Chapter 7 of Maseches Kilayim opens with the law of havrachah. Havrachah is the act of taking the end of a vine shoot and bending it back down into the ground, burying it in the earth so that it will grow up in another spot - sometimes in a better spot. The name of the act comes from the word berech (knee): just as a knee bends, so too a person bends the vine and brings it back into the ground.
"Hamavrich es hagefen ba'aretz":
The language of the Mishnah: "Hamavrich es hagefen ba'aretz, im ein afar al gabah sheloshah tefachim - lo yavi zera aleha" - one who bends a vine into the ground, if there is not three tefachim of earth over the buried portion, may not sow seeds above it. The reason: with three tefachim of earth the roots of the seeds do not work their way down to the vine; but with less than that we are concerned that they will find their way downward into the vine, and kilayim will result.
The Mishnah adds: "Afilu hivrichah bidla'as o besilon" - the law applies even when he buried the vine inside a hollowed out gourd, or inside a silon, a pipe made of earthenware and clay. For both of these are destined to be perforated by the roots of the seeds that are sown, and they offer no real protection to prevent the plant from penetrating into the vine.
The cases in which sowing is permitted:
A metal pipe: a pipe made of metal is not penetrated by the roots, and therefore sowing is permitted.
Sowing alongside the vine: this too is permitted, because the roots of the seeds do not spread out to the sides but rather descend straight downward, and in this way they miss the vine.
"Hivrichah besela": one who bends the vine into a rock or into a stone - even though there is only three fingerbreadths of earth over it, he may bring seed above it, because the stone forms a barrier against any growth that emerges from the seed. In practice not even three fingerbreadths are needed, and the Mishnah Rishonah explains that any amount suffices.
"Arkuvah shel gefen":
An arkuvah describes a vine that came up out of the ground a bit, and immediately after it emerged the person bent it, brought it back into the ground and replanted it - so that its shape is like the shape of an arkuvah, a bent knee.
In such a case the Mishnah says: "Ein modedin lah ela min ha'ikar hasheini" - when a person comes to measure the six tefachim that one must distance from a single vine before planting, he measures only from the second base, the place where the arkuvah emerges at its far end. In other words: the vine comes up out of the ground, returns into the ground and comes up again, and the measuring is done from the second place where it comes up.
And when does this law apply? The Mishnah Rishonah explains that it applies specifically when the first place from which the vine emerged is buried and is not visible, and only the second place is visible. But if the first base is visible to the eye, one must count and measure from the first place where it emerged from the ground as well. And the reason the Mishnah spoke of the second place is only because we assume that the first place is not visible.
In summary: in this Mishnah we learned the law of havrachah - bending a vine shoot and returning it into the ground. One may not sow above it unless there is three tefachim of earth over it, and even bending it into a gourd or into an earthenware pipe does not help, since these become perforated by the roots. Sowing is permitted, however, in the case of a metal pipe, alongside the vine, and where he bent it into a rock - there three fingerbreadths of earth suffice, and according to the Mishnah Rishonah any amount suffices. We also learned the law of an arkuvah of a vine, that one measures only from the second base, and this applies specifically when the first base is buried and not visible.
In the next Mishnah we will continue our study of the laws of the vine in this chapter.