Academic Reading — All-Types Test 13

Reading passage

How Plants Survive the Desert A Life at the arid margins of the world demands a particular kind of ingenuity, and the plants that colonise deserts have arrived at it by many separate routes. Rainfall in these regions is not merely scarce but wildly unpredictable, sometimes failing for years and then arriving as a single violent downpour. A plant rooted to one spot cannot flee such conditions, so it must instead be built to endure them. The result is one of the most inventive chapters in the story of evolution: a catalogue of forms, chemistries and behaviours shaped almost entirely by the problem of water. Remarkably, botanists find the same solutions appearing again and again on continents that have been separated for tens of millions of years, a sign of how few workable answers the desert actually permits. B The most familiar of these solutions belongs to the succulents, among them the cacti of the Americas and the fleshy euphorbias of Africa. Their swollen stems and leaves act as internal reservoirs, storing the brief windfall of a rainstorm and releasing it slowly through the long drought that follows. To guard this hoard, a succulent seals its surface with a thick waxy skin and reduces the number of pores through which moisture can escape. Many species have dispensed with broad leaves altogether, shrinking them into spines that lose almost no water while deterring the thirsty animals that would otherwise tear the plant open for its stored liquid. A pale, reflective coating on some cacti turns away a portion of the fierce sunlight, keeping the tissue beneath a few crucial degrees cooler. C Retaining water is only half of the difficulty; taking it in at all poses its own puzzle. Most plants open their pores by day to absorb the carbon dioxide that photosynthesis requires, but in the desert the midday heat would turn open pores into ruinous leaks. A number of succulents have solved this by inverting the ordinary schedule. They keep their pores shut while the sun is high and open them only after dark, when the air is cooler and evaporation slows. The carbon dioxide gathered at night is locked into an acid and held until daylight, when it is quietly fed into the plant's chemistry. This nocturnal strategy is costly and slow, but in a landscape where every drop is precious it is a bargain worth making. D Not all desert plants store water; some simply chase it. The mesquite and a handful of other deep-rooted shrubs send taproots plunging many metres downward in search of moisture that never reaches the surface, tapping hidden reserves far below the reach of their neighbours. Others take the opposite approach, spreading a dense mat of shallow roots just beneath the soil so that they can seize the lightest shower before it evaporates. A single plant may combine both systems, hedging against a climate that offers no guarantees about where the next water will come from. E The most audacious strategy of all is to avoid the drought entirely. The desert ephemerals, often dismissed as mere wildflowers, pass the dry years as dormant seeds locked in the soil, indifferent to the heat above them. When a rare heavy rain finally soaks the ground, chemical signals in the seed coat release their grip and the plants race through their entire life in a matter of weeks, flowering and setting a new generation of seed before the moisture is gone. Their seeds are cautious gamblers: a downpour too light to sustain a full life cycle will not trigger germination, and the seed waits, sometimes for a decade, for a rain it can trust. F Taken together, these adaptations reveal a landscape far from empty. What looks to the casual traveller like barren ground is in fact a finely tuned community, each species occupying its own narrow way of making a living from an unreliable sky. Understanding how such plants survive is more than a botanical curiosity, for as dry regions expand across the globe, the tricks that desert flora have refined over millions of years may hold lessons for the crops on which a warming world will increasingly depend.