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 (a type of bromeliad) 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