Stylidium velleioides

trigger plant


Stylidium velleioides

A.R.Bean 1999

pronounced: sty-LID-ee-um vell-eye-OY-deez

(Stylidiaceae — the trigger plant family)


common name: trigger plant

native 4Stylidium is derived from the Greek στυλος (stylos), a column, referring to the united stamens and style; velleioides refers to the similarity of parts of this species to Velleia spathulata.

Seven members of the Stylidium genus were collected by Banks and Solander in 1770, first at Botany Bay and then at the Endeavour River, Cooktown. The genus has a distinctive reproduction structure. Pollination is achieved by the use of the touch-sensitive ‘trigger’, that consists of the male and female reproductive organs fused into a floral column that snaps forward quickly when touched, covering the visiting insect in pollen. The insect is stunned, but not harmed. Because both the male and the female organs are present in the column, the stamen and the stigma take turns in dominating the column’s function. The anthers develop first and then are pushed aside by the developing stigma. The delayed development of the stigma prevents self-pollination. Different species have evolved the trigger mechanism in different locations, with some attacking the pollinating insect from above, and others from below. The column’s response to touch is very fast – in as little as 15 milliseconds. After firing, the column resets to its original position in anywhere from a few minutes to half an hour, depending on the species and the temperature – in lower temperatures the return is slower. The trigger is able to fire many times before it stops responding to stimuli. Most Stylidium species are pollinated by small solitary bees and the nectar-feeding bee flies Bombyliidae.

The plants are also carnivorous to some extent, because the glandular trichomes that cover the scape and flower can trap, kill and digest small insects with enzymes produced by the plant. The insects trapped in this way are too small to serve any role in pollination. It is not clear whether this capability is for supplementing nutrients, or simply a defence mechanism to prevent insects from damaging flower parts.

Stylidium velleioides is endemic to tropical Queensland, from Mount Surprise in the Gulf Savannah south to St Lawrence, near Mackay. Its typical habitat is along creek banks and seepage areas in mainly eucalypt woodlands. The plants photographed were found growing in a swampy area of Bolger Bay, on the western side of Magnetic Island, where Melaleuca and Pandanus are prevalent.

It is an herbaceous annual, growing from 15 – 30 cm tall. The leaves, up to about 30 per plant, are obovate, and form either a terminal rosette when stems are present, or a basal rosette in the absence of stems. The leaves are about 8 – 33 mm long by 3 – 9 mm wide. The inflorescences are about 15 – 25 cm long, producing tiny bright pink flowers about 5 mm across, with darker pink bands, having 4 petals, zygomorphic in nature, and usually blooming in late spring.

Some of the species collected by Banks and Solander were sketched by Sydney Parkinson on board the Endeavour, and were later engraved for inclusion in Banks’ Florilegium. In the early 19th century, the French botanist Charles François Antoine Morren wrote one of the first descriptions of the trigger plant anatomy. The number of species of Stylidium now stands at about 300, although quite a few of them are still awaiting description.

Photographed in Bolger Bay 2016
Page last updated 13th April 2019