APERTURES: are any thinning, thickening or other modification of the wall of pollen or spores that serve as an exit for its contents or to allow shrinking and swelling of the grain in response to changes in moisture content. For pollen these are the furrows (colpi) and pores. below, left The arrangement and number of pores and furrows are a chief criteria for identifying pollen classes. For eudicots, the pollen grains have a symmtrical form consisting of a polar axis ("po" at left) and an equatorial plain ("eq" at left).
This page provides a basic introduction to some features of pollen morphology. For a more complete treatment, see
PORES are circular aperatures no greater than twice as
long in one dimension than perpendicular to that dimension.
Generally, they are arranged symmetrically around
the equatorial plain of the grain, three pores being the "basic form"
for eudicots. However, other arrangements of pores exist
in various pollen classes.
The pores may be simple openings in both layers of the pollen wall, left
or they can result from the absence of part of the pollen wall.
Ordinarily, the term annulus ("a" at left)
refers to a thickening of the outer wall, for example in grasses.
The term endopore ("e" at left)
is used for a thinning or thickening of the inner wall, such as in
mulberry pollen (thickening) or Linanthus (Polemoniaceae) (absence of the inner layer).
The most prominent pore type (Onagraceae) consistes of a large
chamber between the separated inner and outer layers of the pollen wall.
FURROWS OR COLPI are elongate apertures. In eudicots the elongation is parallel to the polar axis and the number of furrows is three (tricolpate), however other pollen classes have the furrows arranged in other ways. As with the pore above, left the furrow can be a simple opening (maple) or it may be covered with a persistent membrane ("m" at left) (oak). The membrane may be covered with sculpturing elements ("L" at left), islands of tectum (Acanthaceae), or completely covered with an operculum ("o" at left) These three conditions also apply to pores, as does the more general term "margo," for which annulus and endopore are special cases. For example, the pores of grass pollen have an operculum which includes both the inner and outer pollen wall. The thin margo ("n" at left) results from the thinning of the wall adjacent to the furrow, as in willow (Salix). Thickened margos also exist, as in the roses (Rosa)("r" at left), which have a relatively thin furrow membrane. An intruding outer wall also can be seen in the pores of Cannabis.
COLPORATE grains contain a furrow with a pore in the center. Most tricolpate grains show some modification of the center of the furrow (at the equatorial plain). This varies from a slight constriction or thinning of the wall at the sides of the furrow to the presence of a distinct pore. Pollen grains with both a distinct furrow and pore are termed "colporate." The simplest pore is a circular rupture in the furrow membrane left as in Artemisia. Thinnings (Larrea) and thickenings (Prosopis) around the pore also occur. Frequently, the pore extends beyond the furrow margine. In some cases, the "pores" may extend laterally (parallel to the equatorial plain) and coalesce (thereby being to long to be true pores). These equatorially-elongated pores may be called "transverse furrows" They are common in the carrot family (Umbelliferae), Transverse furrows with thickened margines, coalesce to form an equatorial ring are called "costae equatorialis."
Owen Davis 12/99