Animals as well as humans can suffer pain, deprivation, and unwanted death. Vegetables cannot. Hence there is a very fundamental and relevant sense in which we cannot harm a vegetable. Anything we do to a head of lettuce or the bloom of a flower can be harmful (or beneficial) to one or more sentient beings who feed on these or otherwise enjoy them. The head of lettuce and the flower, however, feel nothing and regret nothing so far as we know. An exception for vegetables is thus consistent with the categorical imperative; an exception for humans with respect to eating animals is not.
(Julian H. Franklin, Animal Rights and Moral Philosophy [New York: Columbia University Press, 2005], 45 [endnote omitted])