Harmomegathic Effect Wodehouse Effect

All living pollen grains are able to absorb and release water; thus, each living grain exists in two morphologically different states: the dry and the hydrated condition. Harmomegathic mechanisms, e.g., infolding of the pollen wall, accommodate the change of the osmotic pressure in the cytoplasm during hydration or dehydration.

Harmomegathic effect

Cistus creticus Cistaceae left: spheroidal right: dry pollen prolate, lobate

Galium rotundifolium Rubiaceae left: oblate right: dry pollen prolate, lobate

Vriesea pabstii Bromeliaceae left: oblate right: dry pollen boat-shaped

Lamiastrum montanum Lamiaceae left: spheroidal right: dry pollen prolate, outline elliptic

Harmomegathic effect

Cistus creticus Cistaceae left: spheroidal right: dry pollen prolate, lobate

Galium rotundifolium Rubiaceae left: oblate right: dry pollen prolate, lobate

Vriesea pabstii Bromeliaceae left: oblate right: dry pollen boat-shaped

Lamiastrum montanum Lamiaceae left: spheroidal right: dry pollen prolate, outline elliptic

Galium Pollen Photo

The main purpose of the harmomegathic effect is to protect the male gametophyte against desiccation during pollen presentation and dispersal, and is often related to pollination biology.

In mature anthers, pollen is turgescent before shedding. After anther dehiscence and during pollen presentation, water loss takes place and the pollen grain becomes typically infolded, depending on aperture form and/or number, and specific wall thinnings or thickenings. The pollen grain in proper dry state represents the genuine harmomegathic effect and its shape is very often typical for a family and/or genus and is therefore of systematic relevance.

The harmomegathic effect is to some degree reversible. Rehydrated pollen with water uptake at the stigma, or under laboratory conditions, is again turgescent and largely recalls the shape before shedding. A second dehydration does not necessarily result in the typical dry shape but, if pollen walls are sufficiently stable, the harmomeg-athic effect can be induced several times in the same way. In the case of thin walls, the susceptible internal structure may become irreversibly damaged, and the harmomeg-athic effect may result in differing shapes, often randomly.

The harmomegathic effect is also observed in pollen taken from herbarium material, and to some degree in fossil material (HALBRITTER and HESSE 2004).

Infolding of the pollen wall after ace-tolysis is mostly not comparable with that in dry state.

The harmomegathic effect depends predominantly on the various characters of the pollen wall. Several pollen features (harmomegathic factors) collectively influence the mode of infolding and cannot be considered separately:

— apertures (the most important character): their position, number and form.

— pollen wall structure: thinned or thickened regions; in particular, internal girdles or endoapertures. If the ektexine is considerably reduced, its role is taken over by other wall strata, namely, by a thick endexine or intine. On the other hand, if the exine is extremely rigid, then the harmomegathic effect is only marginal.

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