TOEFL Complete the Words Practice Questions

A cross section of a tree trunk from a temperate forest usually shows a series of concentric rings. These ba form bec the la of divi cells un the ba produces dist kinds o wood a the sea change. In spring, growth is rapid, and the wood laid down is light and porous. By late summer the new wood is denser and darker. The sharp boundary between one year's dark wood and the next year's pale wood allows scientists to count a tree's age and even to detect past droughts in unusually narrow rings.

The Venus flytrap grows in boggy soils that contain very little nitrogen. It ma up f this shor by catc insects i hinged lea lined wi sensitive ha. The ja snap sh only if two of them are touched within about twenty seconds, a rule that keeps the plant from wasting energy on raindrops or falling debris. Once prey is sealed inside, glands secrete digestive enzymes, and over roughly ten days the plant absorbs the nutrients released from its victim's body.

In lawns and meadows, mushrooms sometimes appear in a nearly perfect circle known as a fairy ring. The vis mushrooms a merely temp outgrowths o an imm underground w whose thr spread out in ev direction fr a single starting point. As the colony advances, it consumes the organic matter in fresh soil, leaving the exhausted interior behind. New mushrooms sprout only along the active edge, so the circle widens a little each year. Some rings are thought to be several hundred years old.

In 1928, the Scottish scientist Alexander Fleming made one of medicine's most famous accidental discoveries. On retu from hol, he disco that unwa spores h contaminated o of h culture pla. Around t intruding fun, the bacteria had been destroyed, while colonies farther away looked normal. Months later he named the unknown substance penicillin, but more than a decade passed before a team at Oxford purified enough of it to treat patients.

A mushroom releases vast numbers of microscopic spores from the thin gills beneath its cap. The mech that cata them i powered sim by mois condensing fr the surro air. A the sp where ea spore is attached, a tiny bead of water forms, while a thin layer of liquid spreads across the spore's surface. When the two suddenly merge, the abrupt shift in the spore's balance flings it about a tenth of a millimeter into the open space between the gills, from which it falls out of the cap and drifts off on the breeze.

Unlike most mushrooms, truffles form their fruiting bodies underground. Because t spores l sealed be ground, th cannot ri the bre as fre as sur spores. Ins, a mat truffle releases a strong scent that attracts squirrels, deer and other animals, which dig it up and eat it. The spores pass unharmed through the animal's gut and are deposited, often some distance away, in its droppings. People have long taken advantage of this attraction, training pigs and dogs to sniff out the prized delicacies.