(Devarim 24:21) "When you cut your vineyard, do not glean it after you": From here R. Eliezer ruled: A vineyard which is all oleloth (gleanings) belongs to the owner. R. Akiva says: It belongs to the poor, it being written (Vayikra 19:10) "And you shall not glean your vineyard."
"do not glean it": Which are "gleanings"? Those which have neither "arm" nor "dripping." If they have "arm" but not "dripping" or "dripping," but not "arm," they belong to the owner; if not, they belong to the poor. ("do not glean it) after you": whence we derive that they are subject to shikchah.
"after you": whence we derive that they are subject to peah. From here they ruled: What is shikchah in an arbor? All that he cannot stretch out his hand for and take. And in runners (grapes growing in a row on isolated vines), whatever he passes by.
"to the stranger, to the orphan, and to the widow shall it be": It is written here "stranger," "orphan," and elsewhere (Ibid. 19) "stranger," "orphan." Just as there (it is shikchah) only when it contains less than two sa'ah, so, here.