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Annals of Botany 89: 133-144, 2002
© 2002 Annals of Botany Company

Wettable and Unsinkable: The Hydrodynamics of Saccate Pollen Grains in Relation to the Pollination Mechanism in the Two New Zealand Species of Prumnopitys Phil. (Podocarpaceae)

JOSHUA SALTER*,1, BRIAN G. MURRAY1 and JOHN E. BRAGGINS1

1School of Biological Sciences, The University of Auckland, Private Bag 92019, Auckland, New Zealand

 * For correspondence. Fax 64 93737416, e-mail j.salter{at}auckland.ac.nz

Received: 2 July 2001; Returned for revision: 2 August 2001; Accepted: 8 October 2001.

The pollination mechanism of most genera of the Podocarpaceae involves inverted ovules, a pollination drop and bisaccate pollen grains. Saccate grains have sometimes been referred to as ‘non-wettable’ due to their buoyant properties, while non-saccate pollen grains have been described as ‘wettable’. The hydrodynamic properties of saccate pollen grains of seven podocarp species in five genera, Dacrydium Sol. ex G. Forst., Dacrycarpus (Endl.) de Laub., Manoao Molloy, Podocarpus L‘Hér. ex Pers. and Prumnopitys Phil. have been tested in water, together with saccate and non-saccate pollen of four other conifer genera, Cedrus Trew (Pinaceae), Cephalotaxus Siebold & Zucc. ex Endl. (Cephalotaxaceae), Cupressus L. (Cupressaceae) and Phyllocladus Rich. ex Mirb. (Phyllocladaceae), and spores of three fern species and one lycopod species. All four spore types studied were non-wettable, whereas the bisaccate and trisaccate pollen types, like all other conifer pollen types, were wettable, enabling the grains to cross the surface tension barrier of water. Once past this barrier, grain behaviour was governed by presence or absence of sacci. Non-saccate and vestigially saccate grains sank, whereas saccate grains behaved like air bubbles, floating up to the highest point. In addition, the grains were observed to float in water with sacci uppermost, consistent with the suggestion that distally placed sacci serve to orientate the germinal furrow of the pollen grain towards the nucellus of an inverted ovule. Observations of pollen grains in the pollen chambers of naturally pollinated Prumnopitys ovules confirmed this. The combination of buoyancy and wettability in saccate pollen has implications for the efficiency of the typical podocarp pollination mechanism.

Key words: Podocarpaceae, Prumnopitys, Prumnopitys ferruginea (G. Benn. ex D. Don) de Laub., Prumnopitys taxifolia (Sol. ex D. Don) de Laub., saccate pollen, pollen hydrodynamics, pollination mechanisms, conifer pollen, fern spores, lycopod spores.


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