It seems that trees can talk to each other. Two American biologists at the University of Washington carried out an interesting 51 in the early 1980s. They placed 52 of harmful insects on the branches of some trees in order to discover 53 trees defend themselves from attack. They found that the chemical 54 of leaves on the tree 55 the insects were soon began to change. The leaves changed so much 56 it was soon impossible for the insects to eat such 57 . The insects began to die 58 starvations. 59 , the scientists' most surprising 60 was that the chemical composition of leaves changed on neighboring trees as well as on those already 61 by insects. Those trees were not yet under 62 and yet they were getting ready to defend themselves 63 attack. It seems that the trees were 64 passing information to each other. The trees were so 65 apart that there was no 66 contact between their branches. 67 were they close enough for their roots to touch. A similar American experiment was 68 out on a different kind of tree. 69 the same results were obtained. No one knows for certain how the trees pass information to their neighbors but some scientists think that they may 70 chemicals into the air to warn their fellows of danger.
【答案解析】 根据语境信息，这里应该为already affected by insects，意为“已经受虫子影响的树木”。affect“影响”，affected为其过去分词形式做后置定语修饰前面的代词those。brought为bring“带来”的过去分词形式。impose强加。effect起作用；达到目的。