Only special plantscan survive in the terrible climate of a desert,for these are regions where theannual range of the soil temperature can be over 75℃.Furthermore,during the

summer there are fewclouds in the sky to protect plants from the sun's ray.Another problem is thefact that there are frequently strong winds which drive small-sharp particles

of sand into theplants,tearing and damaging them.The most difficult problem for all forms ofplant life,however,is the fact that the entire annual rainfall occurs during afew

days or weeks inspring.

Grasses and flowersin desert survive from one year to the next by existing through the long,hot,dry season in the form of seeds.These seeds remain inactive unless theright

amount of rainfalls.If no rain falls,or if insufficient rain falls,they wait until the nextyear,or even still the next.

Another factor thathelps these plants to survive is the fact that their life cycles are short.Bythe time that the water from the spring rains disappears—ust a few weeks afterit

falls一such plants no longer need any.

The perennials（多年生植物）have special features whichenable them to survive as plants for several years. Thus,nearly all desertperennials have extensive root systems

below ground and asmall shoot system above ground.The large root network enables the plant toabsorb as much water as possible in a short time.The small shoot system,on

the otherhand,considerably limits water loss by evaporation（蒸发）.

Another feature ofmany desert perennials is that after the rainy season they lose their leaves inpreparation for the long,dry season,just as trees in wetter climates losetheirs in

preparation for thewinter. This reduces their water loss by evaporation during the dryseason.Then,in the next rainy season,they come fully alive once more,and grownew

branches,leaves andflowers,just as the grasses and flowers in desert do.The last paragraphis mainly about__________.

A.the comparisonbetween plants in different areas

B.the growth of newleaves in the rainy season

C.water loss byevaporation in the dry season

D.the changes in theleaves of the perennials