Peah, Chapter 2, Mishnah 4. In the previous Mishnah we learned that with trees, with the fruit of trees and with what is called an orchard, the only thing considered a separation dividing one orchard from another - turning one orchard into two - is a fence, that is, a wall. Our Mishnah comes to teach that there are trees so significant, unusually tall and with many branches, that even a fence is not enough to divide between them.
The law of carob trees - "kol haro'in zeh es zeh":
The Mishnah says: "V'lecharuvin, kol haro'in zeh es zeh" - as long as the carob trees see one another, so that a person standing beside one can see the next one, they are all considered a single field, even if there is a fence between them. Once they see one another, this is one field, one grove, and only a single peah is given from all of them.
The practice of Rabban Gamliel's household:
Rabban Gamliel said: "Nohagin hayu beis abba, nosnin peah achas lazeisim shehayu lahem b'chol ruach, v'lecharuvin kol haro'in zeh es zeh". In other words, this was the practice of his father's house, the house of the Nasi:
With olive trees: they gave a single peah from the olive trees they had in each direction. The northern side of the city, where they had olive trees, and likewise the southern, western and eastern sides - each direction was considered a separate field and a separate grove, and from each one they gave a single peah. And it appears that this is the law even if the trees did not see one another, since the entire direction is considered one field.
With carob trees: even Rabban Gamliel's household followed the words of the first Tanna, that the carob trees must see one another. Only with olive trees did this exceptional law apply, that even if they do not see one another, but stand in the same direction on one side of the city, they are considered a single field.
The opinion of Rabbi Eliezer beRabbi Tzadok:
Rabbi Eliezer beRabbi Tzadok says in the name of Rabban Gamliel: "Af lecharuvin shehayu lahem b'chol ha'ir" - that is, not as was stated at first, that the measure for carob trees is that they see one another, but rather all the carob trees they had anywhere in the city are counted for a single peah, even if they do not see one another. According to this, the carob trees are considered a single field even more than the olive trees: the olive trees are considered divided into fields according to the direction in which they stand, each direction a field in its own right, whereas with carob trees the entire city is considered one, according to the practice of Rabban Gamliel's household.
In summary: our Mishnah teaches that there are trees between which even a fence does not create a division. According to the first Tanna, with carob trees the measure is "all that see one another." Rabban Gamliel's household practiced giving a single peah for the olive trees in each direction, and with carob trees they required that the trees see one another. Rabbi Eliezer beRabbi Tzadok, however, reports in his name that even the carob trees throughout the entire city are considered one, even without their seeing one another.