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.