I wndered how far I would get down this thread before someone brought up the 'Carrots have feeling too' argument. If you are an 'ethical' vegetarian the aim is to reduce suffering as much as possible. It is clear to me that animals feel pain because the more evolved ones show very similar reactions to pain and distress as we humans do. Even further down the scale, science can show that animals have nervous systems and the only point of those is to feel. Plants do not have nervous systems.

So if these plants actually do feel pain in some way which is unknown to us, given the aim of lessening suffering there are two options open. The first is suicide which causes extreme mental suffering to everyone who cares about you. The other option is vegetarianism.

What do cows, pigs, sheep etc eat - plants! It takes about sixteen pounds of plant protein to produce a pound of beef. So eating that pound of beef not only makes you responsible for the suffering of the cow (and the term humane slaughter is a nonsense), but also the supposed suffering of the plants that went to feed the cow, not to mention the suffering of the people who could have eaten those plants if they hadn't been fed to a cow.