I know the dictionary answer is vetch, an alfalfa-like plant. Is he making a comparison between "wheat" and "tares?" I realize this sounds like a homework question, but I'm over 60, and maybe a little nerdy.
We have among us 600 years, good & sound, left in the bank of life. Take 30 of them—the richest birth-day gift ever offered to poet in this world—& sit down & wait. Wait till you see that great figure appear, & catch the far glint of the sun upon his banner; then you may depart satisfied, as knowing you have seen him for whom the earth was made, & that he will proclaim that human wheat is worth more than human tares, & proceed to organize human values on that basis.
Mark Twain to Walt Whitman on the occasion of Whitman's 70th birthday, May 24, 1889