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".

Morphologically, the arrangement of the vegetative organs of Utricularia is quite peculiar in view of the general norm of the  flowering plants. Following Taylor (1989), the general vegetative morphology of Utricularia is described as follows:

Utricularia does not have roots but in many (terrestrial) species structures that resemble and function as roots exist. These organs are termed rhizoids (Taylor 1989). A vertical stem is present only in a few species. In the majority of species, horizontal shoots called stolons form the framework of the vegetative part of the plant (Taylor). Some aquatic species produce air shoots, arising from the stolons. The true function of this structure is not well understood.

Utricularia has the green, flat, sometimes feather-like organs that perform photosynthesis. There are some disputes as to what to call them (whether these organs should properly be called leaves). "It seems reasonable to me", Taylor (1989) writes, "that plants which have, in defiance of the general rule, no radicle or roots and no true cotyledons, may be allowed to have leaves which also disobey the rules... "

In some aquatic species, a dense cluster of leaves is formed at the growth point of the stolon at the end of the growing season. This is (called) a turion, a term for hiberncula (winter buds) often used for aquatic species. Turions are resistant to low temperature and desiccation.

The traps of all known carnivorous plants are considered (to be) a modified leaf, morphologically speaking, with Utricularia being the only exception.

Positioning of the traps in Utricularia is highly diverse in relation to the other vegetative organs of the plant, and this offers significant taxonomic characters (Taylor).

In some species the traps arise only from the stem or the peduncle base, while in some others the trap appears at the tip of the leaf. In terrestrial species, the traps may arise on leaves, stolons, and also on rhizoids. In many aquatic species, the traps are formed laterally on the leaves or leaf segments (Taylor).  

Inflorescence

Inflorescence is always racemorse (Taylor 1989), meaning the flower scape.....

Corolla is always two-lipped. (bilabiate) without an obvious tube (unlike Pinguicula). The upper lip is small er than the lower. The lower lip of the corolla always terminates in a spur, as in Pinguicula. The expanded part (limb) of the lower lip has (at its base) a palate. The front end of the lower lip ??? is either entire or lobed, depending on the species.

The calyx of Utricularia is always two-lobed, except in subgenus Polypompholyx which has a four-lobed calyx. The calyx lobes, in many cases, provide useful taxonomic characters in species where the corolla morphology is often variable (Taylor). In cases where corolla shapes have a high degree of variation, the calyx lobes are, of all the floral organs, probably the most useful in providing taxonomic character, according to Taylor (1989).

Many species produce a reduced-size flower that self-pollinates without opening. This form of flower is called cleistogamous. In some species, cleistogamous flowers coexist with normal, open, chasmogamous flowers. In a terrestrial U.S. species, Utricularia subulata, cleistogamy is quite common. In an aquatic species Utricularia geminiscapa, a short-stemmed cleistogamous flower is produced underwater along with an open, chasmogamous flower that protrudes above the water surface.   

The bladder interior ?? contains two kinds of glands, bifids (two-armed glands) and quadrifids (four-armed glands).... the relative disposition of the arms of the quadrifids provides diagnostic taxonomic character...

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 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 (1 m??) or more in length. Their flower scapes may attain the height of 1m. Some of these flowers compete with those of orchids in their beauty and elegance.

The traps range in size from 5 mm at the largest end to a microscopic 0.3 mm. These sacs are highly sophisticated mechanical traps capable of catching tiny water animals with amazing efficacy. The traps come complete with a self-resetting mechanism  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 traps are common for all species of bladderworts, though there are some clear differences between terrestrial and aquatic species. Each trap has some antenna-like hairs on one side of the trap opposite the attaching stem. These hairs are non-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 a 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 the 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 a trigger lever. When a small water animal, 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. Giving in to the outside water pressure, the door 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 normal convex shape. The door swings shut in a fraction of a second, closing the entrance once again. All this happens in 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 set again in preparation for the next catch. This resetting is the result of continuous pumping of water out of the trap interior.

Water Pumping Mechanism

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 the 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, trapped animals are digested and absorbed by quadrifids, and the nutrients are carried away to the rest of the plant. The digestive enzymes, believed to be secreted from the quadrifids, have been detected inside the trap -- at least in younger traps. After the first prey is captured, the bacterial actions are seen to dominate in the digestive process and the trap often appears dark purple in color. In an animal-rich environment, it is not unusual for each trap to capture several water animals, filling the tiny trap completely.

As we have seen, the water tightness is essential for the proper function of the suction-trap mechanism of the bladderworts. Let us take a look at the mechanical subtlety of this structure. Referring to this elaborate trap structure, F. E. Lloyd, who contributed immensely to our present knowledge of the trap mechanism, comments, " ... But most to be wondered at are the traps which present an astounding 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. A slight change in the 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 the 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 seal the door further to prevent 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 mounting pressure outside.

Suppose a tiny water animal touches the tip of a trigger lever. By the lever action, the door edge 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, the downward push of the lever is most effective in triggering the trap.

Besides pumping of water by the 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.