问答题

Practice 4　　As I mentioned last week, I’ve recently returned from Australia. While I was there, I visited a eucalyptus forest that, in February, was the scene of an appalling wildfire. Perhaps naively, I had expected to find that many trees had been killed. They hadn’t. They had blackened bark, but were otherwise looking rather well, many of them wreathed in new young leaves. This prompted me to consider fire and the role it plays as a force of nature.　　Fossil charcoals tell us that wildfires have been part of life on Earth for as long as there have been plants on land. That’s more than 400 million years of fire. Fire was here long before arriviste plants like grasses; it pre-dated the first flowers. And without wanting to get mystical about it, fire is, in many respects, a kind of animal, albeit an ethereal one. Like any animal, it consumes oxygen. Like a sheep or a slug, it eats plants. But unlike a normal animal, it’s a shape-shifter. Sometimes, it merely nibbles a few leaves; sometimes it kills grown trees. Sometimes it is more deadly and destructive than a swarm of locusts.　　The shape-shifting nature of fire makes it hard to study, for it is not a single entity. Some fires are infernally hot; others, relatively cool. Some stay at ground level; others climb trees. Moreover, fire is much more likely to appear in some parts of the world than in others. Satellite images of the Earth show that wildfires are rare in, say, northern Europe, and common in parts of central Africa and Australia. (These days many wildfires are started by humans, either on purpose or by accident. But long before our ancestors began to throw torches or cigarette butts, fires were started by lightning strikes, or by sparks given off when rocks rub together in an avalanche.)　　Once a fire gets started, many factors contribute to how it will behave. The weather obviously has a huge effect: winds can fan flames, rains can quench them. The lie of the land matters, too: fire runs uphill more readily than it goes down. But another crucial factor is what type of plants the fire has to eat.　　It’s common knowledge that plants regularly exposed to fire tend to have features that help them cope with it—such as thick bark, or seeds that only grow after being exposed to intense heat or smoke. But what is less often remarked on is that the plants themselves affect the nature and severity of fire.