Longman - TOEFL® IBT - CDROM Material
ASL01
Listen to a lecture in a botany class.
Today, we're going to be talking about insectivorous plants. As you know, we have a field trip tomorrow. We'll be going to the botanical gardens, and we're going to be seeing some insectivorous plants there.
Do you understand what insectivorous plants are? Insectivorous plants are plants that trap and assimilate insects in order to sustain life. You may understand the meaning of insectivore from related words such as carnivore or herbivore: a carnivore eats meat, and an herbivore eats plants, while an insectivore devours insects. But—unlike carnivores and herbivores, which are animals—the insectivores that we're going to discuss today are plants.
All insectivorous plants contain chlorophyll and have roots, so they don't get all of their nutrients from the insects they take in. Insectivorous plants tend to live in soil that lacks enough nitrogen for them to exist, so they consume their insect victims in order to get the nitrogen they need. There're many types of insectivorous plants in the world—there're perhaps 500 known species of insectivorous plants worldwide. Of these many types of insectivorous plants, we're going to look at one of the better known ones in depth.
The insectivorous plant that we're going to discuss in detail is the Venus flytrap, which is native to North America. Now look at the drawing of the Venus flytrap on the screen. The Venus flytrap catches insects by suddenly snapping the ends of one of its trap leaves around an insect. You can see in the diagram that a Venus flytrap has a number of trap leaves and that each trap leaf is divided into two parts. Inside the two parts of the trap leaf are three sensory bristles, sensory bristles which resemble tiny hairs. When an insect touches the bristles inside a trap leaf, the two surfaces of the leaf shut instantaneously, and the insect's trapped inside the parts of the leaf. The Venus flytrap then discharges a digestive liquid into the leaf in order to assimilate the insect and obtain the nitrogen the plant needs in order to survive.
That's all for today on the Venus flytrap. I hope that from our discussion of the Venus flytrap you've developed a clearer understanding of how this and other insectivorous plants function. You'll need it for the trip tomorrow.
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