1 . Lichens (地衣)

Lichens look like splashes of paint left behind by a careless painter. Unlike many plants, they do not require soil to grow. They grow on trunk of trees in steaming tropical rain forests, on farmers’ fenceposts, on the bricks of big-city buildings, and on old gravestones. Lichens can tolerate extremes of climate. They grow on rocks in hot springs, on wind-swept mountaintops, and on stones in the driest deserts. In the Arctic, lichens are the principal source of food for reindeer. Whole mountainsides in Antarctica appear green and orange because of the presence of lichens; they are one of the few plants that can survive there. They are among the oldest of known plants. Recently, scientists discovered lichen fossils on a rock in a mine in southwest China that date back 600 million years.

When conditions become harsh, lichens become dormant (休眠). If there is not enough moisture, they simply dry up, but a short rain or even a heavy dew gives them new life. When growing on rock surfaces, lichens produce acids that dissolve (溶解) the minerals, contributing to the process of weathering by which rocks are slowly turned to soil. This property enables lichens to be pioneers. They appear on barren rock rubbed clean by glaciers, fires, lava flows, or floods, beginning the process of soil formation that allows mosses (苔藓) and other plants to later take root. But, despite their hardiness, lichens are extremely sensitive to airborne particles(颗粒). That’s why they serve as an early warning system for air pollution.

It is the acids lichens produce that give them their distinctive colors. Lichens are often spoken of in the same breath as mosses, and some lichens are even called mosses, but true mosses are all distinctively green, whereas lichens appear in many vivid colors. At one time, acids from lichens were used to make dyes, such as the purple dye, the blue dye, and the red dye, and they are sometimes still used that way today. Some lichens, such as oakmoss, contain oils that produce fragrant odors used in scented soaps, cosmetics and perfumes. Some lichens are also known to have antibiotic properties to kill bacteria.

So definite are the form, color, and characteristics of these organisms that for hundreds of years lichens were constantly under scientists’ microscope.

1. What characteristic of lichens is mainly talked about in paragraph 1?

A．They grow only on rock surfaces.

B．They live primarily in cold places.

C．They have adapted to a wide variety of environments.

D．They live in remote locations far from human communities.

2. The author refer to lichens as pioneers in paragraph 2 because they __________.

A．have their primitive structure

B．grow in areas before other plants do

C．are found in remote parts of the world

D．develop so early in the history of the planet

3. According to paragraph 3, humans use lichens in the following ways EXCEPT __________.

A．as a means of coloring clothing

B．as a type of medicine

C．as a source of food

D．as an ingredient in perfume

4. Which of the following cases would be probably dealt with in the next paragraph?

A．Lichens are important in Canada because of their abundance in the north.

B．Extracts of lichens were sold as herbal medicines to facilitate hair growth.

C．Glacier Park’s vast array of lichens indicate relatively good air quality.

D．A German botanist first found lichens are composed of two life forms.