The rostellum is a projecting part of the column in Orchidaceae flowers, and separates the male androecium from the female

gynoecium Gynoecium (; ) is most commonly used as a collective term for the parts of a flower that produce ovules and ultimately develop into the fruit and seeds. The gynoecium is the innermost whorl of a flower; it consists of (one or more) ''pistils' ...

, commonly preventing self-fertilisation. In many orchids, such as '' Orchis mascula'', the

pollinia A pollinium (plural pollinia) is a coherent mass of pollen grains in a plant that are the product of only one anther, but are transferred, during pollination, as a single unit. This is regularly seen in plants such as orchids and many species of mil ...

or pollen masses, are connected by stipes down to adhesive discs attached to the rostellum which forms cups keeping the discs or balls sticky. Fertilisation of Orchids figure 28

In ''

Catasetum ''Catasetum'', abbreviated as Ctsm. in horticultural trade, is a genus of showy epiphytic Orchids, family Orchidaceae, subfamily Epidendroideae, tribe Cymbidieae, subtribe Catasetinae, with 166 species, many of which are highly prized in hortic ...

'' flowers the rostellum projects forward at each side as an "antenna", and the pollen masses are connected by a bent stalk or pedicel to a sticky disc kept moist at the back of the flower. When an insect touches an "antenna", this releases the bent pedicel which springs straight and fires the pollinium, sticky disc first, at the insect.

Charles Darwin Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all species of life have descended ...

described in ''

Fertilisation of Orchids ''Fertilisation of Orchids'' is a book by English naturalist Charles Darwin published on 15 May 1862 under the full explanatory title ''On the Various Contrivances by Which British and Foreign Orchids Are Fertilised by Insects, and On the Good ...

'' how he "touched the antennæ of C. callosum whilst holding the flower at about a yard's distance from the window, and the pollinium hit the pane of glass, and adhered to the smooth vertical surface by its adhesive disc."

References

* *{{Citation , last =Darwin , first = Charles , year =1862 , title =On the various contrivances by which British and foreign orchids are fertilised by insects, and on the good effects of intercrossing , location = London , publisher = John Murray , url =http://darwin-online.org.uk/content/frameset?viewtype=text&itemID=F800&pageseq=1 , accessdate =2009-02-07 , postscript = Orchid morphology