Kilayim, Chapter 2, Mishnah 7. Continuing the principle we have learned, that the laws of keeping different species separate in a field are essentially a matter of appearance - how the thing looks to an observer - and are only rabbinic, our Mishnah lists several cases that were permitted, because in them the problem of appearance does not arise.
The corner of a triangle entering the adjacent field:
A triangular area sown with wheat, whose pointed corner enters into a field of barley - this is permitted. The reason: it looks like the end of his field at the tip of the triangle, like two entirely different fields, even though it is possible that one man owns both of them. The very separation and distinction between them gives them the appearance of two separate fields.
"Shelo chittim ve'shel chavero min acher" - his own is wheat and his fellow's is another species:
His field is sown with wheat, and in his fellow's adjacent field another species is growing, let us say barley - "mutar lismoch lo me'oso hamin" - he is permitted to place alongside it that same species. That is, he may plant his own barley right up against his fellow's barley field, even though there is now wheat in his own field next to barley. The reason: as far as appearance goes, the part sown with barley looks like a continuation of his fellow's field, and the dividing line appears to run between the two species - which is not the case, for in truth the dividing line runs elsewhere, and within his own private field wheat and barley are found together. But since the barley is adjacent to his fellow's barley, it looks like a continuation of it, and it is therefore permitted.
"Shelo chittim ve'shel chavero chittim" - his own is wheat and his fellow's is wheat: a furrow of flax:
His field is sown with wheat, and his fellow's adjacent field is also sown with wheat - and even so, "mutar lismoch lo telem shel pishtan" - he is permitted to place next to his fellow's field, right between the two fields, a furrow, that is, a single row of flax, but not a row of any species other than flax.
Why specifically flax? The early commentators offered several reasons:
The Rambam: It is not a person's way to sow a single row of flax, since such a small quantity is of no use.
The Rosh: A person does not want to place flax next to his grain, because flax tends to damage grain.
According to either reason, the impression created is that he planted the flax only in order to test how well it grows - whether it will make for a good crop in the future or not - and not because he wants to sow two species together. Another species, by contrast, is not permitted, since it does not have these characteristics, even if his intention is merely to test how it grows. It is worth noting that this permission to place a row of flax applies only at the edge of his field, between the two fields, and not in the middle of the field.
The views of Rabbi Shimon and Rabbi Yosei:
Rabbi Shimon: The permission applies both to flax seed and to other species, and they may be placed adjacent to each other between the fields. And some understand his words in exactly the opposite way - that he takes the stricter position and says that even flax seed is forbidden, just as other species are forbidden, disagreeing with the first Tanna.
Rabbi Yosei: He goes further and permits such a test even in the middle of the field - a test of viability, how well the flax will succeed, in a single furrow and a single row. This is permitted, because onlookers will understand that the intention is not to create kilayim, but simply to run a brief test of how the flax grows. Therefore it is permitted even in the middle of his own field, and not only between his field and his fellow's field, as the first Tanna held.
In summary: In this Mishnah we learned that whatever does not present the appearance of kilayim is permitted: the corner of a triangle of wheat entering a barley field, planting a species identical to the one growing in his fellow's field alongside it, and a furrow of flax between two fields of wheat, which looks like nothing more than a test. We also examined the dispute among the Tannaim as to the scope of the permission - Rabbi Shimon regarding other species and flax seed, and Rabbi Yosei regarding permitting the test even in the middle of the field.