Academic Reading — All-Types Test 28

Reading passage

Surviving the Desert: How Plants Cope with a Shortage of Water A. Deserts are among the harshest environments on Earth, yet a remarkable variety of plants manages to grow and reproduce in them. Water is the central problem. Rain may fall only a few times a year, and when it does, much of it evaporates or drains away before roots can absorb it. At the same time, the plants face intense sunlight, wide swings in temperature between day and night, and soils that are often thin and poor in nutrients. To persist under such conditions, desert plants have evolved an array of features that reduce water loss, maximise water uptake, and allow growth to be timed around the brief periods when moisture is available. These features are not random; each represents a workable solution to the same underlying challenge of living where water is scarce and unpredictable. B. One widespread strategy is succulence, the storage of water in swollen tissues. Cacti are the best-known example, but many unrelated groups have independently developed similar bodies. A barrel cactus, for instance, can hold a large volume of water in its fleshy stem, drawing on this reserve during the long dry months. The outer surface of such a plant is typically covered by a thick, waxy layer called the cuticle, which seals in moisture and slows evaporation. The stem is often ribbed or pleated, allowing it to expand like an accordion after rain without splitting, and to contract gradually as the stored water is used. In many succulents the leaves have been reduced to spines. These spines lose almost no water, deter thirsty animals, and cast a little shade over the plant surface, all of which help conserve the precious reserve within. C. Because leaves are the main site of water loss, controlling how and when the plant breathes is critical. Plants exchange gases through tiny pores called stomata, which must open to admit the carbon dioxide needed for photosynthesis. Yet every moment the stomata are open, water vapour escapes. Most desert plants keep their stomata firmly shut during the heat of the day and rely instead on a special photosynthetic pathway. Under this system, the stomata open only at night, when the air is cooler and more humid, and the plant takes in carbon dioxide then, storing it in a chemical form until daylight. When the sun rises the pores close, but photosynthesis can still proceed using the carbon dioxide gathered in the dark. This nocturnal timing dramatically cuts the amount of water lost, though it also limits how quickly the plant can grow. D. Roots offer another route to survival, and desert plants pursue two contrasting approaches. Some send down a single deep taproot that reaches far below the surface to tap moisture held in the ground long after the topsoil has dried. The mesquite, a hardy desert tree, is famous for roots that may extend many metres downward in search of a reliable supply. Other plants spread a dense, shallow network of roots just beneath the surface, sometimes covering an area far wider than the visible plant above. This arrangement lets them capture the thin film of water from a passing shower almost as soon as it lands, before it can evaporate or sink away. Each pattern suits a different source of water, and some species combine elements of both, hedging against the uncertainty of when and where rain will arrive. E. Not every desert plant endures the drought directly; some simply avoid it. After a rare heavy rain, the desert floor can transform within days as countless seeds germinate, grow, flower and set new seed in a matter of weeks. These plants, sometimes called ephemerals, pass the long dry intervals as dormant seeds buried in the soil, protected by tough coats that will not sprout until enough water has fallen to see the plant through its short life. In this way an ephemeral sidesteps the very conditions that its neighbours must resist. The seeds themselves may lie in wait for years, and the brief, spectacular bloom that follows a good rain is really the visible moment of a strategy built mainly on patience. F. Taken together, these adaptations reveal that there is no single formula for desert survival. Storing water, closing the stomata, reshaping the roots and waiting out the drought as a seed are all valid responses to the same shortage, and different plants have arrived at different combinations of them. What unites them is the constant pressure to make the most of scarce and irregular moisture. The desert, far from being empty, is a place where the struggle for water has produced some of the most ingenious designs in the plant kingdom, each one a quiet record of the difficulties overcome.