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 mya. 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 12mya 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 (though they come in various shapes) and attach to the submerged stolons by slender stalks. 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