16.

  1. The diseases of willow trees.
  2. The smell of willow.
  3. The possibility of tree communication.
  4. The need to preserve the wilderness.

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17.

  1. Suspicious.
  2. Hostile.
  3. Interested.
  4. Unconcerned.

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18.

  1. They began to speak.
  2. They made a strange smell.
  3. They became infested.
  4. They cured themselves.

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19.

  1. By speaking to the researchers.
  2. By allowing the researchers to treat them.
  3. By altering their own chemistry.
  4. By eating the caterpillars.

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20.

  1. Trees may be more complex than previously believed.
  2. The trees were observed communicating orally.
  3. Trees only communicate in small groups.
  4. What appears to be communication is probably a coincidence.

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W:You'll never believe what I heard on "Ecowatch" last night.
M:Try me.
W:Supposedly some researchers have found evidence for communication among trees.
M:Please. You must be kidding.
W:I swear it's what they said. Apparently those ecologists were studying a group of willow trees that were infested with caterpillars. They were in the process of trying to cure the trees when ...
M:When the trees started talking to them?
W:No, listen. One particular tree cured itself. It managed to change the chemistry of its leaves so that the caterpillars couldn't stand the taste.
M:Hmm, I'm impressed that the trees know how to take care of themselves, but that doesn't really mean they ...
W:If you'd let me finish then you'd understand. After the first tree had successfully altered the chemistry of its leaves, it produced a strange smell； shortly there after the surrounding trees which had not yet become infected began to undergo similar metamorphoses -- their leaves also began to produce this strange smell.
M:Gee, I have to admit, that is impressive. Still, it could just be a coincidence. They could have developed the same response that the first tree had independently.
W:Yeah, I thought the same thing. It's a fascinating idea nevertheless.