Carnivorous Plants Website
Carnivorous Plants in the Wilderness
by Makoto Honda
Carnivorous Plants Story  

  

 

 

6.  Bladderworts   GENUS Utricularia                     Back to Contents

 

General

In the ponds and swamps, one may find floating, rootless plants with hundreds of tiny, balloon-like sacs attached to their green branching stems. They are bladderworts, yet another kind of carnivorous plants. These sacs in the water are sophisticated miniature traps designed to provide animal meals for these rootless floaters.There are about 214 species of bladderworts worldwide, occurring in practically every part of the globe except in arid regions and Antarctica. Some are terrestrial species found in moist-to-wet, often acid soils, and in sphagnum moss ?? , while others are aquatic, preferring to be floating freely in quiet waters in bogs and lakes. Many terrestrial species in the tropics are epiphytic. Some species exhibit an intermediate lifestyle, capable of adapting to either terrestrial or semi-aquatic habitats depending on the amount of rainfall of the season. Of all the bladderworts in the world, terrestrial species, by far, are the majority. (about 80%) Thirty-some species of bladderworts are found in the continental U.S.

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.

Inflorescence

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.

 

Suction Trap

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.

Mechanical Delicacy

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.

 

 

One hour or so later, another water creature appeared in the scene. No doubt the same bladder has already been reset once again for another catch.

Aa long as there is a room left i.n the bladder, the trap continues to reset itaelf and capture prey. Trapped creature'j inside the bladder are gradually digested --- which takes a week or so "- and the nutrients are absorbed through the bladder walls and are used for various growth and ireproductive activities of the plants.
 

I wiia looking through tha
vl-ew-findei" of my camera.
A ti-ny water creature
appeared in the view.After
moving around the bluildaru.
the unwary swi-ininer approached
one of the traps.

The watar creature came very g;

close to the trap entrance , j^
;uid, .InBtu.ntly, it diaappyured |p
ft'om the scene --- or ao i.t ^

..ippeared to the naked eye. gw

^-

a,npty a mointeirl ugo.

A stand of ynllo-flowBr butler- llndnrwn tpr l,r;ip'< .ir-r:' ;ilao r'\l"i?rrr'tl l.o
worts in a Florida pond ( U. ni blia) a", bladders. Each bladder m",isur°s 2
Thin HpflciRn hfin worldwide din- mm acroBa in this npfclHf). (1). glhba)
tribution.

 

 

Somo aquaUc bl qrlrlprworhs producf "n.ont''!" in ord'"" to n upper-I: now")" nt.fm"
rininff: above 'the wntar' aurrncR. Tho pinnta nbo'/s nrs nnl.lvft to th" nnut.h-
firintprn U.S. Thp n tnr-ohnpnfl {"\.nah [nnnaiirB 10 om In (Unmctcr, Tn OI(P ('"nnlton
fiwnmp In OnorRin, Mfiy. (U, lnriftl.,i)

Another nqunt.i.c appc IBH ti-., ^-l.;^'^''''^ '^\^'sl^:•^:"^Y•i^S^^<;^i'A$y;l^/'''^t'!'^;^w'-^'^^?;';';^;'^^ -"
wl th purp1 " riownrn, In i*'^1^^^^ "^IL^.^,.^'l^y;lfrK;';:-i!^i;<i^^';;/'•'v•,^l.^:•^^
North CnroUnn i Mny, F-C^K: ^Sft^^^waKrt&S®®^^''' ^fv^^^^&N^'iS'^^K,..
(U, purpurea) tesS'w^ ^A^.S3iW^K,3y:-..S^^^^^^ .:.". -•'""^ ,'
 


In the ponds and ditches, you may find a floating, rootless plant with hundreds of tiny, balloon-like sacs attached to its stems. This is the bladderwort, yet another kind of carnivorous plant. Those tiny sacs in the water are, in fact, sophisticated suction traps designed to provide the plant with animal meals. These bulbous sacs are also referred to as bladders, after which the plant is named. The size of the bladder is microscopic, varying anywhere from 1/2 mm to 5 mm depending on the species. There are as many as 200 species of bladderworts found in various parts of the world. Some bladderworts are aquatic, normally floating just below the water surface, while many others grow on the wet surface of marshes and swamps. Thirty-some species are known in the United States.

These hairs are mainly ornamental and probably are intended to attract tiny water animals to the trap. The trap entrance is located just below these hairs. A small door at the entrance, which opens only inwardly, rests on the latch and is sealed water-tight. On the outer surface of the door grow tiny, stiff hairs called a trigger bristle.

When the trap is set, the inside water pressure is lower than the outside because the water is constantly being pumped out from the trap interior through the glands found in the bladder walls.  Due to this pressure difference, the side walls of the bladder is somewhat sucked inwardly (convex) when the trap is viewed from above.

Suppose a tiny water animal approaches the trap, probably seeking a shelter in the bladderwort jungle, or merely passing by. The unwary visitor swims closer and closer to the trap entrance. The moment the water animal touches one of the trigger bristles attached to the door, the latch of the door is released, and the door suddenly swings open inward by the outside water pressure. The trap bulges as the water rushes in, and the water animal is sucked into the trap along with the rushing body of water. The door snaps shut behind the prey. The trap now looks bulbous with a bladderful of water, and the prey is caught helplessly inside the trap. All this happens in a matter of a fraction of a second.

Over a period of thirty minutes to an hour, the trap automatically reset itself and the same bladder is ready again for another catch. This automatic resetting takes place because the water is constantly being removed from the trap interior by the water-pumping action of the glands in the bladder walls. It is not unusual for a single bladder in animal-rich waters to capture several tiny water creatures until the bladder is completely full.

While this greedy trapping spree is going on, the animals captured inside the bladder slowly begin to decompose, often owing to a large part to bacteria. The dissolved nutrients of the decomposed prey are quickly absorbed through the glands and are carried away to the other parts of the plant. This digestion process takes a week or so, depending on the size and amount of the prey. As the digestion progresses inside the trap, the color of the bladder often turns dark purple. This is how the rootless bladderwort obtain its nutrients from the animal prey.

Bladderwort blossoms are quite showy (though individual flowers are generally small ) especially when the ponds and marshes are covered with thousand of tiny flowers. A flower blooms at the tip of a slender flower stem coming out of the water surface. Yellow is the dominant color, though purple flowers are also common.

Let us examine the structure of the bladderwort trap. Each bladder has some antenna-like hairs on one end of the trap.

 

 

The bladderworts do not possess roots, though organs that resemble and function as roots usually exist in many species. They are termed rhizoids. In many species, horizontal shoots termed stolons form the framework of the vegetative part of the plant.

The bladders (traps) are formed on various vegetative organs depending on the species. The trap arise from stolons, rhizoids, leaves, ....

Introduction  VenusFlytrap  Sundews  PitcherPlants  CobraPlant  Butterworts  Bladderworts