Closure of the Utricularia trap is faster than that of Dionaea. The door in the front of the bladder is attached to the top of the opening and swings open inward. The door is very elastic and when closed it rests on the edge of the door opening. There are projecting hair-like structures near the top of the door. (Fig. 6-4) These projections funnel the prey into the vicinity of the trap door.
The trap in all species is set by removing most of the water in the bladder. Water is removed by internal glands which excrete it. The removal of water inside the bladder results in lower pressure on the inside of the trap than on the outside. Consequently, the walls cave in giving the trap sides a concave shape. In this state, the door is forced tightly against the opening and no water enters the trap. If something touches or brushes against one or more of the trigger hairs on the door, the trap is set off. The door springs open, water gushes in carrying with it the prey. (Fig. 6-5) The force of the gush of water is often sufficient to jerk the whole plant. Apparently when the trap is set an extremely unstable equilibrium in pressure is set up, keeping the door closed. Therefore, only a very small force is required to set off the trap. Trapping usually occurs within 1/50 of a second. Traps usually reset themselves within 15-45 minutes.
While these plants normally capture very small animals, a trap can capture larger or longer creatures by sucking them in a little at a time. That is, after the trap is sprung part of the animal is sucked in, the elastic door closes around it, creating a water tight seal so that the trap can reset itself and again spring to suck in another part of the prey. Movement of the plant parts comprising the trap in the genera Dionaea and Aldrovanda is, in part, a growth phenomenon, whereas in Utricularia spp. it is the result of mechanical action.
Here, as in the case of some other carnivorous plants, digestion results from the action of enzymes secreted by the plant together with bacterial activity. There are organisms that live and thrive in the Utricularia spp. trap, feeding on the captured prey without themselves being digested. As wastes accumulate in the bladder, bladder color turns from greenish to dark purple to black and eventually the trap drops off.
Many instructive hours can be spent observing and studying the mechanism of trapping and the structure of the Utricularia trap. To observe trapping of prey and gross structure of the trap a magnification of 2-30 times is sufficient. Binocular or dissecting microscopes are particularly efficacious but a simple magnifying glass will reveal a great deal. Higher magnifications are required for examination of glands, details of the opening and other intricate parts of the trap.
Specimens to be examined are placed in a transparent container such as a petri dish or finger bowl. A pair of tweezers with fine points and a dissecting needle or two make manuevering the specimen into a favorable position for observation easier. Conventional dissecting needles are too large to open the door to the trap. They can be modified by grinding them into a smaller sharper point or by substituting a fine sewing needle in the handle.
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