Most of Utricularia spp. have long stems or stolons with varying degrees ol branching. Some species exceed lengths of 10 ft. (3 m). These thread-like, rootless plants have leaves that vary in size from insignificant to over 10 in. (25 cm) long. There is great variety in leaf sizes and shapes within the genus. There is a species, U. pubesccn which has mucilage on the upper surfaces of its leaves. It is not yet known whether prev captured by the mucilage is digested. Perhaps this plant has developed 2 methods ol capturing and digesting prey.
The leaves of the terrestrial and epiphytic species grow upright. The distinction between stems, branches, and leaves is not clear. Modified stems called rhizoids anchoi some species in the growing medium. (Photo 6-1)
The distinguishing feature of this genus is their bladders or traps, which range from extremely small to lengths of up to xk in. (6 mm) and are all basically oval-shaped. The shape of the bladder is species specific.
It was once believed that the bladders functioned like pontoons to support the plan! and some even thought they extracted air from the water and stored it for the plant's use. But these beliefs were dispelled when it was established that the bladders trapped such prey as tiny animals, insects and baby fish (fry). The bladders grow on leaves of the aquatic species whereas bladders arise from any plant part of non-aquatic species. An unusual characteristic of these plants is that any vegetative part is capable of developing into any other vegetative part.
The racemose inflorescence bears personate flowers with elongated spurs and ,i bilabiate corolla and calyx. (Photo 6-2) The throat of the corolla is usually blocked by a pubescent palate whose degree of exsertion is specific to the species. There are 2 curved stamens which may or may not be covered by the 2-lipped stigma. (Fig. 6-3) Flo we i color varies by species and includes shades of yellow, white, purple, blue and red. In some epiphytic species the corolla diameter can exceed 2 in. (5 cm) and strongly resembles orchid flowers, while in most species the corolla diameter ranges from 0.08 1 in. (0.2-2.5 cm). (Photos 6-3, 6-4)
Both cleistogamous (a flower that does not open and is self pollinated) flowers and chasmogamous (flowers which are pollinated when open) flowers can occur on the same plant of some species. Evidence indicates that the environmental conditions determine which kind of flower or if both will occur on a plant.
Annuals which survive the dry season by forming seeds are sensitive to water level fluctuations. It has been observed that if the water level remains above a critical level the plants will not flower at their usual time and contrariwise if the water level drops
Fig. 6-2 Terrestrial Utricularia with scape.
below the critical level, a plant will flower regardless of how short a time plants have been growing.
Some species such as U. inflata produce tubers on the terminal ends of branches during periods of low moisture and/or temperatures. These tubers will germinate upon the arrival of suitable growing conditions.
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.
Fig. 6-5 Utricularia trapping mechanism. The trap is set by removal of water from the bladder which results in lower pressure within the trap. A longitudinal section reveals the door is forced tightly against the opening. The trap is set-off when the trigger hairs on the door are touched. The door springs open and water rushes in, carrying with it prey in the vicinity. The door then closes entrapping the prey.
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