Academic Reading — Test 5

Reading passage

In the waterlogged bogs and acidic heathlands where pitcher plants flourish, the soil offers very little of the nitrogen and phosphorus that most plants depend upon to grow. These nutrients, normally drawn up through the roots, are scarce because the cold, oxygen-poor and acidic ground slows the decay that would otherwise release them. Faced with this shortage, pitcher plants have arrived at an unusual solution. Rather than relying solely on the soil, they supplement their diet by capturing small animals, chiefly insects, and breaking down their bodies to obtain the chemical building blocks they cannot easily collect from below ground. In this sense the plants are not predators in the ordinary meaning of the word, for they do not hunt or pursue; instead they wait, and let their prey come to them. The trap itself is a modified leaf that has, over a long span of evolutionary time, curled and fused into a hollow tube or jug. This vessel is often brightly marked, and many species produce nectar around the rim, or peristome, to lure visitors. Colour, scent and the promise of a sugary reward all work together to draw insects towards the opening. Some pitchers add a further deception: translucent patches in the upper wall let through light, so that an insect which has entered may mistake these glowing windows for an exit and exhaust itself trying to escape through solid tissue. The lure, then, is built on signals that insects already trust, turned against them. Once an insect alights on the rim, the second stage of the trap takes over. The peristome is frequently wet and extremely slippery, especially in humid conditions, and the smooth, waxy surface offers almost nothing for an insect's feet to grip. Downward-pointing hairs line the inner wall in many species, allowing easy passage in one direction only and steadily directing the captive deeper into the tube. At the bottom waits a pool of liquid. An insect that loses its footing tumbles into this fluid, and the inward-pointing hairs and slick walls make climbing out almost impossible. The plant has expended no energy in the chase; gravity and a few clever surfaces have done the work. The liquid at the base is not merely water. In most pitcher plants it contains digestive enzymes, secreted by glands in the lower wall, which slowly dissolve the soft tissues of the drowned insect. These enzymes break proteins and other compounds into simpler molecules small enough to be absorbed through the pitcher's lining. In some species the plant produces the entire enzyme mixture itself; in others, a community of bacteria and other tiny organisms living in the pool assists with, or even dominates, the breakdown. The fluid is frequently acidic, a condition that both aids digestion and discourages the prey's escape. Through this process the nitrogen and phosphorus locked inside the insect's body are released and taken up, supplying nutrients the roots could never have gathered from the surrounding peat. Remarkably, not every creature that enters a pitcher is doomed. Certain insects have evolved to live within the very traps that kill others. The larvae of some mosquitoes and midges, for example, complete their development in the pitcher's fluid, feeding on the trapped and decaying prey while remaining unharmed by the enzymes. A few spiders spin webs across the mouth of the pitcher and steal insects before they fall, and some small frogs shelter inside. This hidden community means that a single pitcher can function as a miniature ecosystem, hosting residents that depend on the plant's hunting yet contribute little or nothing to its meals. The relationship is not always one of straightforward loss for the plant, however, for the waste produced by these tenants can itself be a source of usable nutrients. The carnivorous habit is costly. Building and maintaining elaborate traps demands resources, and the leaves that form them are generally less efficient at photosynthesis than ordinary foliage. For this reason carnivory tends to pay off only where the soil is genuinely impoverished and sunlight is plentiful, conditions under which the extra nutrients gained outweigh the energy spent. Where bogs have been drained or enriched with fertiliser run-off from nearby farmland, pitcher plants often decline, outcompeted by faster-growing neighbours that no longer suffer the nutrient shortage the pitchers were so well adapted to exploit. The trap that once gave them an advantage becomes a burden when the ground beneath them changes.