Kilayim, chapter 5, mishnah 1. The mishnah deals with the law of a "kerem shecharav" - a vineyard that has become ruined. This differs from the law of the bald patch that we learned, since here we are not dealing with one isolated bare patch, but with many vines that have been destroyed in various places throughout the vineyard.
An impoverished vineyard:
Even though the vineyard has been ruined, if one can gather within it ten vines in an area normally sown with grain - a field of that size - and they are planted in the proper formation, meaning two vines opposite each other with a fifth vine emerging from them, or a third vine in the second row, as we mentioned earlier, this is defined as a "kerem dal," a weak and impoverished vineyard. As long as this minimum measure of ten vines remains and they are arranged in parallel rows in this manner, the vineyard has not lost its name.
Since it retains the status of a vineyard, one may not sow near it except at a distance of four amos, and a distance of six tefachim does not suffice:
A single vine: the required distance is six tefachim.
A vineyard: the required distance is four amos - and this is the law of an impoverished vineyard.
A vineyard planted in disarray:
A vineyard whose vines grow in scattered places and are not planted in rows at all: if within the disarray one can identify vines that are aligned opposite one another - even though they are not planted in straight rows - and one can measure two vines opposite three, meaning two rows of two with one more behind them, as was explained above, this is considered a vineyard with all its laws.
But if it does not even have this measure, and the vines are scattered in completely random places in a way that one cannot measure and establish that there are vines aligned opposite one another, it does not have the status of a vineyard but only the law of individual vines, and it suffices to plant outside of six tefachim.
The view of Rabbi Meir:
Rabbi Meir disagrees and holds: "Ho'il vehu nireh k'tavnis hakeramim - harei zeh kerem" - since it looks like the pattern of vineyards, it is a vineyard. That is, since it has the general appearance of a vineyard and its manner of arrangement, it is halachically considered a vineyard even though one cannot find in it three opposite two, and one must keep a distance of four amos from it.
In summary: In this mishnah we learned three cases: a ruined vineyard in which ten vines per beis se'ah remain planted in the proper formation - this is a "kerem dal" and its law is that of a vineyard, requiring a distance of four amos; a vineyard planted in disarray in which one can align two opposite three - a vineyard; and if it does not even have this measure - the law of individual vines applies to it and its required distance is six tefachim, while according to Rabbi Meir it too is a vineyard, since it looks like the pattern of vineyards.