IELTS Practice Test 2

Reading Passage: The Brainless Problem-Solver

A Few organisms challenge our categories as thoroughly as Physarum polycephalum, a slime mould that thrives on rotting logs and damp leaf litter in woodland around the world. Despite its name, and its habit of growing in the same places as mushrooms, it is not a fungus; biologists group it with the amoebae, single-celled organisms that engulf their food. What makes Physarum remarkable is the scale of that single cell. In its feeding stage, known as the plasmodium, it is a bright yellow sheet containing millions of nuclei with no internal walls dividing them, and under favourable conditions it can spread wider than a dinner plate. In the wild it feeds on bacteria and fungal spores; in the laboratory, researchers have found that it will happily accept porridge oats, which has made it one of the cheapest experimental organisms to keep.

B Physarum has no muscles, no nerves and no limbs, yet it can travel a centimetre or more in an hour. It does so by pumping. The plasmodium is threaded with a branching network of tubes, and the cytoplasm inside them is pushed first one way and then the other by rhythmic contractions of the tube walls, reversing direction roughly every minute or two. Because slightly more material moves forward than back, the whole cell creeps towards whatever it is exploring. The network is constantly being rebuilt as it goes. Tubes that carry a strong, steady flow thicken and become permanent channels, while those that carry little flow narrow and are eventually abandoned. This simple rule — strengthen what is used, discard what is not — turns out to have surprising consequences.

C In one of the best-known experiments, a plasmodium was cut into pieces and spread through a plastic maze, where the pieces fused into a single cell filling every corridor. When food was then placed at two of the maze's exits, the mould withdrew from the dead ends over the following hours and kept only a thick tube along the shortest route between the two food sources. A later team took the idea further. They arranged oat flakes on a damp surface in the pattern of the cities surrounding a capital, with the mould starting from the capital's position. Within about a day the mould had linked the oats with a web of tubes that, when compared with the region's actual railway network, proved similarly efficient: it used a comparable total length of track and coped about as well when a single link was cut. Nobody had designed the result. It emerged from countless local decisions about which tubes to keep, and that is precisely what has attracted the interest of engineers, who wonder whether similar rules could help to plan networks that grow and repair themselves.

D Physarum has no brain, yet some of its behaviour looks remarkably like memory. As it travels, it leaves behind a thin layer of slime, and it tends to avoid ground that is already coated, so it does not waste energy searching the same patch twice. Some researchers describe the trail as an external memory: rather than storing information inside itself, the mould writes it onto its surroundings. In one test, plasmodia had to escape from a U-shaped barrier to reach food; those placed on a surface already smeared with slime, where their own trail could no longer guide them, were far more likely to remain trapped. In another study, a mould crossing a narrow track was exposed to three pulses of cold, dry air at regular intervals, each of which slowed it down. Afterwards, it slowed at the moment the next pulse would have arrived, even though none was delivered. Such findings are striking, but many biologists caution against calling them 'learning' or 'thinking' — words that, they argue, carry over assumptions from the study of animals that a single cell may not deserve.

E When its surroundings dry out or food runs short, Physarum does not simply die. It hardens into a dormant, crust-like form called a sclerotium, which can survive for months or even years and returns to active life when moistened. This ability to be stored, posted and revived has made the organism popular well beyond research laboratories. It is now a familiar sight in schools, where students watch it grow towards food over a few days, and artists have used its spreading networks as living drawings. Computer scientists, meanwhile, have written programs that copy its simple rules for growing and pruning tubes, and have used them to suggest layouts for road and communication networks. Whether such models capture what the mould really does is still debated, but Physarum has already shown how much can be achieved by a single cell.