QUESTIONS 21–50: Reading PassagesDirections
Read each passage and answer the questions that follow it. Choose the best answer for each question. Base your
answers only on what you have read in a given passage. You may reread any passage you wish to.Desert plants have evolved very special
adaptations for living in extremely dry conditions.
Most have small, thick leaves, an adaptation that limits
water loss by reducing surface area relative to volume.
( 5 ) During the driest months, some desert plants shed
their leaves. Others, such as cacti, subsist on water the
plant stores in its fleshy stems during the rainy season.
Some send out long, deep taproots in order to reach
underlying water. Others have developed shallow,
( 10 )widespread root systems, which allow them to take
advantage of very occasional but heavy rainfalls.
Some plants have ways of actively protecting their
water supplies. The creosote bush, for instance,
produces a powerful poison that discourages the
( 15 )growth of competing root systems.
In addition, desert plants must survive similar,
water-related difficulties imposed on them by their
food-making process, photosynthesis. In order to
produce food, plants must use carbon dioxide from the
( 20 )air around them. When absorbing carbon dioxide,
however, plants lose significant amounts of water,
because the same tiny openings on the plant surfaces
serve both purposes. Desert plants solve this problem
in various ways. The food-producing periods of many
( 25 )plants are extremely short; some of these plants
survive long, dry periods in the form of seeds, while
many others simply drop their leaves and are bare for
much of the year. Still other plants have solved the
problem by developing cell mechanisms that
( 30 )control the opening and closing of the stomates, the
tiny openings on the plant surfaces. These guard cells