TOEFL Speaking Task 4 Practice 030Nature

Narrator:Listen to part of a lecture in a biology class.

Professor:We usually think of plants trying to get as much sunlight as possible. In most cases, it's true. Plants grow toward light. But what's interesting is, in some cases, plants do the opposite. Sometimes plants actually grow away from or avoid light. Let's talk about a couple of reasons why. Some plants need to find other objects like trees to cling to and grow on for support. These plants are attracted to the dark shadows that tall objects cast on the ground. By growing along the ground in a dark shadow, these plants eventually reach the tall object and then grow upward along it, being supported by it. For example, some vines do this. Vines grow in shadows until they reach a tree or building, then the vine will climb or grow up that object. Other plants avoid light in order to conserve moisture. Usually a plant's leaves face upward, so their whole surface is exposed to the sun's rays. However, since intense sunlight makes plants lose water, some plants are in danger of drying out from too much exposure. During the day, when the light gets really intense, these plants change the position of their leaves to decrease their exposure to the sun and therefore retain moisture. For example, a species of bean plant does this. When the bean plant senses that the sunlight is too strong in the middle of the day, it turns its leaves sideways. The sun only hits the sides of the leaves, the thin edge. This way, the bean plant avoids drying out.