Stylidium

Stylidium (also known as triggerplants or trigger plants) is a genus of dicotyledonous plants that belong to the family Stylidiaceae. The genus name Stylidium is derived from the Greek στύλος or stylos (column or pillar), which refers to the distinctive reproductive structure that its flowers possess. Pollination is achieved through the use of the sensitive 'trigger', which comprises the male and female reproductive organs fused into a floral column that snaps forward quickly in response to touch, harmlessly covering the insect in pollen. Most of the approximately 300 species are only found in Australia, making it the fifth largest genus in that country. Triggerplants are considered to be protocarnivorous or carnivorous because the glandular trichomes that cover the scape and flower can trap, kill, and digest small insects with protease enzymes produced by the plant. Recent research has raised questions as to the status of protocarnivory within Stylidium. The majority of the Stylidium species are perennial herbs of which some are geophytes that utilize bulbs as their storage organ. The remaining small group of species consists of ephemeral annuals. Members of the genus are most easily identified by their unique floral column, in which the stamen and style are fused. The column—also commonly called a 'trigger' in this genus—typically resides beneath the plane of the flower. Stylidium flowers are zygomorphic, which means they are only symmetrical in one plane. Flowers usually bloom in the late spring in Australia. Species of the genus Stylidium represent a very diverse selection of plants. Some are only a few centimeters tall, while others can grow to be 1.8 meters (5.9 feet) tall (S. laricifolium). One typical plant form is a dense rosette of leaves close to the ground that gives rise to the floral spike in the center. Plant forms range from wiry, creeping mats (S. scandens) to the bushy S. laricifolium. Flower morphology differs in details, but ascribes to a simple blueprint: four petals, zygomorphic in nature, with the trigger protruding from the 'throat' of the flower and resting below the plane of the flower petals. Flower size ranges from many species that have small 0.5 cm (0.20 in) wide flowers to the 2–3 cm (1–1 in) wide flowers of S. schoenoides. Flower color can also vary from species to species, but most include some combination of white, cream, yellow, or pink. Flowers are usually arranged in a spike or dense raceme, but there is at least one exception to the rule: S. uniflorum, as its name suggests, produces a single flower per inflorescence. Leaf morphology is also very diverse in this large genus. Some leaves are very thin, almost needle-like (S. affine), while others are short, stubby, and arranged in rosettes (S. pulviniforme). Another group of species, such as S. scandens (climbing triggerplant) form scrambling, tangled mats typically propped up on aerial roots. The column typical of the genus Stylidium is sensitive and responds to touch. The change in pressure when a pollinating insect lands on a Stylidium flower causes a physiological change in the column turgor pressure by way of an action potential, sending the column quickly flying toward the insect. Upon impact, the insect will be covered in pollen and stunned, but not harmed. Because the column comprises the fused male and female reproductive organs of the flower, the stamen and stigma take turns in dominating the function of the column—the anthers develop first and then are pushed aside by the developing stigma. This delayed development of the stigma prevents self-pollination and ensures that cross pollination will occur between individuals of a population. Different species have evolved the trigger mechanism in different locations, with some attacking the pollinating insect from above and others from below (a 'punch in the gut' to the insect). The response to touch is very quick in Stylidium species. The column can complete its 'attack' on the insect in as little as 15 milliseconds. After firing, the column resets to its original position in anywhere from a few minutes to a half hour, depending on temperature and species-specific qualities. The column is able to fire many times before it no longer responds to stimuli. The response time is highly dependent upon ambient temperature, with lower temperatures relating to slower movement. Stylidium species are typically pollinated by small solitary bees and the nectar-feeding bee flies (Bombyliidae).