Peter Singer cares a lot about animals and he is against speciesism, the discrimination on the basis of species. However, I've never heard that he had said something about plants.
What makes plants or fungi different for Singer? Isn't differentiating between different organisms on basis of biological kingdom?
Peter Singer identifies wrong with pain. It's not particularly clear that plants experience anything similar to what we normally call "pain" whereas it seems like many types of animals do experience identifiably similar pain.
So it's discriminating based on what he takes to be the relevant feature of morality: pain. And this is linked to some form of sentience and consciousness.
