Utricularia macrorhiza left and middle Brother Alfred Brousseau © 1995 Saint Mary's College of California, right © 2004 Robert Sivinski

The Uncommon Bladderwort

Jane Mygatt
UNM Herbarium

In the hidden realm of shallow pools and tranquil streams dwells an ogre of the plant kingdom. Armed with an underwater flotilla of pear-shaped bladders or "utricles," it engulfs myriad unsuspecting minute passers by. This ogre, the common bladderwort Utricularia macrorhiza Le Conte, has evolved an ingenious adaptation to survive in a nitrogen-deficient environment.

A member of the bladderwort family (Lentibulariaceae), the common bladderwort is a rootless (some say ruthless) perennial aquatic herb and the only native carnivorous plant in our state. The generic name, Utricularia, is derived from "utriculus," the Latin diminutive of "bag". The numerous small bladders are modified hollow leaves with sensitive bristles on "trap door" entrances. When the primary prey (Lilliputian crustaceans and insect larvae) trigger the bristles, the door opens inward, expanding the bladder. The force of this expansion sucks the prey in and the trap door shuts within 1/35th of a second. The prey then dies and its decaying remains are digested.

An unoccupied trap is somewhat flattened and curves inward because of water tension. Water pressure is lower inside the bladder than in the surrounding water. After the trap has been triggered, it can take 30 minutes for the excess water to pass to the exterior of the bladder, restoring the water tension and resetting the trap. There may be hundreds of tiny bladders on a single plant, attached to submerged leaves by short slender stalks. These bladders measure just a few millimeters wide.

Being aquatic and devoid of roots would pose nutritional problems if it were not for the numerous bladders digesting a nitrogen-rich supply of aquatic animals. Biologists are uncertain whether digestion occurs through enzymes secreted by the bladders or by bacterial breakdown. It is presumed that the decomposed nutrients are absorbed through the cells of the bladders.

In July, attractive flowers rise completely above water level, effecting insect pollination. The yellow bilabiate flowers resemble those of the figwort family (Scrophulariaceae) from which the bladderworts are derived. The capsules bear numerous small seeds and germination takes place in the water.

Methods of capturing prey reach some of the greatest heights of specialization in bladderworts. A carnivorous lifestyle is present in more than 500 species of flowering plants worldwide, all bearing modified leaf structures for the capture and digestion of animal prey. Our fascination with these botanical ogres increases as we learn more about their unusual tactics in nature's "Little Shop of Horrors".

Utricularia macrorhiza © 2001 Jane Mygatt.