Cats are carnivores, with no interest in vegetable food, and yet they will occasionally chew on plants. People watched his cat roam in the yard, which contains several poisonous plants, including dieffenbachia and allamanda. Happily, his cat has sniffed at these but never bitten into them.
In fact, outdoor cats very rarely chew on poisonous plants, but sometimes bored indoor cats do bite into houseplants, and some of the common ones—dieffenbachia and philodendron, for example—are poisonous. While few cats are ever poisoned this way, it might give you peace of mind to ask your vet for a list of poisonous shrubs and houseplants, plus information on emergency treatment.