Kilayim, Chapter Seven, Mishnah One
The first three mishnayot of our chapter deal with an ancient vineyard farming technique called bending or havracha in Hebrew. Basically one takes a vine bends it into the ground and then takes the other head of the vine out of the ground in another place from where it will grow a new vine. The advantage gained through this technique is that he now gets a new vine that is in a sense nursed by its mother vine, at least for the time being. This is less risky and simpler than starting a new vine from scratch. Using this technique one can also allow more vines in his vineyard. The mishnah deals with the implications that this technique has on the laws of kilayim.
1) If one has bent a vine into the ground [and then brought it back up elsewhere], then if there is not soil over it to the height of three handbreadths, he may not sow seed above it, even if he bent [and conducted it underground] through a gourd or through a pipe.
2) If he bent it [and conducted] it through rocky soil, then even if there is not soil over it to the height of three handbreadths, it is permitted to sow seed above it.
3) As for a knee-like vine [formed by burying and conducting it underground], they dont measure [for how much space to work it] except from its second root.
Section one: The issue here is whether one can plant seeds on top of the vine that is being conducted underground. The mishnah states that if there are not three handbreadths of soil above the vine he may not sow seeds there. This rule holds true even if he conducted the vine through a dried-out gourd, which would have been used as a pipe, or a regular pipe made out of clay. Since the seeds can pierce the walls of the gourd or the clay pipe, they are not sufficient to cause a break between the seeds and the vine.
Section two: However, if the soil is rocky and the seeds will not be able to penetrate it and thereby come into contact with the vine, then he does not need three handbreadths of separation.
Section three: When the vine is taken out of the ground after having been conducted from there from the mother vine, it will bend down and form a knee-like point which is called an avrucha. When measuring how much space the vine needs to be worked, they measure from the second spot where the vine seems to come out of the ground, at this joint, and not from the root where it first comes out of the ground.