Because plants cannot move or talk, most people believe that they have no feelings and that they cannot receive signals from outside. However, this may not be completely true. People who studied plants have found out that plants carry a small electrical charge  . It is possible to measure this charge with a small piece of equipment called "galvanometer". The galvanometer is placed on a leaf off the plant, and it records any changes in the electrical field of the leaf. Humans have a similar field which can change when we are shocked or frightened. A man called Backster used a galvanometer for his studies of plants and was very surprised at his results. He found that if he had two or more plants in a room and he began to destroy one of them - perhaps by pulling off its leaves or by pulling it out of its pot - then the galvanometer on the leaves of the other plants showed a change in the electrical field. It seemed as if the plants were signalling a feeling of shock. This happened not only when Backster started to destroy plants, but also when he destroyed other living things such as insects  . Backster said that the plants also knew if someone had destroyed a living thing some distance away, because they signalled when a man who had just cut down a tree entered the room. Another scientist, named Sauvin, achieved similar results to Backster's. He kept galvanometers fixed to his plants all the time and checked regularly to see what the plants were doing. If he was out of the office, he telephoned to find out about the signals the plants were sending. In this way, he found that the plants were sending out signals at the exact times when he felt strong pleasure or pain. In fact, Sauvin could cause a change in the electrical field of his plants over a distance of a few miles simply by thinking about them. Backster was surprised at the results of his studies because   _  .
he destroyed an insect he destroyed a plant by pulling off its leaves he found that plants could express feelings of shock he found that plants could move and speak after all