ST_009系列托福综合写作

ST_009

综合写作

During the day, most plants position their leaves so the leaves can collect sunlight. However, at night, some plants fold their leaves. There is no general agreement about why this occurs, but there are several competing theories as to why it might be beneficial for plants to fold up their leaves at night.

Protection Against Cold

First, some scientists believe that plants fold their leaves at night to protect themselves against the cold. An open leaf has a large surface area exposed to the air, which means it will readily lose heat on a cold night. A folded leaf has less of its surface exposed to the air, and will lose less heat. By folding their leaves at night, plants may protect their leaves from freezing.

Protection Against Infection

Folding leaves may be a mechanism to keep the leaves from getting wet if it rains. Leaves are vulnerable to diseases caused by fungal infection. When leaves are wet, the chance of infection is

greater, since fungal spores (small particles through which fungi reproduce) require water to spread to new locations. If the leaves stay dry, the risk of fungal infection is reduced.

Elimination of Nighttime Light

Finally, many plants need to keep track of the time of year in order to produce flowers at the right time. These plants often keep track of the season based on how much sunlight there is in a day. Plants' sense of time can be distorted when their leaves are exposed to sources of light at night, such as moonlight. The purpose of folding leaves may be to limit leaf exposure to nighttime light, which may help plants track the seasons more accurately.