Tosefta sur Péa 6:9

Two gavels separated from one another are [considered to be] Shikcha (forgotten sheaves), [but] three [gavels] are not [considered to be] Shikcha. Two sheaves separated from one another are [considered to be] Shikcha (forgotten sheaves), [but] three [sheaves] are not [considered to be] Shikcha. Two grapevines separated from one another (i.e. from separate tree trunks) are [considered to be] Shikcha (forgotten sheaves), [but] three [grapevines] are not [considered to be] Shikcha. Two [grape] berries [lying next to each other on the ground] are [considered to be] Peret (individual fallen grapes), [but] three [grape berries lying next to each other on the ground] are not [considered to be] Peret. Two stalks [of grain] separated from one another in the usual fashion are [considered to be] Leket (fallen stalks), [but] three [stalks of grain] are not [considered to be] Leket. These are the words of Bet Hillel. Rebbi Yossi says, “Chananyah, the son of the brother of Rebbi Yehoshua says, ‘Any [type of produce] where the property of the poor person (i.e. some kind of gift to the poor) can come in the middle [of two sets of produce, one standing and one detached, both of which are one Seah in volume], for example grain and vineyard, does not combine [to form two Seahs and therefore both parts are still considered to be Shikcha]. However, any [produce] where the property of the poor person (i.e. some kind of gift to the poor) cannot come in the middle [of two sets of produce, one standing and one detached, both of which are one Seah in volume], for example fruits of a tree, does combine [to form two Seahs and therefore both parts are not considered to be Shikcha].’”