Passage 1: How Desert Plants Survive
Deserts cover roughly a third of the Earth's land surface, and at first sight they seem almost empty of life. Yet a closer look reveals a surprising variety of plants, many of which have survived in these harsh environments for thousands of years. Defining a desert is not straightforward, but most scientists agree that the key factor is not heat but dryness: a desert is generally an area that receives less than 250 millimetres of rain per year. Some deserts, in fact, are bitterly cold for much of the year. Rain, when it does arrive, often falls in short, violent storms, and much of it runs off the hard ground or evaporates before plants can use it. What all of them share is that water is scarce and unpredictable, and every plant that grows there must find a way of dealing with this problem.
Broadly speaking, desert plants follow one of three strategies. Some avoid drought altogether by growing only when water is available. Others store water inside their bodies to use during dry periods. A third group simply tolerates dryness, surviving long periods in which their tissues lose most of their moisture. Some of these plants can appear completely dead for months and yet turn green again within hours of being watered. Many species combine elements of more than one strategy, but the division is a useful way of understanding the extraordinary range of forms found in arid regions.
The drought avoiders are mostly small annual plants, which complete their entire life cycle in a few weeks. Their seeds can lie in the soil for many years, protected by a hard outer coat. Remarkably, many of these seeds contain chemicals that prevent them from germinating after a light shower, which would not provide enough moisture for the plant to grow and produce seed of its own. Only a heavy rainfall washes these chemicals away. When this happens, a desert that has looked lifeless for a decade can be covered in flowers within days, an event that attracts large numbers of insects and, increasingly, tourists.
The best-known water storers are succulents, such as cacti. Their thick stems are made up of a spongy tissue that can hold large quantities of water, and many have folds, or ribs, that allow the stem to expand like an accordion after rain and shrink again as the water is used. Instead of broad leaves, which would lose moisture rapidly, most cacti have spines. These are in fact highly modified leaves. They protect the plant from thirsty animals, and they also trap a thin layer of still air around the stem, which reduces water loss. In some species, a dense covering of spines also provides shade, keeping the surface of the plant several degrees cooler than the surrounding air.
Succulents also have an unusual way of taking in carbon dioxide, which all plants need for photosynthesis. Most plants open small pores in their leaves, known as stomata, during the day. In the desert, however, this would cause the plant to lose a great deal of water through evaporation. Many succulents therefore open their stomata only at night, when the air is cooler and more humid. They store the carbon dioxide in the form of an acid and then use it the following day, when sunlight is available, while their pores remain tightly closed. This process is less efficient than ordinary photosynthesis, which is one reason why most cacti grow very slowly; a cactus only a metre tall may be more than fifty years old.
Roots play an equally important role. Some desert shrubs send a single taproot straight down to reach water deep underground; in one well-studied species, roots have been found more than thirty metres below the surface. Cacti, on the other hand, usually have shallow roots that spread widely just under the surface, allowing them to absorb water quickly after even a brief storm. Within hours of rain, some cacti grow new fine roots, known as rain roots, which die back again once the soil dries out.
Desert plants are not only of interest to botanists. As droughts become more frequent in many farming regions, researchers are studying these species to find ways of making food crops more resistant to dry conditions. In several countries, farmers have already begun to grow certain cacti as animal feed, since they require only a fraction of the water needed by grass. Others are investigating whether the ability to take in carbon dioxide at night could one day be transferred to crops such as rice or wheat. Perhaps the most valuable lesson of desert plants, though, is a simple one: in an environment where resources are scarce, success depends less on growing quickly than on wasting nothing.
