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.
A.the strong winds
B.the strong sun'sray
C.the long summerdays
D.the high soiltemperature
参考答案与解析：
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