Passage 2 Can trees talk? Yes, but not in words. Scientists have reason to believe that trees do communicate with each other. Not long ago, researchers learned some surprising things. First a willow tree attacked in the woods by caterpillars ( 毛虫 ) changed the chemistry of its leaves and made them taste so terrible that they got tired of the leaves and stopped eating them. Then even more astonishing, the tree sent out a special smell---a signal causing its neighbors to change the chemistry of their own leaves and make them less tasty. Communication, of course, doesn’t need to be in words. We can talk to each other by smiling, raising our shoulders and moving our hands. We know that birds and animals use a whole vocabulary of songs, sounds, and movements. Bees dance their signals, flying in certain patterns that tell other bees where to find nectar ( 花蜜 ) for honey. So why shouldn ’ t trees have ways of sending message? It can be concluded from the passage that caterpillars do not feed on leaves that ______.