Peah, Chapter 6, Mishnah 8. To understand the Mishnah before us, let us begin with the words of the Yerushalmi, which explains the source of this law.
The basis of the law in the Yerushalmi:
The verse says regarding shichechah (forgotten sheaves): "When you reap your harvest in your field and you forget a sheaf in the field." From the wording of the verse the Yerushalmi derives that the law of shichechah applies specifically to a sheaf whose surroundings are already harvested - a bundle around which the grain has already been cut. But a sheaf surrounded by standing grain, that is, grain still uncut which was not forgotten, does not become shichechah. It emerges that standing grain which was not forgotten serves as a kind of safety mechanism: it prevents the forgotten sheaf next to it from being considered shichechah.
The wording of the Mishnah:
"Hakamah matzeles es ha'omer ve'es hakamah" - standing grain that was not forgotten saves for the owner both the sheaf that was forgotten and the standing grain that was forgotten. When a forgotten bundle lies next to standing grain that was not forgotten, or forgotten standing grain stands next to it, neither of them becomes shichechah, thanks to that standing grain which was not forgotten.
"Veha'omer eino matzil lo es ha'omer velo es hakamah" - a bundle that was not forgotten does not provide this protection at all: not to another sheaf that was forgotten, and not to standing grain that was forgotten. This highlights that this is a property unique to standing grain alone; a bundle has no such power.
The amount of standing grain that saves:
The Mishnah asks: "Eizo hi kamah shehi matzeles es ha'omer?" - how much standing grain is needed for it to have the power to save the forgotten sheaves from becoming shichechah? To this the Mishnah answers: "Kol she'einah shichechah afilu kelach echad" - a single stalk of standing grain that was not forgotten is enough, and it has the power to save a forgotten sheaf or forgotten standing grain from becoming shichechah.
In summary: We learned from the Yerushalmi that the law of shichechah was stated specifically about a sheaf whose surroundings are harvested, and not about a sheaf surrounded by standing grain, and from here that standing grain saves. The Mishnah establishes that standing grain which was not forgotten saves both the sheaf and the standing grain, while a sheaf saves nothing at all - neither a sheaf nor standing grain. And the amount of standing grain that saves - even a single stalk.