.

Pteridaceae, fossil trilete tetrad mark polar view

Cryptogramma crispa Pteridaceae trilete tetrad mark

Tetrad mark in spores

Polypodium sp. Polypodiaceae, fossil monolete tetrad mark polar view

Sphagnum sp. Sphagnaceae, fossil trilete tetrad mark polar view indet.

Pteridaceae, fossil trilete tetrad mark polar view

Cryptogramma crispa Pteridaceae trilete tetrad mark

Toxoid pollen germination

Cephalotaxus sp. Cephalotaxaceae exine shedding prior to pollen tube formation fresh pollen in water Instant pollen tubes

Scabiosa caucasica Dipsacaceae

Morina longifolia Morinaceae

Toxoid pollen germination

Cephalotaxus sp. Cephalotaxaceae exine shedding prior to pollen tube formation fresh pollen in water Instant pollen tubes

Scabiosa caucasica Dipsacaceae

Morina longifolia Morinaceae

Number, type and position of apertures are genetically determined and usually fixed within a species;however, it may sometimes vary (e.g., number of apertures in stephanoaperturate pollen grains).

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