Bladderworts are likely to disappoint amateur botanists on nature excursions eager to see a carnivorous plant in action, “devouring” its victim. The trap in question, the bladder, is rarely larger than 3 mm in diameter, and often captures only microscopic prey. Moreover it acts underwater and, lastly, the whole process happens very quickly, in about 1/460 of a second. Without very special techniques, it is absolutely impossible to see the plant in action. What makes it so interesting, though, is to understand how its very refined trap mechanism works.
Slack (1979) estimates that there are about 250 species in this genus. Although most of them are aquatic, many are terrestrial. They grow in very wet soil, for they need water for the bladder to function. In tropical rainforests, some of them are epiphytic. Bladderworts have no roots. Where they are anchored to the substrate, it is often by rhizoids. The main stem bears fine, frequently divided stolons. Leaves, when they are present, are generally very narrow and difficult to distinguish from the stems.
Within the genus, the leaves present many modifications, depending on the species (Lloyd 1942). Their bladders appear to have originated with a modified leaf segment. Their inflorescence is also quite variable. The flowers, single or in clusters, are two-lipped. In Québec there are eight bladderwort species, seven of which are strictly aquatic, while Utricularia cornuta is found in marshy habitats.
The trap
The bladder resembles a bean flattened on the sides, attached to the plant by a small stalk connected to its underside. On the narrower part of the bladder is an opening closed by a roughly circular, flexible structure, of which the upper half circle forms the hinge. On the sides of the opening are often two ramified antennae forming a sort of tunnel, which guides the prey toward the opening. A few long hairs, or bristles, attached to the bottom of the door serve as triggers. In a resting state, the bladder has a globular shape with two convex sides. The door is held firmly closed and sealed by mucilage and an external membrane, the velum, pressed against the threshold.
The trap is set by glands that expel much of the water, deforming the bladder and drawing its side walls inward. The suction keeps the door in a precarious balance that is broken by the slightest contact with the trigger hairs. Peduncular glands on the door secrete sweet mucilage that appears to attract aquatic organisms ranging in size from microscopic protozoa to mosquito larvae. As soon as they brush against the trigger hairs, the door is immediately sucked inward.
Since Lloyd (1942), it has been generally recognized that the triggering of the door by the hairs is only a physical phenomenon. Nonetheless, some researchers believe that there is also some kind of electrical stimulus, like the one that triggers the two “jaws” of a Venus flytrap (Sydenam, 1973). Once the door opens, the animal is sucked into the bladder along with the water. And when the suction ceases, the bladder resumes its globular form and the door closes again. To reset the trap, the water inside is once again expelled by a system of glands, while other, enzyme-secreting, glands absorb the nutrients from the digested prey.
Bladderwort species
Purple-flowered species
Utricularia resupinata Greene
- Common name: Inverted bladderwort
- French name: Utriculaire résupinée
- Description: Delicate horizontal stem bearing linear leaves up to 3 cm long. It is distinguished from other bladderworts by a tube-shaped bract surrounding the flower stalk and by its odd-looking flowers, which are tipped backward and facing upward (resupinate).
- Habitat: Shallow water and along shorelines
- Distribution in Québec: Outaouais, Laurentians, Eastern Townships. Included on the list of rare plants in Québec (Bouchard et al., 1981 and 1983).
- Global distribution: Eastern North America, from Québec to Florida
Utricularia purpurea Walter
- Common name: Eastern purple bladderwort
- French name: Utriculaire pourpre
- Description: Submerged plant, often occurring in masses, with a long, leafless, brownish stem with whorled stolons. The flower stalk is held 7 to 10 cm above the water, bearing a reddish-purple flower with yellow markings at the base.
- Habitat: Stagnant water, boggy lakes
- Distribution in Québec: Laurentians only. Included on the list of rare plants in Québec (Bouchard et al. 1983).
Yellow-flowered species
Terrestrial plants
Utricularia cornuta Michaux
- Common name: Horned bladderwort
- French name: Utriculaire cornue
- Description: The 3 to 30 cm flower stalk bears a cluster of 1 to 5 flowers with spurs, forming the main structural element of the plant. There are only a few bladders, borne on a very small underground horizontal stem.
- Habitat: Wet peat bogs, sandy sites or wet muddy shorelines
- Distribution in Québec: Widespread
- Global distribution: North America
Aquatic plants
Utricularia geminiscapa Benjamin
- Common name: Hidden-fruit bladderwort
- French name: Utriculaire à scapes géminés
- Description: The 15 to 25 cm stem bears not only coloured flowers on its stalks but also another type of flowers that do not open (cleistoglamous), located mainly at the base of the stalk. The leaves, up to 2 cm long, are entire.
- Habitat: Ponds and slow-moving streams
- Distribution in Québec: Outaouais, Abitibi, Laurentians, Eastern Townships, Lower St. Lawrence and North Shore, but very sporadic (Haber, 1979). Included on the list of rare plants in Québec (Bouchard et al., 1981 and 1983).
- Global distribution: North America
Utricularia vulgaris Linnaeus
- Common name: Greater bladderwort
- French name: Utriculaire vulgaire
- Description: Horizontal stem 30 to 100 cm long, floating just beneath the surface, sometimes ramified. Highly divided leaves with toothed capillary segments. The stalk is inserted at the apex of the stolons, and bears 3 to 8 flowers.
- Habitat: Stagnant water, muddy streams, marshes, boggy lakes
- Distribution in Québec: Widespread
- Global distribution: North America, Europe, North Africa, Asia, Siberia
Aquatic plants with stems spreading on muck
Utricularia gibba Linnaeus
- Common name: Humped bladderwort
- French name: Utriculaire à bosse
- Description: Small plant with leaves with capillary, entire segments. 3 to 10 cm stalk bearing flowers with two almost equal lips.
- Habitat: Shallow and often boggy lakes
- Distribution in Québec: Outaouais, Mauricie, Laurentians, Eastern Townships, North Shore. Included on the list of rare plants in Québec (Bouchard et al., 1981 and 1983).
- Global distribution: North and Central America, Caribbean
Utricularia minor Linnaeus
- Common name: Lesser bladderwort
- French name: Utriculaire mineure
- Description: Small plant closely resembling U. gibba, distinguished mainly by its flowers, on which the lower lip is much longer than the upper one. Each leaf bears a bladder. The 1 to 11 cm stalk bears 2 to 7 flowers (generally 6).
- Habitat: Shorelines, shallow stagnant water
- Distribution in Québec: Widespread
- Global distribution: North America, Europe, Asia
Utricularia intermedia Hayne
- Common name: Flat-leaved bladderwort
- French name: Utriculaire intermédiaire
- Description: Stem with 2 types of stolons, one bearing bladders and the other with palmately divided, toothed leaves. The 10 to 20 cm stalk bears up to 6 flowers.
- Habitat: Shallow stagnant water
- Distribution in Québec: Widespread
- Global distribution: North America, Europe, Asia
References:
- Bouchard, A., D. Barabé, M. Dumais and S. Hay. "Liste préliminaire des plantes vasculaires rares du Québec", Bulletin de la SAJIB, vol. 6, no. 2, 1981, p. 44-48.
- Bouchard, A., D. Barabé, M. Dumais and S. Hay. Les plantes vasculaires rares du Québec, Syllogeus, no 48, 1983, 79 p.
- Haber, E. "Utricularia geminiscapa at Mer Bleue and Range Extensions in Eastern Canada", Can. Field Nat, vol. 93, 1979, p. 391-398.
- Lloyd, F.E. The Carnivorous Plants, Mass., Chronica Botanica Company, 1942, 352 p.
- Slack, A. Carnivorous Plants, Mass., MIT Press, 1979, 240 p.
- Sydenam, P.H. et G.P. Findlay. "The Rapid Movement of the Bladder of Utricularia sp.", Aust. J. Biol. Sci., vol. 26, 1973, p. 1115-1126.
