Triggerplant
2007 Schools Wikipedia Selection. Related subjects: Plants
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 Curtis's Botanical Magazine print of Stylidium scandens.
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Triggerplants (also trigger plants) are a group of dicotyledonous plants that belong to the genus Stylidium (family Stylidiaceae). Most of the approximately 300 species are only found in Australia, making it the fifth largest genus in that country. Pollination is achieved through the use of the sensitive "trigger" (male and female reproductive organs fused into a floral column) that snaps forward quickly in response to touch, harmlessly covering the insect in pollen. Triggerplants can also be described as being protocarnivorous because the glandular trichomes that are just below the flower can trap and kill small insects, but cannot digest them.
Characteristics
The majority of the triggerplants 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.
The triggerplants are most easily identified by their unique floral column, in which the stamen and style are fused. The column--also called a "trigger" in this genus--typically resides beneath the plane of the flower. Triggerplant flowers are zygomorphic, which means they are only symmetrical in one plane. Flowers of triggerplants usually bloom in the late Spring in Australia.
Morphology
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 tall ( S. laricifolium). The typical plant form is a dense rosette of leaves close to the ground that gives rise to the floral spike in the centre. Atypical plant forms ranges from wiry, creeping mats ( S. scandens) to the bushy S. laricifolium.
Flower morphology differs in details, but ascribes to a simple blueprint: four petals ( S. hispidium has five), zygomorphic in nature, with the trigger protuding 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 wide flowers to the 2-3 cm wide flowers of S. schoenoides. Flower colour 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 again 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). One of the unusual specimens is S. scandens (Climbing triggerplant) in which the leaves have a modified tip in the shape of hook that helps it climb on top of surrounding vegetation.
Pollination mechanism
The column in triggerplants is sensitive and responds to touch. The change in pressure when a pollinating insect lands on a triggerplant 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 is essentially 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 triggerplants, completing its "attack" on the insect in 15 milliseconds. After firing, the trigger resets to its original position in anywhere from a few minutes to a half hour, depending on temperature and species-specific qualities. The trigger 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.
Carnivory
Members of the genus are considered to be protocarnivorous (or subcarnivorous). They have glandular trichomes projecting from sepals, leaves, flower parts, and the scape. The tip of the trichome produces a sticky muscilage--a mixture of sugar polymers and water--that is capable of attracting and suffocating small insects. Unlike true carnivorous species of plants, triggerplants are thought to not produce digestive enzymes and must rely on bacteria, fungi, or other decomposition processes to release the nutrients stored in the muscilage-bound prey, though more research must be done to support that hypothesis. The insects captured by the glandular trichomes are much too small to serve any role in pollination. It is unclear, however, whether these plants evolved the ability to trap and kill insects as an adaptation to low environmental nutrient availability or simply a defensive mechanism against insects damaging flower parts.
There is also a correlation between triggerplant location and proximity of known carnivorous species, like sundews (Drosera), bladderworts (Utricularia), the Albany pitcher plant (Cephalotus follicularis), and the rainbow plant (Byblis). While this alone does not prove that triggerplants are themselves carnivorous, the hypothesis is that the association arose because triggerplants and the known carnivorous plants obtain scarce nutrients using the same source (captured insects). Preliminary proof is given that the trapping mechanisms of two of the triggerplant-associated plants are the same (the tentacles of Byblis and Drosera), though this may be only a coincidence and further research must be done.
Distribution and habitat
Most species of triggerplants are endemic to Australia. In Western Australia alone, there are more than 150 species, at least 50 of which are in the area immediately around Perth. There are at least four species of Stylidium that are not confined to the Australian continent: S. tenellum is found in Myanmar, Melaka, and Tonkin; S. kunthii in Bengal and Myanmar; S. uliginosum in Queensland, Sri Lanka, and the south coast of China; and S. alsinoides in Northeast Australia, Queensland, and the Philippines. The cladistic group Stylidium contains more than 230 individual species (more than 300 species may exist, but many specimens have not yet been formally described), making it the fifth largest genus in Australia.
Triggerplant habitat includes grassy plains, open heaths, rocky slopes, sandplains, forests, and the margins of creeks and water holes. Somes species, such as S. eglandulosum, can even be found in disturbed areas like near roads and under powerlines. Others (i.e. S. coronmiforme) are sensitive to disturbance and are considered rare because of their extremely specific habitat.
Botanical history
Discovery and description of new Stylidium species has been occurring since the late 18th century, the first of which was discovered in Botany Bay in 1770 and described by Joseph Banks and Daniel Solander. Later, in the early 19th century, the French botanist Charles Morren wrote a monograph on the triggerplant anatomy, illustrated by many botanical artists including Ferdinand Bauer. Around the same time, British botanist Robert Brown described (or "authored") several Stylidium species, including S. adnatum and S. repens. More species began to be described as more botanists explored Australia more thoroughly.
In 1958, Rica Erickson wrote Triggerplants, describing habitat, distribution, and plant forms ( ephemeral, creeping, leafy-stemmed, rosette, tufted, scale-leaved, and tropical). It was Erickson that began placing certain species into these morphologically-based groups, which may or may not resemble true taxonomic divergences. It was not until the 1970s and 1980s that research of the trigger physiology was begun in the lab of Dr. Findlay of Flinders University. Douglas Darnowski added to the growing library of knowledge on triggerplants when he published his book Triggerplants in 2002, describing an overview of habitat, plant morphology, carnivory, and research done to date. Following its publication, he attempted to assist in the organization of the International Triggerplant Society, which has not been formally created yet.
As of 2002, only 221 Stylidium species were known. There are now over 300 species, many of which are awaiting formal description.
Cultivation
Most triggerplants tend to be hardy species and can be easily cultivated in greenhouses or gardens. They are drought resistant, hardy to cold weather, and the species diversity in this genus gives gardeners a wide variety of choices. Most species that are native to Western Australia will be cold hardy to at least -1 to -2°C. The few that can be found all over Australia, like S. graminifolium, will tolerate a wider range of habitat since their native ranges includes a great diversity of ecoregions. Some species of triggerplants are suitable for cultivation outdoors outside of the Australian continent including most of the United Kingdom and as far north as New York City or Seattle in the United States.
Cultivation from seed may be difficult or easy, depending on the species. The more difficult species to grow include the ones that require a period of dormancy or smoke treatment to simulate a bushfire. Triggerplants should be grown in a medium that is kept moist and has a relatively low concentration of nutrients. It should also be noted that triggerplants appear to be sensitive to disturbance of their root systems. Minimization of such disturbance will likely result in healthier plants.