Kilayim, Chapter 2, Mishnah 3. The Mishnah deals with someone who sowed his field with wheat and then changed his mind, and now wishes to sow barley in its place. How does he rid himself of the wheat that has already been sown?
Waiting until it "becomes wormy":
He must wait until the wheat kernels he sowed become "wormy" - from the word for worm, meaning that they have sent out small roots resembling worms. In moist soil this takes about three days, and in dry soil somewhat longer. Once they have begun to take root, he must turn the soil over with a hoe, and only then may he sow.
The reason for this: had he not waited until they took root, the kernels would retain their power of growth even after the soil was turned over. But once they have taken root, uprooting them by turning the soil over halts the growth of the wheat, and from that point on he is permitted to sow the barley.
If it has sprouted:
If the wheat has already sprouted, and there is a leafy growth coming up out of the ground, it has reached that very same stage. He should not say: I will sow the barley and immediately afterward turn the soil over, since the barley will not be harmed because it has only just been sown, and by turning the soil over I will uproot the wheat. He may not follow this order: first he turns the soil over, and only afterward does he sow.
The novel point of the Mishnah:
Even though the wheat has already become wormy, sprouted and come up out of the ground, and one might have been tempted to say that since it is visible to the eye he will not forget to turn the soil over - unlike the beginning of the Mishnah, which deals with seeds hidden in the ground that cannot be seen, where it is obvious that he must turn the soil over first - even so, one may not rely on his remembering. First he turns the soil over and uproots the wheat sprouts, and only then does he sow the barley.
"Kamah yehei choresh":
Turning the field over is a form of plowing, and therefore the Mishnah asks how much of the field he must uproot. Its answer: "K'talmei ha'revi'ah" - he should do so in a manner resembling the furrows made after the rains have fallen, whose width was generally known in the days of the Mishnah, and that is the measure he is required to uproot.
Abba Shaul says: "Kedei shelo yeshayer rova l'veis se'ah" - he must uproot the entire field, provided that he does not leave behind an unuprooted area of a quarter kav in a beis se'ah, that is, in an area in which one normally sows a se'ah of seed. It emerges that one twenty-fourth of the ground may be left unplowed, and that suffices according to Abba Shaul. True, this amount will still produce wheat in the new barley field, but there is nothing wrong with that, since as long as the measure is one twenty-fourth or less, it is permitted with regard to kilayim.
In summary: In this Mishnah we learned how to rid oneself of the wheat seed that was sown earlier: waiting until it becomes wormy and takes root, turning the soil over, and only afterward sowing the barley. Even when the wheat has already grown and is visible to the eye, the order may not be changed and one may not rely on remembering. The measure of plowing is gauged as "k'talmei ha'revi'ah," and according to Abba Shaul provided that he does not leave over a quarter kav in a beis se'ah, which is one twenty-fourth, a measure that is permitted with regard to kilayim.