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NARRATOR:Listen to part of a lecture in an environmental science class.
FEMALE PROFESSOR:Many organisms have developed the ability to survive in harsh environmental conditions-extreme heat or cold, or, very dry conditions...Like, plants in the desert-your textbook doesn't have much about the specifics on desert plants, but I think that desert plants are great examples of specialized adaptations to extreme environmental conditions.
So with desert plants, there are basically three different adaptive strategies.And I should point out that these strategies are not specific to any particular species-many different species have developed each of the adaptations.So, first off, there are succulent plants.
There are many different species of succulent plants, but they all can absorb and store a lot of water.Obviously, opportunities to get water in the desert are few and far between.Generally, rains are light and short, so the rain doesn't seep too far down into the soil...and there's a limited window of time for any plant to get the water before it evaporates.But succulent plants have a spread-out and shallow root system that can quickly pull in water from the top inch of soil, though the soil has to be saturated, since succulents aren't good at absorbing water from soil that's only a little moist.Succulent plants also are well suited to retaining water-important in an environment where rainy days are rare.Succulent plants can store water in their leaves, in their stems, or in their roots. And to keep that moisture from evaporating in the hot desert sun, most succulent plants have a waxy outer layer that makes them almost waterproof when their stomates are closed.They also preserve water by minimizing their surface area-the more of the plant that's out in the sun, the more potential there is to lose stored-up water-and that means that most succulent plants have few, if any, leaves.
Now besides succulent plants, there are also drought-tolerant plants.Drought-tolerant plants are like bears in a way.You know how bears mostly sleep through the winter?They can survive without eating because their metabolism slows down.Well, drought-tolerant plants also go into a dormant state when resources-in their case, water-runs short.A drought-tolerant plant can actually dry out without dying.I said before that most desert rains are light and brief, but occasionally there's a heavy one.
Drought-tolerant plants revive after one of these significant rainfalls-and they're able to absorb a good bit of the rainfall due to their deep roots.Actually the root system for drought-tolerant plants is more extensive than the root systems of many plants that live in wetter climates.Drought-tolerant plants can even absorb water from relatively dry soil because of their deep roots, in contrast to succulent plants.
The third adaptive strategy is to avoid the drought conditions altogether.[Small laugh] Yes, there are plants that do this-annual plants.An annual plant will mature and produce seeds in a single season that will become the next generation of annual plants.In desert conditions, annual plants grow in the fall or spring to avoid the heat of summer and the cold of winter.Of course, these plants could face a serious problem if a particular fall or spring happened to be very dry-they would have difficulty growing and could die before producing seeds.But they have a mechanism to prevent one year of low rainfall from wiping them out.Not all seeds an annual plant puts out will grow the following year.Some seeds remain dormant in the ground for several years.It's a type of insurance that protects the annual plants from a season of poor growing conditions, of unfavorable weather.
旁白：听一段环境科学课的演讲。
教授：很多生物进化出了在严酷的环境状况下生存的能力，比如极热或极冷、或非常干旱的环境...就像沙漠里的植物一样，你们的课本上没有很多具体介绍沙漠植物的内容，不过我认为沙漠植物是专门适应极端环境状况的很好的例子。
沙漠植物主要有三种不同的适应策略。我应该指出的是，这些策略不是针对任何具体品种的植物的---很多不同种类的植物都进化出了各自的适应策略。首先是肉质植物。
肉质植物有很多不同的品种，但它们都能吸收和储存很多水分。显然，在沙漠能获得水的机会少之又少。一般那里的降雨很少而且时间很短，所以雨水不会渗到地下很深的地方，而且任何植物要在雨水蒸发前得到这些水的时间也是有限的。但是肉质植物的根系伸展得很开，也很浅，可以很快地从表层土中吸收水分，不过泥土必须吸饱水，因为肉质植物不擅长从只是有点湿润的土壤中吸收水分。肉质植物还很适合留住水分，这在一个雨天很稀少的环境中是很重要的。肉质植物可以把水分储存在它们的叶子、茎干或根系中。为了防止水分在炎热的沙漠太阳下蒸发，大部分肉质植物都有一个像蜡一样的外层，这个外层在它们的气孔关闭时可以把它们变得几乎不透水。它们还通过最小化它们的表面区域来保留水分---植物暴露在太阳下越多，流失储存的水分的可能性就越大---而这就意味着大部分肉质植物即使有叶子，也只会有一点点叶子。
除了肉质植物，还有耐旱植物。耐旱植物在某种程度上和熊很像。你们都知道熊多半会睡过一整个冬天。它们不进食也能存活是因为它们的新陈代谢慢了下来。当水源不足的情况时，耐旱植物也会进入休眠状态。抗旱性植物可以在不死亡的情况下变干。我之前说过大部分沙漠降雨都很小而且时间很短，但偶尔也会有一次大雨。
在这种重要的降雨之后，耐旱植物会复活。而且得益于它们扎得很深的根，它们可以吸收很多雨水。事实上，耐旱植物的根系比生活在湿润气候下的很多植物的根系延展得更加广阔。耐旱植物因为它们深深的根甚至可以从相对较干的土壤中吸收水分，与肉质植物形成了鲜明的对比。
第三个适应策略就是完全避开干旱的环境。是的，确实有植物会这么做，它们就是一年生植物。一年生植物仅在一季中就能成熟并产籽，这些籽会成为下一代的一年生植物。在沙漠环境中，一年生植物会在秋天或春天生长，以避开夏季的酷热和冬季的严寒。当然啦，如果某一年的秋天或春天碰巧特别干旱的话，这些植物会遇到很严重的问题---它们会无法生长，而且可能会在能产籽之前就死去。但是它们有一个机制，能够防止一年的低降雨量导致它们灭绝。不是所有一年生植物散出的种子都会在来年发芽的。一些种子会在地上保持休眠状态好几年。这是一种保险，可以保护一年生植物不受生长环境不好的一季，恶劣天气的影响。