1 . Directions: Fill in each blank with a proper word chosen from the box. Each word can be used only once. Note that there is one word more than you need.

A. adaptable        B. classic          C. determined   D. effectively   E. fundamentally F. immediately     G. sideways       H. stretched       I. survival        J. underside       K. upright

Why Are Trees Tree-shaped?

While running through a leafy park days ago, I passed dozens of tree species of different sizes and ages, and I was suddenly struck by how     1     similar they were. Wide trunks split into smaller branches in the same sort of way. You could guess the height of a tree just by looking at how wide its trunk is. We would notice     2     if someone drew a tree with branches that were too wide or a trunk that was too thin. What makes a tree look like a tree? Maximising the sunlight hitting the tree’s leaves is obviously critical for the tree’s     3    . But the size and shape of the branches is largely     4     by a different standards: a tree must be strong enough to stay standing     5    , possibly for centuries. Imagine an oak tree, with branches reaching     6     off the main trunk. Each branch has to support its own considerable weight, so the wood in its lower half is slightly compressed and the top half is being     7    . That top layer of a branch takes the weight of the branch. And a particular type of wood has grown there to do that job. It’s called reaction wood, because it formed as reinforcement (增强) to take the increasing weight as the branch grew. The trees that reinforce on top the branches are the “lollipop” ones:     8     branching species like oaks. But there is another way of solving that problem. Some trees grow their reinforcing reaction wood on the     9     instead, so the branch is pushed up, rather than pulled from above. It works, but not as     10    , so these trees can’t support large side branches. The ones that reinforce underneath have a thick trunk with much smaller side branches, and they grow to be conical—some examples of these trees are pines and firs.

oak pine fir

2 . Directions:Complete the following sentences by using the proper form of the words or expressions   given in the frame.Each one can only be used once.

A. capable        B. effectiveness       C. employing        D.exposure          E.famously           F. joining          G. limitation        H. minimal       I.precisely        J.recognizing       K.worthwhile

Clean Air Act

The air in modern homes and offices is pretty clean,but not as clean as it might be.Often it contains small amounts of volatile(挥发性的）,poisonous,organic compounds.Long-term    1     to these is a bad thing,so clearing them out of the air people breathe is widely accepted as     2    

Finding an effective way to do so has proved difficult.But Stuart Strand, Long Zhang and Ryan Routsong, of the University of Washington,in Seattle,think they have succeeded,As they report in Environmental Science and Technology, their method involves     3     a gene from a rabbit into a popular indoor plant nicknamed Devil's vine—a type of ivy hat is so called because it is    4     difficult to kill.

The idea of     5     plants to de- pollute   the atmosphere inside buildings has been around for decades-but has met with only qualified success. One experiment involving unmodified spider plants,for example,showed that they are indeed     6     of removing formaldhyde (甲醛) from the air.The     7     is that to make much of a difference in a space as large as a house would require turning most of the rooms into spider-plant forests.

Dr Strand, Dr Zhang and Mr Routsong thus sought something suitably transgenic ( 转 基 因 的),but that does not flower indoors. The plant they settled on was Devil's vine,     8     because of its toughness.With the help of a bacterium,they were able to ferry the rabbit version of the gene into the plant's chromosomes(染色 体),and thus to,engineer a type of Devil's vine able to produce an air-cleaning substance. To test the     9     of their idea, the researchers put their modified ivy to work inside greenhouses filled with air containing high levels of harmful substances.The plants performed well,reducing the harmful substances in air to     10    .