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Annals of Botany 89: 653-664, 2002
© 2002 Annals of Botany Company

REVIEW

Types of Pollen Dispersal Units in Orchids, and their Consequences for Germination and Fertilization

Ettore Pacini*,1 and Michael Hesse2

1University of Siena, Department of Environmental Biology, Via P.A. Mattioli 4, I-53100 Siena, Italy and 2University of Vienna, Institute of Botany, Department of Ultrastructure and Palynology, Rennweg 14, A-1030 Vienna, Austria

* For correspondence. E-mail pacini{at}unisi.it

Received: 7 December 2001; Returned for revision: 1 February 2002; Accepted: 6 March 2002.

The various pollen dispersal units (PDU) found in orchids are discussed together with possible evolutionary trends and the consequences for germination and fertilization. Orchids with monad and tetrad pollen form more complex dispersal units by means of pollenkitt, elastoviscin, a callosic wall, common walls or a combination of these. Evolutionary trends include (1) from pollenkitt to elastoviscin; (2) from monad to tetrads and multiples of tetrads; (3) from partially dehydrated (<30 %) to partially hydrated (>30 %) pollen; and (4) from monad pollen to PDUs with many pollen grains. The biological consequences concern both male and female reproductive systems. Some features of the male side are present in all orchids irrespective of the pollen dispersal unit, whereas other characters are found only in orchids with pollinia; the same applies for the female counterpart. Pollen grains of orchids with pollinia germinate at least 24 h after pollination because the pollen grains/tetrads must swell and make space for the growth of pollen tubes.

Key words: Review, pollen, compound pollen, pollen dispersal, pollination, Orchidaceae.


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