| Stylidium | |
|---|---|
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| Flowers ofStylidium graminifolium | |
Scientific classification | |
| Kingdom: | Plantae |
| Clade: | Tracheophytes |
| Clade: | Angiosperms |
| Clade: | Eudicots |
| Clade: | Asterids |
| Order: | Asterales |
| Family: | Stylidiaceae |
| Subfamily: | Stylidioideae |
| Genus: | Stylidium Sw. |
| Species | |
Seeseparate list. | |
| Synonyms | |
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Stylidium (thetriggerplants ortrigger plants) is a genus ofdicotyledonousplants that belong to the familyStylidiaceae. The genus nameStylidium is derived from the Greekστύλος orstylos (column or pillar), which refers to the distinctive reproductive structure that its flowers possess.[1] Pollination is achieved through the use of the sensitive "trigger", which comprises the male and female reproductive organs fused into afloral column that snaps forward quickly in response to touch, harmlessly covering the insect in pollen. Most of the approximately 300species are only found inAustralia, making it the fifth largest genus in that country. Triggerplants are considered to beprotocarnivorous orcarnivorous because the glandulartrichomes that cover thescape and flower can trap, kill, and digest small insects withprotease enzymes produced by the plant. Recent research has raised questions as to the status of protocarnivory withinStylidium.[2]
The majority of theStylidium species areperennial herbs of which some aregeophytes that utilizebulbs as their storage organ. The remaining small group of species consists ofephemeralannuals.[3]
Members of the genus are most easily identified by their uniquefloral column, in which thestamen andstyle are fused. The column—also commonly called a "trigger" in this genus—typically resides beneath the plane of the flower.Stylidium flowers arezygomorphic, which means they are only symmetrical in one plane.[4] Flowers usually bloom in the late spring in Australia.[5]
Species of the genusStylidium 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 denserosette of leaves close to the ground that gives rise to thefloralspike in the center. Plant forms range from wiry, creeping mats (S. scandens) to the bushyS. laricifolium.[5][6]
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 ofS. 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 denseraceme, but there is at least one exception to the rule:S. uniflorum, as its name suggests, produces a single flower perinflorescence.[6]
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 asS. scandens (climbing triggerplant) form scrambling, tangled mats typically propped up onaerial roots.[6]
The column typical of the genusStylidium is sensitive and responds to touch. The change in pressure when a pollinating insect lands on aStylidium flower causes a physiological change in the columnturgor pressure by way of anaction potential, sending the column quickly flying toward the insect.[7] 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 andstigma take turns in dominating the function of the column—theanthers develop first and then are pushed aside by the developing stigma. This delayed development of the stigma preventsself-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).[5][8]
The response to touch is very quick inStylidium 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.[9]Stylidium species are typically pollinated by small solitary bees and the nectar-feeding bee flies (Bombyliidae).[10]
Stylidium species with glandulartrichomes on theirsepals, leaves, flower parts, orscapes have been suggested to beprotocarnivorous (or paracarnivorous). The tip of the trichome produces a stickymucilage—a mixture of sugar polymers and water—that is capable of attracting and suffocating small insects.[6] The ability to trap insects may be a defensive mechanism against damage to flower parts. However, trichomes ofS. fimbriatum have been shown to produce digestive enzymes, specificallyproteases, like othercarnivorous plants. Adding species ofStylidium to the list of plants that engage in carnivory would significantly increase the total number of known carnivorous plants.[11]
The insects captured by the glandular trichomes are 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.[6]
There is also a correlation between location ofStylidium species and proximity of known carnivorous species, likesundews (Drosera),bladderworts (Utricularia), theAlbany pitcher plant (Cephalotus follicularis), and therainbow plant (Byblis). While this alone does not prove thatStylidium species are themselves carnivorous, the hypothesis is that the association arose becauseStylidium species and the known carnivorous plants obtain scarce nutrients using the same source, namely captured insects. Preliminary proof is given that the trapping mechanisms of two associated plants are the same (thetentacles ofByblis andDrosera), though this may be only a coincidence and further research must be done.[6] Recent research has raised questions as to the status of protocarnivory withinStylidium.[2]
MostStylidium species areendemic to Australia. InWestern Australia alone, there are more than 150 species, at least 50 of which are in the area immediately aroundPerth. There are at least four species ofStylidium that are not confined to the Australian continent:S. tenellum is found inMyanmar,Melaka, andTonkin;S. kunthii inBengal and Myanmar;S. uliginosum inQueensland,Sri Lanka, and the south coast ofChina; andS. alsinoides in Queensland and thePhilippines. The cladistic groupStylidium contains more than 230 individual species (more than 300 species exist, but many specimens have not yet been formally described),[12] making it the fifth largest genus in Australia.[3]
Stylidium habitat includes grassy plains, openheaths, rocky slopes, sandplains, forests, and the margins of creeks and water holes.[5] Somes species, such asS. eglandulosum, can even be found in disturbed areas like near roads and under powerlines. Others (i.e.S. coroniforme) are sensitive to disturbance and are considered rare because of their extremely specific habitat.[6]
Even though many species ofStylidium may coexist in the same location, natural hybrids between species have not often been reported. Both natural hybridisation in the field and artificial hybridisation in cultivation are rare.[10] The first natural hybrid,S. petiolare ×S. pulchellum, was reported bySherwin Carlquist in 1969 betweenCapel andBoyanup in Western Australia.[13]
Discovery and description of newStylidium species has been occurring since the late 18th century, the first of which was discovered inBotany Bay in 1770 byJoseph Banks andDaniel Solander during their travels in the Pacific withJames Cook aboard theEndeavour.[12] Seven species were collected by Banks and Solander, some of which were sketched bySydney Parkinson on board the Endeavour and were later engraved in preparation for publication inBanks' Florilegium. Later, in the early 19th century, the FrenchbotanistCharles François Antoine Morren wrote one of the first descriptions of the triggerplant anatomy, illustrated by many botanical artists includingFerdinand Bauer. Around the same time, British botanistRobert Brown described (or "authored") severalStylidium species, includingS. adnatum andS. repens. More species began to be described as more botanists explored Australia more thoroughly.
In 1958,Rica Erickson wroteTriggerplants, 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 thesemorphologically-based groups, which may or may not resemble truetaxonomic divergences. It was not until the 1970s and 1980s that research of the triggerphysiology was begun in the lab of Dr. Findlay ofFlinders University. Douglas Darnowski added to the growing library of knowledge onStylidium when he published his bookTriggerplants in 2002, describing an overview of habitat, plant morphology, carnivory, and research done to date. Following its publication, he co-founded the International Triggerplant Society.[14]
As of 2002, only 221Stylidium species were known.[15] There are nowover 300 species, many of which are awaiting formal description.
MostStylidium species 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, likeS. graminifolium, will tolerate a wider range of habitat since their native ranges includes a great diversity ofecoregions. Some species of triggerplants are suitable for cultivation outdoors outside of the Australian continent including most of theUnited Kingdom and as far north asNew York City orSeattle in theUnited States.[6]
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 abushfire.Stylidium specimens should be grown in a medium that is kept moist and has a relatively low concentration of nutrients. They appear to be sensitive to disturbance of their root systems. Minimization of such disturbance will likely result in healthier plants.[6]
Media related toStylidium at Wikimedia Commons