Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the best answer to each of…

Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the best answer to each of the following questions from 31 to 40.

On the surface, a desert appears to be one of the harshest environments on Earth yet a remarkable variety of plants have adapted to thrive in this dry climate. Despite their origins in different locations around the globe, desert plants have developed similar strategies for surviving in arid environments. Some plants have adapted in such a way that it is hard to tell them apart even though they belong to very different biological families, one evolution of certain survival strategies which are shared across geographic and biological boundaries.

In general, the survival strategies of desert plants can be divided into two kinds: adaptation for quick use of ephemeral resources ("maximum variance behavior") and adaptation for the best use of poor but more permanent resources ("minimum variance behavior"). The former strategy involves adaptation to environmental changes, such as seasonal availability of water. This is observed in desert annuals and perennials alike. Such plants tend to grow rapidly and produce many seeds under the right conditions. The latter strategy involves adaptation to the worst possible conditions, which can be seen in succulents, true xerophytes, and grasses. These plants usually grow slowly, use water efficiently, and exhibit passive cooling.

Water scarcity and heat are the key factors limiting the survival of desert plants. [I] Depending on strategies and physical adaptations, they can be divided into either succulents or drought- resistant plants. For the most part, succulents have evolved extensive, shallow root systems to quickly absorb water during brief periods of heavy rainfall. [II] Water is then stored in the fleshy tissue of their thick trunks or lobes, as well as in the roots. The cactus is a good example of a succulent. [III] To retain water, succulents have waxy coatings on their skin and a modified system of respiration. The stomata (surface pores), through which the plant takes in carbon dioxide and releases oxygen, open only at night when temperatures are cooler and less water from the plant will evaporate. [IV] Unlike most other plants, a succulent stores all or most of its chlorophyll, the chemical essential to photosynthesis, in its stem, skin, or other outer tissues, rather than concentrating it in the leaves. Doing this places it in a strong, well- watered part of the plant, rather than in an appendage susceptible to drying out and dropping off.

Drought- resistant plants, or xerophytes, come in many forms including true xerophytes, deciduous plants, and grasses. Xerophytic shrubs, such as the five species of creosote bush, are the most abundant type of vegetation in most arid environments. They are able to withstand desiccation in severe droughts without dying. Some have evolved small leaves with special coatings to reduce loss of water through evaporation. Others have replaced leaves with thorns or spines. While succulent roots form shallow, dense webs, xerophytes tend to develop deep root networks that pull water out of soil other desert plants cannot reach. For example, the roots of the mesquite bush, said to have the deepest root systems among desert plants, can reach depths of up to eighty feet.

Succulents and xerophytes, on the one hand, have physically adapted to gather and retain water to survive long periods of drought. Drought- avoiding plants, on the other hand, escape unfavorable conditions by perishing. These include annuals and perennials. Because profuse seed development is crucial to the survival of most annual species, they tend to produce far more flowers than other types of desert plants. The desert marigold of the American southwest, for example, has adapted to seasonal changes in rainfall by growing rapidly, bursting into a brilliant display of yellow flowers, and then dropping a cascade of seeds before dying. In some cases, annuals complete their entire life cycle in a matter of a few weeks or months. Their seeds may remain inactive for up to ten years while waiting for water conditions. Perennials, such as the ocotillo, may go dormant during dry periods, spring to life when it rains, and then return to dormancy in a process that may occur up to five times per year.

(Adapted from Expert on Cambridge IELTS practice tests)

According to paragraph 4, thorns on xerophytes ___________.