Why did an officer want to give the dog a piece of mushroom?

In a police station, an officer managed to get some fresh mushrooms. He was so happy that he agreed to share the mushrooms. When their breakfast arrived the next day, each officer found some mushrooms on his plate.
   "Try the dog with a piece first, "suggested a careful officer who was afraid that the "mushrooms" might be poisonous.
   The dog seemed to enjoy its mushrooms, and the officers then began to eat their meal, feeling quite satisfied with its good taste.
   After an hour, however, they were so frightened when the gardener rushed in to announce that the dog was dead. Immediately, the officers jumped into their cars and rushed to the nearest hospital. Stomach pumps were used and the officers had a very unpleasant time getting rid of the remains of the mushrooms.
   When they returned, they sat down and started to discuss the whole matter. Each man described the pains that he felt and they agreed that these had grown worse on the way to the hospital.
   The gardener was called in to give a detailed account of the way in which the poor dog had died.
   "Did it suffer much before it died?" asked one of the officers, feeling very pleased that he had escaped a painful death himself. The gardener looked rather surprised. "No. "he said. "It was killed instantly when a car hit it."