What is "rostellum"

Rostellum \Ros*tel"lum\, n.; pl. Rostella. [L. See Rostel.] A small beaklike process or extension of some part; a small rostrum; as, the rostellum of the stigma of violets, or of the operculum of many mosses; the rostellum on the head of a tapeworm.

n. 1 A small beak-like process or extension; a small rostrum. 2 (context botany English) A projecting part of the column in the flower of an orchid that separates the male stamen from the female gynoecium. 3 (context biology English) A retractable protruding part at the anterior end of a soft-bodied tapeworm; the scolex from which it protrudes is often armed with hooks which serve to keep it in place attached to the host.

thumb|upright=1.8|right|alt=photograph of a spire of purple flowers, and drawing showing the working parts of the flower|Cutaway drawing of Orchis mascula showing the rostellum projecting forward from the column to form cups which keep the adhesive balls sticky. The rostellum is a projecting part of the column in Orchidaceae flowers, and separates the male stamen from the female gynoecium, 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 described in Fertilisation of Orchids 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."

Rostellum (meaning "small beak", from the Latin for " beak"; pl. rostella) in helminthology is a protruding part of the anterior end of soft-bodied tapeworms. It is a retractable, cone-like muscular structure that is located on the apical end of the scolex, and in most species is armed with hooks, the organs of attachment to the host's intestinal wall. It is a parasitic adaptation in some cestodes for firm attachment in the gastrointestinal tract and is structurally different from one species to another (or even absent is some species), thereby becoming an important diagnostic feature.