Chapter 12. Man-eating Plants

Man-eating Plants

Read the article below. Then answer the questions that follow.

Unlike "Audrey II" in the cult classic musical Little Shop of Horrors, real-life Venus flytraps are much too small to ingest humans. But flytraps' jaw-like leaves to snap closed faster than you can sing "please don't eat me!"

In the cult classic musical The Little Shop of Horrors, two florists fall in love only to be eaten alive by a singing Venus flytrap named "Audrey II." Although real-life Venus flytraps (Dionaea muscipula) are too small to ingest humans, their jaw-like leaves snap closed faster than you can sing “please don’t eat me.”

While at rest, Venus flytrap leaves look like an open clamshell. In response to touch or movement, their hairy green “jaws” snap shut in about 100 milliseconds. Scientists say this is one of the fastest movements in the plant kingdom. But how do carnivorous plants, which have no muscles or nerve cells, respond faster than their prey, which have agile feet or even wings? Let’s take a look at what makes the Venus flytrap and other carnivorous plants capable of predatory movement.

Water ways

Plant cells consist mainly of water, and the turgor pressure exerted on their cell walls helps maintain shape and structure. Turgor pressure is tightly regulated by cell membrane machinery that controls concentration gradients across the cell membrane and thus the directional flow of water by osmosis.

This may be why Venus flytraps are convex when open, but concave when closed. One theory suggests that turgor pressure holds the jaws open. When water flows from the inner tissue towards the outer leaf, the “jaws” close around their prey.

The Venus flytrap's "jaws" snap shut in response to stimuli, trapping its prey. The entire process takes about 100 milliseconds.

While it is likely this motion could not happen without the flow of water inside the leaf, scientists debate whether turgor pressure alone can account for the rapidity. In other cases of plant movement, changes in turgor pressure are relatively slow. For example, the Sensitive Plant (Mimosa pudica) responds to human touch by changing the directional flow of water. However, the gradual folding of its leaves is far slower than the flytrap, which outpaces flies and spiders. Why can the Venus flytrap move more quickly?

Flytraps on film

High-speed video has shown that geometry also plays a role in the Venus flytrap’s quick response. Researchers at Harvard University used fluorescent dots to mark the external surface of the flytrap’s “jaws.” Video recordings taken under ultraviolet light traced the silhouettes of the markers before and after closure. This helped researchers estimate the spatial distribution of strain during closure.

It turns out that flytrap “jaws” change geometry, transforming from a convex to a concave shape during intermediate phases of movement. About 20% of displacement occurs in the first 1/3 of a second, while 60% of the movement occurs during an intermediate 1/10th of a second. The final 20% of the movement occurs in 1/3 of a second.

What happens during the intermediate phase? Picture holding a stretched rubber band. When you let go of the rubber band, energy is released. In a similar way, the sudden reversal of turgor pressure in the inner leaf induces the rapid, sudden snap of the jaw. In order to snap shut, the elastic energy inside the leaf has to surpass a threshold. This threshold is reached as water flows into the plant tissue, stretching long, lengthwise-oriented cells on the outer surface of the flytrap as well as cells located inside the “jaw.” These create an elastic strain much like that in a rubber band. When the stored elastic energy surpasses a threshold, the snap is quick.

True to its name, the bladderwort features bladder-like traps lining their stems. When prey stimulate hair on the bladder, a trapdoor on the bladder opens quickly, pulling water and prey inside.

What moves faster than a flytrap?

Bladderworts like Utricularia stellaris are plants that live in water or water-logged soils and digest insect larvae from the water. They do so with small bladder-like traps that line their stems. These ball-shaped bladders pull prey inside through a trapdoor. A recent study conducted at the Tata Institute in India demonstrates this rapid movement is driven by a pressure gradient.

The bladder pumps out water so that the pressure inside the bladder becomes much lower than that of the surrounding water. This causes the walls to squeeze inward giving a slightly concave appearance. When prey stimulate hair on the bladder, the trapdoor opens rapidly and water (and prey) rush inside. This happens in about 500 microseconds (μs), carrying prey inside faster than they can swim away.

The power of movement in plants

The Sensitive Plant, Venus flytrap, and bladderwort are not the only plants that move. In the late 19th century Charles Darwin studied the movement of common plants like petunias and irises. Many plants follow a discernable pattern of motion over the course of a day, although this slow movement is hardly noticeable to people. While changes in turgor pressure are involved in some instances, movement is also regulated by plant hormones.

© 2015 WH Freeman and Company.

12.1 Comprehension & Application Questions

Answer the following questions to demonstrate your understanding of the article.

Question 12.5

High-speed video taken by Harvard researchers revealed that the shape of the Venus flytrap’s “jaws” change from 4xYKI3C5KOlgax1xpJg8t5icv8Kk1ku0 to fd5SRW9OmyVeczolTgbBJAMwt0FwwO0a when capturing prey.

2

Correct. As the article points out, the “jaws” of the flytrap change geometry in response to stimuli, transforming from a convex to concave shape in less than 1 second.

Try again. As the article points out, the “jaws” of the flytrap change geometry to trap prey. What shape would be better suited to trapping prey?

Incorrect. As the article points out, the “jaws” of the flytrap change geometry in response to stimuli, transforming from a convex to concave shape in less than 1 second.