Utricularia
Utricularia, commonly and collectively called the bladderworts, is a genus of carnivorous plants consisting of approximately 233 species. They occur in freshwater and wet soil as terrestrial or aquatic species across every continent except Antarctica. Utricularia are cultivated for their flowers, which are often compared with those of snapdragons and orchids, especially amongst carnivorous plant enthusiasts.
All Utricularia are carnivorous and capture small organisms by means of bladder-like traps. Terrestrial species tend to have tiny traps that feed on minute prey such as protozoa and rotifers swimming in water-saturated soil. The traps can range in size from. Aquatic species, such as U. vulgaris, possess bladders that are usually larger and can feed on more substantial prey such as water fleas, nematodes and even fish fry, mosquito larvae and young tadpoles. Despite their small size, the traps are extremely sophisticated. In the active traps of the aquatic species, prey brush against trigger hairs connected to the trapdoor. The bladder, when "set", is under negative pressure in relation to its environment so that when the trapdoor is mechanically triggered, the prey, along with the water surrounding it, is sucked into the bladder. Once the bladder is full of water, the door closes again, the whole process taking only ten to fifteen milliseconds.
Bladderworts are unusual and highly specialized plants, and the vegetative organs are not clearly separated into roots, leaves, and stems as in most other angiosperms. Utricularia lack a root system.
Bladder traps are recognized as one of the most sophisticated structures in the plant kingdom.
Description
The main part of a bladderwort plant always lies beneath the surface of its substrate. Terrestrial species sometimes produce a few photosynthetic leaf-shoots. The aquatic species can be observed below the surfaces of ponds and streams.Plant structure
Most species form long, thin, sometimes branching stems or stolons beneath the surface of their substrate, whether that be pond water or dripping moss in the canopy of a tropical rainforest. To these stolons are attached both the bladder traps and photosynthetic leaf-shoots, and in terrestrial species the shoots are thrust upward through the soil into the air or along the surface.The name bladderwort refers to the bladder-like traps. The aquatic members of the genus have the largest and most obvious bladders, and these were initially thought to be flotation devices before their carnivorous nature was discovered.
Etymology
The generic name Utricularia is derived from the Latin utriculus, a word which has many related meanings but which most commonly means wine flask, leather bottle or bagpipe.Flowers
Flowers are the only part of the plant clear of the underlying soil or water. They are usually produced at the end of thin, often vertical inflorescences. They can range in size from wide, and have two asymmetric labiate petals, the lower usually significantly larger than the upper. They can be of any colour, or of many colours, and are similar in structure to the flowers of a related carnivorous genus, Pinguicula.The flowers of aquatic varieties like U. vulgaris are often described as similar to small yellow snapdragons, and the Australian species U. dichotoma can produce the effect of a field full of violets on nodding stems. The epiphytic species of South America, however, are generally considered to have the showiest, as well as the largest, flowers. It is these species that are frequently compared with orchids.
Certain plants in particular seasons might produce closed, self-pollinating flowers; but the same plant or species might produce open, insect-pollinated flowers elsewhere or at a different time of year, and with no obvious pattern. Sometimes, individual plants have both types of flower at the same time: aquatic species such as U. dimorphantha and U. geminiscapa, for example, usually have open flowers riding clear of the water and one or more closed, self-pollinating flowers beneath the water. Seeds are numerous and small and for the majority of species are long.
Distribution and habitat
Utricularia can survive almost anywhere where there is fresh water for at least part of the year; only Antarctica and some oceanic islands have no native species. The greatest species diversity for the genus is seen in South America, with Australia coming a close second. In common with most carnivorous plants, they grow in moist soils which are poor in dissolved minerals, where their carnivorous nature gives them a competitive advantage; terrestrial varieties of Utricularia can frequently be found alongside representatives of the carnivorous genera–Sarracenia, Drosera and others–in very wet areas where continuously moving water removes most soluble minerals from the soil.Utricularia has a variety of life forms, including terrestrial, lithophytic, aquatic, epiphytic, and rheophytic forms which are all highly adapted for their environments.
About 80% of the species are terrestrial, and most inhabit waterlogged or wet soils, where their tiny bladders can be permanently exposed to water in the substrate. Frequently they will be found in marshy areas where the water table is very close to the surface. Most of the terrestrial species are tropical, although they occur worldwide.
Approximately 20% of the species are aquatic. Most of these drift freely over the surface of ponds and other still, muddy-bottomed waters and only protrude above the surface when flowering, although a few species are lithophytic and adapted to rapidly moving streams or even waterfalls. The plants are usually found in acidic waters, but they are quite capable of growing in alkaline waters and would very likely do so were it not for the higher level of competition from other plants in such areas. Aquatic Utricularia are often split into two categories: suspended and affixed aquatic. Suspended aquatics are species which are not rooted into the ground and are free-floating, often found in nutrient poor sites. Conversely, fixed aquatics are species which have at least some of their shoots rooted into the ground. These plants often have dimorphic shoots, some which are leafy, green, and often bladderless which float in the water, and others which are white and coated with bladders that affix the plant to the ground. Utricularia vulgaris is an aquatic species and grows into branching rafts with individual stolons up to one metre or longer in ponds and ditches throughout Eurasia.
Some South American tropical species are epiphytes, and can be found growing in wet moss and spongy bark on trees in rainforests, or even in the watery leaf-rosettes of other epiphytes such as various Tillandsia species. Epiphytic Utricularia are often known for their orchid-like flowers and are the most ornamentally sought after. Rosette-forming epiphytes such as U. nelumbifolia put out runners, searching for other nearby bromeliads to colonise.
There are also a few lithophytic species which live on wet surfaces of cliffs and mossy rocks and rheophytic species which live in shallow rivers and streams.
The plants are as highly adapted in their methods of surviving seasonally inclement conditions as they are in their structure and feeding habits. Temperate perennials can require a winter period in which they die back each year, and they will weaken in cultivation if they are not given it; tropical and warm-temperate species, on the other hand, require no dormancy. Floating bladderworts in cold temperate zones such as the UK and Siberia can produce winter buds called turions at the extremities of their stems: as the autumnal light fails and growth slows down, the main plant may rot away or be killed by freezing conditions, but the turions will separate and sink to the bottom of the pond to rest beneath the coming ice until the spring, when they will return to the surface and resume growth. Many Australian species will grow only during the wet season, reducing themselves to tubers only long to wait out the dry season. Other species are annual, returning from seed each year.
Dispersal and life form evolution
The ancestral line of Utricularia is thought to have been terrestrial. From terrestrial forms, epiphytic forms evolved independently three times and aquatic life forms arose four times in genus Utricularia.Biogeographic patterns associated with the boreotropic hypothesis lists the origin of Lentibulariaceae to temperate Eurasia or tropical America. Based on fossilised pollen and insular separation, the last common ancestor of Genlisea–''Utricularia clade was found to be a South American lineage that arose 39. Utricularia probably diverged from its sister genus 30 mya and subsequently dispersed to Australia, represented by subgenus Polypompholyx, and to Africa. There were most likely other transcontinental dispersals, one of which is represented by sect. Nelipus. The colonization of Utricularia to North America probably occurred 12 mya from South America. The dispersal of Utricularia'' to Eurasia probably occurred through the Bering Strait via long-distance dispersal 4.7 mya.
Carnivory
Physical description of the trap
Authorities on the genus, such as botanists Peter Taylor and Francis Ernest Lloyd, agree that the vacuum-driven bladders of Utricularia are the most sophisticated carnivorous trapping mechanism to be found anywhere in the plant kingdom. The bladders are usually shaped similarly to broad beans and attach to the submerged stolons by slender stalks.Bladders are hollow underwater suction cups, also known as utricles, that possess a valve with bristles that open and close.
The bladder walls are very thin and transparent but are sufficiently inflexible to maintain the bladder's shape despite the vacuum created within. The entrance, or 'mouth', of the trap is a circular or oval flap whose upper half is joined to the body of the trap by very flexible, yielding cells which form an effective hinge. The door rests on a platform formed by the thickening of the bladder wall immediately underneath. A soft but substantial membrane called the velum stretches in a curve around the middle of this platform, and helps seal the door. A second band of springy cells crosses the door just above its lower edge and provides the flexibility for the bottom of the door to become a bendable 'lip' which can make a perfect seal with the velum.
The outer cells of the whole trap excrete mucilage and under the door, this is produced in greater quantities and contains sugars. The mucilage certainly contributes towards the seal, and the sugars may help to attract prey.
Terrestrial species, like U. sandersonii have tiny traps with a broad beak-like structure extending and curving down over the entrance; this forms a passageway to the trapdoor and may help prevent the trapping and ingestion of inorganic particles. Aquatic species, like U. inflata tend to have larger bladders—up to —and the mouth of the trap is usually surrounded not by a beak but by branching antennae, which serve both to guide prey animals to the trap's entrance and to fend the trap mouth away from larger bodies which might trigger the mechanism needlessly. Epiphytic species have unbranched antennae which curve in front of the mouth and probably serve the same purpose, although it has been observed that they are also capable of holding a pocket of water in front of the mouth by capillary action, and that this assists with the trapping action.