Harmony and Conflict in the Living World - Alexander F. Skutch [57]
Although structurally similar, the sessile glands on the leaf's upper surface have different functions. Just within the leaf's margin runs a band of alluring glands from which the secretion, apparently sweet, attracts ants and other small creatures; digestive glands, responsible for the leaf's red color, occupy the major part
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Venus's-flytrap, Dionaea muscipula, plant with traps open and
closed on prey
of its upper face. When, after enjoying the exudate of the marginal glands, a small visitor moves farther inward, it can hardly avoid touching one of the six bristles, each of which is a multicellular structure with, near its base, a transverse layer of sensitive cells, where it readily bends. To spring the trap, the creature must either touch the same bristle twice or each of two bristles once, with an interval of no less than three-quarters of a second but no more than twenty seconds. When stimulated in this manner, the valves may, in optimal conditions, fold together in less than half a second, although often the movement is much slower. It is effected, not by a hinged midrib, but by the rapid stretching of the underside of each valve.
If the prey is appropriate, the valves tighten upon it and may remain closed for from five to ten or more days, opening after the corpse is fully digested, to become ready for another capture. When the trap closes without prey, as it can be made to do by mechanical or electrical stimulation, it reopens in about twenty-four hours, to become again responsive. When an insect, a bit of
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albumen, or some other source of protein is enclosed, the sessile glands pour out a proteolytic enzyme in an acid medium that inhibits bacterial action. The trap does not reopen until it is again dry, the glands having absorbed all the liquids. Wind blows away the insect's hollow exoskeleton. A trap may catch up to three insects, one at a time.
The only other species known to have a trap similar to that of Dionaea is Aldrovanda vesiculosa of the same family, a rootless aquatic sparingly found floating just below the surface of quiet acid ponds from southern France to Japan, Australia, and tropical Africa. It is an inconspicuous plant rarely over six inches long and somewhat less than an inch broad (15 by 2 cm.), which remains wholly submerged until it sends into the air short stalks with white flowers. Its slender stem bears closely spaced whorls of eight leaves, each of which consists of a broadly expanded petiole, much like that of Venus's-flytrap, bearing at its end a round blade and four to eight serrated bristles, which project beyond the general contour of the plant and impart a spiny aspect. On the concave upper surface of each semicircular half of the leaf, among the many glands that one expects on carnivorous plants, stand about twenty long, slender, multicellular hairs. Near the middle of each is a sensitive zone that corresponds to that of the more massive trigger-bristle of the flytrap. A light touch on one of these multicellular hairs often causes the valves of a young Aldrovanda to spring together like a mussel shell, but sometimes more touches are needed. Older leaves usually require greater stimulation than newly expanded ones. Although the traps appear capricious or unreliable, a tiny aquatic creature that swims among the crowded trigger-hairs will probably impinge upon enough of them to effect its own incarceration. After closure, the valves press their outer zones more firmly together, enclosing a little pocket in which the prey is