There are many houseplants that are toxic to cats, but fortunately the majority of felines seem to co-exist with them, without being tempted to nibble on them.
However, if Felix is especially curious, his caregivers should avoid growing dieffenbachia, dracaena, philodendron, schefflera, rubber plant, asparagus fern, aloe, peace lily (or any kind of lily actually), or English ivy.
For more information you may peruse a longer list of poisonous plants, on the web site of the Cat Fancier's Association.
It appears that some plants are especially lethal, even if the cat ingests only a tiny piece. However, there are other plants that only cause problems if a big hunk of them is chomped on.
Look for the following symptoms if you suspect a cat has been poisoned: vomiting, diarrhea and a staggering gait. Always try to find a sample of chewed leaves to take with you to the vet to aid the diagnosis, if you suspect poison may have been ingested by the cat.
Remember, too, that seasonal flowers grown indoors, such as amaryllis, paperwhite narcissus and Easter lilies are not good for felines.