Darwin had a fascination for the Venus flytrap, but is it appropriate to conjure up his ghost when talking about it? The carnivorous plant still defies evolutionary explanations, especially now, when a recent paper drew attention to more amazing design features from macro to micro. For some reason, writers still feel a compulsion to mention Darwin’s name when talking about a plant that defies his evolutionary ideas.
Even though the Venus flytrap (Dionaea muscipula) grows only in America (particularly, in the Carolinas), it has long fascinated botanists from around the world. Recently, a team of plant biophysicists from Germany and Saudi Arabia went hunting for the clever plant’s secrets. Publishing in PNAS,1 they concentrated on plant hormones involved in the stimulation of the traps, the fast closure followed by slow constricting closure, and the formation of an “external stomach” on the trap surfaces that digests the prey. They found that signals for trap closure follow different signaling pathways than those for digestion. In sum, “These findings demonstrate that prey-catching Dionaea combines plant-specific signaling pathways… with a rapidly acting trigger, which uses ion channels, action potentials, and Ca2+ [calcium ion] signals.” That’s a lot of cooperation between multiple parts, each exquisitely adapted to their role in the goal of catching bugs.
Students who have played with Venus flytraps in school know that it takes two strokes of the trigger hairs separated by a short pause, or two strokes of separate hairs, to make the trap shut. This prevents needless trap resets from falling leaves or other non-prey objects. But they probably didn’t realize they have just tripped an electrical switch leading to a series of mechanical events: “Insects touching these mechanosenory organs protruding from the upper leaf epidermis of the Venus flytrap activate mechanosensitive ion channels and generate receptor potentials, which induce an action potential.” Prior to closure, the trap has been set by storing elastic energy, allowing it to close within 100 milliseconds (1/10 second). But even when sprung, that’s not all. A whole sequence of coordinated events is set into action:
However, the trap is not completely closed at this moment. To hermetically seal the trap, it requires ongoing activation of the mechanosensitive hairs by the trapped prey. Unless the prey is able to escape, it will further stimulate the inner surface of the lobes, thereby triggering further APs [action potentials]. This forces the edges of the lobes together, sealing the trap hermetically (prey-dependent slow closure) to form an external “stomach” in which prey digestion occurs. The second phase of trap closure is accompanied by secretion of lytic enzymes from the glands covering the inner surface of the bilobed leaf trap. Thus, digestive glands do not secrete until stimulated by natural or artificial prey. Additionally, prey-derived compounds stimulate digestive glands leading to acidification of the external stomach and production of lytic enzymes….
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