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The rostellum is a projecting part of the 
In '' Catasetum'' 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.
column
A column or pillar in architecture and structural engineering is a structural element that transmits, through compression, the weight of the structure above to other structural elements below. In other words, a column is a compression member ...
in Orchidaceae
Orchids are plants that belong to the family (biology), family Orchidaceae (), a diverse and widespread group of flowering plants with blooms that are often colourful and fragrant. Orchids are cosmopolitan distribution, cosmopolitan plants that ...
flowers, and separates the male androecium from the female gynoecium
Gynoecium (; ; : gynoecia) 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 (botany), whorl of a flower; it consists ...
, commonly preventing self-fertilisation. In many orchids, such as '' Orchis mascula'', the pollinia or pollen masses, are connected by stipes down to adhesive discs attached to the rostellum which forms cups keeping the discs or balls sticky.
In '' Catasetum'' 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 Natural history#Before 1900, naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all speci ...
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