The bladderworts are perennial or annual, consisting of long branching stems bearing numerous, tiny, balloon-like sacs -- or bladders -- for which the common name was given. The genus name Utricularia is derived from the Latin word utriculus also referring to a "small bag".

The bladderworts present a rather unique morphology. First of all, the plants are entirely rootless -- completely giving up the normal plant way of obtaining nutrients from the root system. Also, the distinction between stem and leaf is often vague, especially in the aquatic species. The trapping mechanism, the bladder, is a modified leaf or a leaf division morphologically, in general conformity with all trapping structures found in carnivorous plants of other genera. The inner surface of the trap (bladder) corresponds to the upper surface of the leaf division that it represents.

The terrestrial species extends its white stems in the damp soils from which arise green leaves and slender flower stems above the ground. Numerous white bladders are attached to the stem. In aquatic species, branching stems and bladders are also greenish, indicating photosynthetic in function, and the leaves are often feathery and thread-like. During the growing season, aquatic species float near the surface of the still waters with only the flower scapes protruding above the water surface.

The flowers are generally quite colorful and showy for both terrestrial and aquatic species, especially when seen in masses. During the flowering, which occurs from spring to late summer in the U.S. species, one often finds the ponds covered with bright yellow or purple corollas. This seems to be the only time these small , obscure plants choose to announce their existence to the rest of the world. Yellow is the most prevalent flower color for this genus though white to purple or bluish flowers are also common often with yellow or reddish markings.

Although most of the U.S. species are relatively small and often less noticeable than other carnivorous plant species, sometimes even in flower, some South American terrestrial species are massive in size, with leaves reaching one foot or more in length, with their flower scapes attaining the height of 1m. Some of these flowers compete with those of orchids in their beauty.

Branching stems bear numerous sacs which range in size from 5mm at the largest end to a microscopic 0.3mm. These sacs are highly sophisticated mechanical traps with self-resetting mechanism capable of catching tiny water animals with amazing efficacy. The typical prey for these miniature traps includes insect larvae (esp. those of mosquitoes), aquatic worms, water ticks, and other tiny swimmers sharing the same habitat.

The basic structure and function of the trap are common for all species of bladderworts, both terrestrial and aquatic. Each trap has some antenna-like hairs on one side of the trap opposite the attaching stem. These hairs are not irritable (non-sensitive) and are considered ornamental in nature, contributing to attract tiny animal prey to the trap entrance located just below the base of the hairs. The hairs may also serve to protect the entrance from flowing debris in the water. The lower half of the entrance is the semi-circular valve -- or a door -- hinged at the upper semi-circular arc, with the free edge of the door tightly sealed in contact with a firm collar of the lower opening of the entrance, called threshold. The door opens only inwardly. When the trap is set, with the door sealed watertight, the pressure inside the trap is kept lower than the outside. This happens because the water is constantly being pumped out of the trap interior by glands scattered all over the trap wall. Because of this pressure differential, when the trap is viewed from above, the walls are warped inwards and appear concave.

On the lower part of the outer surface of the door grow tiny, stiff hairs. These hairs function as trigger levers. When a small water animals, probably seeking a shelter in the bladderwort jungle or perhaps lured by nectar secretion, touches one of these levers, a delicate mechanical latch of the door is broken. The door, giving in to the outside water pressure, swings open inwardly, causing the water animal to be sucked into the trap along with a bladderful of rushing water. The elastic trap bulges with an in-rush of water, with the side walls of the trap popping up in a convex shape. The door swings shut in a fraction of a second, closing the entrance once again. All this happens in a fraction of a second -- an astonishing 1/30 to 1/40 of a second. Once trapped inside, there is no hope left for the prey.

Over a period of thirty minutes to an hour, the trap mechanism is automatically reset in preparation for the next catch. This resetting is the result of continuous pumping of water out of the trap interior. On the inner surface of the trap are found numerous glands with four projections known as quadrifids. These quadrifids are believed to be responsible for absorbing water from trap interior. Tiny spherical glands seen on the outer surface of the trap are known to engage in active transport of ions from the trap interior. The osmotic pressure (gradient) built up by the ion movement generates the outward flow of water from the trap interior.

Digestion

During a period of several days, the trapped animals are digested and absorbed by quadrifids to be carried away to the rest of the plants. The digestive enzymes, believed to be secreted from the quadrifids, can be detected inside the trap -- at least in the younger traps. After the first prey is captured, the bacterial actions are seen to dominate in the digestive process.

In an animal-rich environment, it is not unusual to observe each trap capturing several water animals. This fills the trap completely and makes the trap colored dark.

As we have seen, the water tightness is essential for the function of suction-trap mechanism of the bladderworts. Let us take a look at the mechanical subtlety of this structure. Referring to this elaborate structure of the trap, F. E. Lloyd, who contributed immensely to our present knowledge of trap structure, comments, " ... But most to be wondered at are the traps which present an astonishing degree of mechanical delicacy depending on a fineness of structure scarcely equaled elsewhere in the plant kingdom."

The surface of the threshold -- against which the door edge rests -- is covered with a pavement epithelium of sessile glands secreting mucilage. There is a slight depression on the middle of the pavement where the cells are most densely packed. The middle of the free edge of the door -- which is strengthened with by dense cells to make a firm edge -- rests in the pavement depression. This slight change in door posture when the trap becomes fully set is reflected in the trigger lever position which is now more erect. Note that only the center of the door edge impinges tightly on the pavement depression, with the other portions of the free edge of the door merely lying flat against the pavement, leaving chinks through which water can enter. This water leakage is prevented by the cuticular membrane attached to the outer edge of the pavement, running completely across the threshold. This thin but firm membranous tissue is called velum, which serves as the second valve of the trap entrance, covering the outer edge of the free margin of the door. That is, when the door swings back right after springing the trap, it pushes against the velum. Mucilage secreted from the stalked glands on the threshold near the door rest also helps to further seal the door from water leakage. This keeps the door water-tight against the increasing outside pressure as the water is continuously being pumped out from within the trap. As long as this delicate mechanical balance of the door latch is undisturbed, the door remains sealed in spite of the amounting pressure outside.

If a tiny water animal touches the tip of the trigger lever, the door edge -- by a lever action -- is pulled out from the pavement depression, thus unlatching the door lock. Giving in to the outside water pressure, the door flaps open inwardly. As expected from its mechanical structure, a downward push of the lever is most effective in triggering the trap.

Besides pumping of the water by wall glands -- which is physiological in nature -- setting, tripping, and resetting of the trap are purely mechanical phenomena, unrelated to growth movement. Therefore, one bladder can repeat the trapping action without any biological growth limitation. Some observer counted 14 times of resetting of the bladderwort trap and this is not the limit.

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