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The most extreme example of vegans I've encounterd is so-called "Free Fall" fruitarians. They harvest their food that naturally falls off the branches, as this is dying naturally of "old age". They will not eat root plants as you are killing the plant by harvesting the root.
I think, pain is proportional to the evolution the organism has achieved. Neural systems are absent or quite rudiment in primitive organisms. They don't have a good neural network to communicate the sense of pain to the neural center(sometimes such centers will be absent). In such organisms response to stimuli end just in some muscle movement. Where as in evolved organisms like mammals pain is multidimensional. If we take humans the pain does not have to be physical, It could be of from emotional or social reasons. Imagine the trauma a lamb will be going through if its mother is getting killed in front of its eyes in a slaughter house....It has been seen in wild elephants in Africa that they still morn the death of their family members years after they reach the sites they got hunted. Many studies suggest that Cetaceans like Dolphins have a better evolved emotional brain than humans. see this link http://www.theecologist.org/blogs_and_comments/commentators/other_c...
The case of plants are entirely different. There pain will not be as complex as an animal having an evolved brain. Of course they respond to touch, light, water or other stimuli but they are very primitive. We hurt plants by eating food sourced from them, but we have no other choice - we have to eat to keep us alive. So we choose the less painful path(for the one who eats and the one who is getting eaten)
The more developed nervous system a living being has, the better it/he/she is able to feel pain. Plants don't have a nervous system. They don't feel pain, they cannot suffer from it the way animals do. Veg people can eat plants without blaming themselves.