TRAP TYPE: Snap Trap
One species, Dionaea muscipula J.Ellis (1768), occupying habitats in the southeastern United States of America (North Carolina, South Carolina).

The steel trap of Dionaea is hardly as powerful as the ones set by trappers for wolves, beavers or bears, but it is just as effective at catching its own small prey. Figure 1 shows the traps ready to spring. Its growth in the wild is restricted to the bogs in the central southeastern coastal plain of the United States. Figure 2 below is a Venus flytrap in its natural environment.

Near the crease where the two leaf "jaws" join there is a series of tiny hairs. If an unwary insect walks across these hairs, touching two or more of them in succession, the leaf will close quickly enough to prevent its escape. Unable to escape between the hair-like teeth at the edge of the leaf (Figure 3), the helpless insect is slowly digested and absorbed by the leaf. Glands on the leaf surface secrete several digestive enzymes that help to decompose the insect. Once the insect has been digested sufficiently, the leaf re-opens for another victim.

The sensitive hairs at the fold of the leaf prevent the leaf from closing every time a drop of rain lands on it, because the leaf requires that two or more of these hairs be triggered in succession (Figure 3). If the leaf does close without a victim, it will re-open in a few hours. According to Lloyd (in George 1962), the traps can only catch about three victims before the leaves turn black and die. And even if the trap fails to catch anything, like when you tease it by touching a hair with a small brush, it can only reopen and close again about seven times! So, don't tease the flytrap!

The mechanism of closing has fascinated biologists for many years. How can an inanimate plant react so quickly to the stimulus of touch? The most widely accepted explanation had been that a rapid change in the turgidity of the cells occurs. That is, there must be a sudden change in the water pressure in the cells – the cells of the bottom part of the midrib, that is. Now we know that it is not nearly so simple, nor is our old explanation valid, although the lower midrib cells do indeed take in more water. In Dionaea, the closing occurs in as little as a half second. Salisbury and Ross (1985) explain the phenomenon as acid growth.

Early theories on the acid growth suggested that potassium or sodium must rush into the lower midrib cells to create an osmotic gradient. That means there is more salt inside than outside the midrib cells, and more water outside those cells than inside. However, Hodick and Sievers (1989) provided evidence that a change in turgor pressure due to movement of potassium or sodium was not the cause, but they were unable to provide any evidence to support an alternative theory.

Nevertheless, we now we have a somewhat clearer picture, but it still is only a collection of circumstantial evidence without direct links to demonstrate cause and effect. First the hairs are triggered, two in succession, and these triggers set up a change in the electrical potential, sending a signal to the lower cells of the midrib. Then a flurry of things happen so fast that we don't know what happens first. The growth hormone IAA appears in the midrib in increased concentrations. Hydrogen ions move rapidly into the cell walls of the midrib in response to action potentials from the trigger hairs (Salisbury and Ross (1985)).

We can only guess what happens here, but a good guess would be that a proton (H⁺) pump moves H⁺ ions out of the midrib cells and into the cell wall spaces between the cells. (Cell walls are really lots of fibers hooked together, creating a network of small capillary spaces.) Hydrogen ions naturally make this area more acid. These hydrogen ions seemingly loosen the cell walls, probably by dissolving the calcium pectate that glues the cellulose together, causing the tissues of the lower side of the midrib to be come flaccid. Calcium (not potassium or sodium as thought earlier) increases inside the cells and the cells absorb water.

It seems reasonable that this calcium might move into the cells by following the charge gradient. After all, if H+ ions left, the cell now has negative charges (electrons) that have no positive partner; the cell has a negative charge. This negative charge will attract positively charged things (positive ions, or cations) from outside the cell, like the Ca⁺⁺ (calcium ions) that were freed from the calcium pectate bonds of the cellulose fibers. Once the calcium enters the cell, it creates an osmotic gradient. There is now a greater proportion of Ca⁺⁺ and smaller proportion of water (H₂O) on the inside of the cells than on the outside of the cells in the cell wall spaces. The result? Water enters the cells by osmosis. Since the cells have become unglued, they are able to expand as they take in water, and hence they grow.

This results in the expansion of the outside of the leaf and the "springing" of the trap. Yes, all this happens at lightning fast speed to make the leaf close. The cells remain at this larger size and the cellulose eventually increases to strengthen the walls. That gets the trap closed, but in a few days, it must re-open. Once the insect is digested, the cells on the upper surface of the midrib will grow, much more slowly, and the leaf will re-open.

As you might imagine, the leaf cannot keep doing this rapid growth trick forever. That is why it is only able to close its trap about seven times during the life of a leaf. But why does it do this at all? In its boggy peatmoss habitat, nutrients are very limited. It can't use the nitrogen in the atmosphere (neither can we!), but it does need some form of nitrogen. Experiments on the Venus flytrap by Roberts and Oosting (1958) suggest that perhaps it is the organic form of nitrogen and phosphorus that is important to the insectivorous plants. And the trapped insects give them just that.