Kilayim, Chapter Five, Mishnah One
This mishnah deals with two vineyards that potentially do not count as vineyards. The first is a vineyard that has been partly ruined, and the second is a vineyard that was not planted in the way in which vineyards are normally planted.
1) A vineyard that has been [partly] ruined: if it is still possible to harvest ten vines within a bet seah, and they are planted according to halakhah, behold this is called a poor vineyard.
2) A vineyard planted in a mixed-up manner, if there remains an alignment of [one line of] two parallel [vines] opposite [a line of] three [vines], it constitutes a vineyard, but if not it is not a vineyard.
a) Rabbi Meir says: since it is in appearance like a vineyard [in general], it is a vineyard.
Section one: If the vineyard was ruined but there are still ten usable vines within a field the size of a bet seah (2500 square cubits, about 25 meters by 25 meters), then these vines constitute a vineyard and all of the rules regarding a vineyard apply. The one caveat is that the vineyard has to have been planted according to halakhah, meaning two vines opposite two vines with a fifth vine forming a tail (see illustrations in 4:6). If there are less than ten vines or they were not planted according to the normal way then they dont constitute a vineyard.
Section two: If a vineyard was planted in a mixed up manner, meaning not in the normal way (which the Mishnah terms according to halakhah) then these vines still constitute a vineyard if there is one section where the proper alignment (2 x 2, with a tail) remains. If no such alignment is found anywhere within this vineyard then it just doesnt count as a vineyard.
Rabbi Meir says that since there are a lot of vines here and it looks like a vineyard, it still counts as a vineyard even if the proper or normal alignment of vines doesnt exist anywhere. Rabbi Meir might be concerned that if the sages did not treat this mixed up vineyard as a real vineyard people would become mixed up and treat real vineyards as if they too were not really vineyards (you might have to read that sentence a few times unless you too want to get mixed up!)