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Cleaning up in ponds

Cleaning up in ponds


Some things in nature are so unexpected that our reaction can only be one of wonder or amazement. Did you know that some plants, innocent in appearance, but vicious in character, lurk in ponds throughout the world? These plants attract, catch and eat aquatic insects, water fleas and young tadpoles, fish fry, tiny worms and very young insect larvae including mosquito wrigglers.

You will understand the dismay of the mobile creatures in these ponds. After all, animals normally eat plants, not the other way around. Unfortunately the plants in question need to supplement their nutrient supply. The ponds and bogs in which they grow, tend to have very acidic water. In such habitats, nutrients (particularly nitrogen) are in short supply. Plants need a great deal of nitrogen because they cannot manufacture proteins without it. With no proteins there are no cells and obviously no entire plants. Utricularia or bladderwort, the plant in question, therefore satisfies this need for nitrogen by feasting on unsuspecting animals.

It is not an easy matter for a plant to catch and digest wriggly, squirmy animals. But the conclusion seems inescapable that Utricularia uses a device rather fancier than would be necessary to do the job. To make use of an obvious pun, bladderwort indulges in over kill! The plant might have used a tangled net, or sticky material to trap animal victims. But this plant uses suction instead. If you were given a choice of making a tangled string, a sticky tape or a vacuum cleaner, which would you choose?

The plants look simple enough, but of course looks can be deceiving. Of 200 described species of this plan t, a tenth grow in North America. When one gets as far north as Alberta however, only three species can be found. Whether large or small, perennial or simply lasting one season, all Utricularia have a highly unusual structure. They have absolutely no roots, not even as an embryo in the seed. Furthermore it is difficult to distinguish between stem and leaf. Masses of small flowers – either purple or yellow – extend above the surface of the water. It is usually the flowers that one notices first.

The really interesting feature of these pond-dwellers, however, is the traps. They look like parts of leaves, only distinctly on the small side. At most, the traps extend 5 mm, but most are 3 mm or shorter. Yet they are interesting enough that people who appreciate plants have, since 1850, devoted considerable attention to them. Charles Darwin was one individual fascinated by these plants.

The traps are usually oval or egg-shaped bladders. At the front end, a trapdoor guards the entrance. Special cells on the trapdoor surface release nectar and mucilage. These smell so good. Numerous small animals make a point to investigate the source of these pleasant aromas. In so doing, they unintentionally brush one of the prominent trigger hairs projecting from the threshold into their pathway.

After this, things happen awfully quickly. The curious intruder wishes he had stayed far away. A doorstop inside shifts, allowing the trapdoor to swing up and inward. Water gushes inward to fill a vacuum in the trap’s interior. The watery inrushing flood sweeps the hapless victim along into the trap. And the view inside is certainly discouraging. The trapdoor swings firmly shut behind the victim, who cannot now escape. To add insult to injury, the victim’s pitiful struggles stimulate special gland cells on the interior wall. These release digestive juices into the cavity and the victim is digested.

When only the skeletal parts of the meal remain, the trap is re-set. Other glands in the trap’s lining absorb the water inside the trap. Once that is gone, a vacuum has been re-established. The convex shape of the sides of the trap produce an engineeringly sound structure which holds the trap door in place. Only stimulation of the outer trigger hairs can unlock that door. If no meal is captured, a trap can be re-set within 15-20 minutes.

These traps are amazingly effective. Each one catches several meals before it fills with skeletal parts and is discarded by the plant. The variety of trap designs among the various species, and the delicacy of the mechanism involved, particularly intrigue those who study plants. These organisms speak to us of design features far above the practical requirements for survival. Like so much that we can observe during a walk in the country, these aquatic vacuum systems remind us of their Creator.

December 1995

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