Nelson-Denny Practice Test 15

Partners on Bare Rock

¹A lichen looks like a single plant: a crusty patch on a gravestone, a leafy gray growth on a branch, or a beard hanging from a fir tree. In fact, it is a partnership between two very different organisms. One partner is a fungus. The other is a photosynthetic partner, usually a green alga or a cyanobacterium, which lives among the threads of the fungus.

²Each partner supplies something the other lacks. The photosynthetic partner uses sunlight to make sugars, some of which pass to the fungus. The fungus, in turn, forms most of the lichen's body. It anchors the lichen, shields the inner cells from intense light and drying, and absorbs water and minerals from rain, dust, and the surface beneath. Together the partners can survive in places where neither could easily live alone.

³This toughness makes lichens important pioneer organisms. On newly exposed rock, such as a cooled lava flow or ground uncovered by a retreating glacier, lichens are often among the first living things to appear. Acids they release help break down the rock surface, and as lichens die and decay, they add bits of organic matter. Over long periods, these processes help produce the first thin soil in which mosses and small plants can take root.

⁴Yet lichens that withstand drought and cold are often sensitive to air pollution. Because they have no roots and take in most of what they need directly from the air and rain, they also absorb pollutants such as sulfur dioxide. Many species disappear from areas where the air is heavily polluted. Scientists therefore survey lichens to judge air quality: a forest with a rich variety of lichens generally has cleaner air than one with only a few hardy kinds.