We continue with Maseches Kilayim, chapter two, and before us is Mishnah 8. The Mishnah deals with two subjects: the prohibition of placing one species alongside a field of another species, and the items found in a field that form a partition between the species - on account of which one may sow the two next to each other.
"Ein somchin lisdeh tevuah chardal v'charia, aval somchin lisdeh yerakos chardal v'charia" - one may not place a row of mustard or safflower next to a grain field, but one may place them next to a vegetable field.
What is the reason for the distinction between a grain field and a vegetable field?
The approach of the Rosh: Mustard and safflower do not damage grain, and therefore an onlooker assumes that they were placed there permanently and that the owner has no objection to their being together - which looks like sowing kilayim. Mustard and safflower, however, do have a damaging effect on vegetables, and for that reason we assume that he placed them there only as a test, to see whether the spot is suitable for planting mustard and safflower, and not on a permanent basis - and therefore it is permitted.
The approach of many other commentators: They understand the matter in exactly the opposite way - mustard and safflower damage grain and do not damage vegetables. And the reason for the prohibition: we are speaking of someone placing them alongside his fellow's field, and his fellow would not allow another person to place mustard and safflower there that will damage his grain. The onlooker will therefore assume that the mustard belongs to the owner of the field himself, and that he wants it so badly that he is willing to give in and absorb the damage to his grain - so it appears to the eye as though he sowed it. But when it is next to a vegetable bed, where there is no damage at all, there is no concern that it will look as though his fellow sowed it there.
Items that form a partition between species:
From here the Mishnah moves on to items found in a field that can serve as a partition separating two different species, on account of which one may sow them next to each other as long as the separating item remains in place: "V'somech labor, v'lanir, v'lagafah, v'laderech, v'lagader shehu gavoah asarah tefachim, v'lecharitz shehu amok asarah v'rachav arbaah, v'la'ilan shehu meiseich al ha'aretz, v'lasela shehu gavoah asarah v'rachav arbaah":
Bor and nir: a field that is not sown, or one that has been plowed and not yet sown.
Gafah: a wall of stones that are not joined together with mortar.
A road.
A wall ten tefachim high.
A trench ten tefachim deep and four tefachim wide.
A tree that spreads over the ground: the word "meiseich" is like the word sukkah - its branches come down along its entire length until the ground and create a partition between what is under the tree and what is outside it.
A rock ten tefachim high and four tefachim wide.
The measurements of the partitions:
Let us return to a field that is bor or nir: for it to be considered a partition, many commentators hold that an area of a beis rova is required, which is about ten and a fifth amos by ten and a fifth amos. The other items naturally have measurements of their own: the road - four amos; the wall - a height of ten tefachim; the trench - a depth of ten tefachim and a width of four. All of these have their measurements specified, whereas regarding bor and nir the commentators say that the measurement is a beis rova.
In summary: in this Mishnah we learned the distinction between placing mustard and safflower alongside a grain field, which is forbidden, and placing them alongside a vegetable field, which is permitted - and two directions of explanation for the reason: according to the Rosh, they do not damage grain and therefore it looks like permanent sowing, while with vegetables they do damage and therefore we assume he is merely testing; and according to other commentators the reverse, that they damage grain, and consequently it appears to the observer as though the owner of the field himself sowed them and gave up on his grain. We also listed the items that form a partition between species - bor, nir, gafah, a road, a wall, a trench, a tree that spreads over the ground and a rock - and we noted their measurements, including the measurement of a beis rova for bor and nir.