Polarity and Symmetry

Mature pollen is shed in dispersal units. The post-meiotic products either remain permanently united or become partly or usually completely disintegrated. In the latter case the dispersal unit is a single pollen grain, a monad; if the post-meiotic products remain united, dyads (a rare combination), tetrads or polyads (massulae, pollinia) are the result. Pollinaria are dispersal units of two pollinia including the sterile, interconnecting appendage.

Tetrad stage

orientation of

microspores

/ \—\

distal poles

shaded green

Polar axis and equatorial plane

microspore's center, perpendicular to the polar axis. Therefore, the equatorial plane divides the pollen grain into a proximal and a distal half.

Isopolar pollen grains have identical proximal and distal poles, thus the equatorial plane is a symmetry plane. In heteropolar pollen grains the proximal and distal halves are different.

Pollen shape and aperture location directly relate to pollen polarity, which is determined by the spatial orientation of the microspore in the meiotic tetrad and can be examined only in the tetrad stage. The polar axis of each microspore runs from the proximal pole, orientated towards the tetrad center, to the distal pole at the outer tetrad side. The equatorial plane is located at the

Poiarity left:

isopolar right:

heteropolar

Poiarity left:

isopolar right:

heteropolar

The various arrangements of the four microspores within permanent or disintegrating tetrads depend on the simultaneous or successive type of cytokinesis and on the type of intersporal wall formation. The spatial arrangement of microspores after simultaneous cytokinesis is usually a tetra-hedral tetrad. This arrangement is of systematic relevance. The spatial arrangement of microspores after successive cytokinesis leads to different tetrad types without any systematic relevance: planar (tetragonal, linear, T-shaped) or non-planar (decussate or tetrahedral).

(probably restricted to Proteaceae, no permanent tetrads).

Tetrad arrangement tetrad tetrahedral

Fagus sp. Fagaceae, fossil (exceptional finding)

tetrad planar

Typha latifolia Typhaceae

Tetrad arrangement tetrad tetrahedral

Fagus sp. Fagaceae, fossil (exceptional finding)

tetrad planar

Typha latifolia Typhaceae

In pollen grains with three apertures, two types of aperture arrangement occur after simultaneous cytokinesis (disintegrating or permanent tetrahedral tetrads). Fischer's law refers to the most frequent arrangement where the apertures form pairs at six points in the tetrad (e.g., Ericaceae, permanent tetrads). Garside's law refers to the unusual arrangement of apertures where they form groups of three at four points in the tetrad

Aperture arrangement

Fischer's law

Garside's law

Aperture arrangement

Fischer's law

Garside's law

Pollen shape refers to the P/E-ratio: the ratio of the length of the polar axis (P) to the equatorial diameter (E). In spheroidal (or isodiametric) pollen grains the polar axis is ± equal to the equatorial diameter. Pollen grains with a polar axis longer than the equatorial diameter are called prolate; grains where the polar axis is shorter than the equatorial diameter are described as oblate.

Pollen shape left: oblate mid: spheroidal right: prolate

Pollen shape left: oblate mid: spheroidal right: prolate

Pollen size varies from less than 10 |jm to more than 100 pm. To indicate the pollen size the largest diameter is used. It also depends on the degree of hydration and the preparation method. Because of this and natural variation, a bandwidth designation is recommended. A diameter indication in the range of, e.g., less than 1 pm is not recommended.

The use of the following size categories may be helpful: very small (<10 pm), small (10-25 pm), medium (26-50 pm), large (51-100 pm) and very large (>100 pm).

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