Mishnayos Kilayim Perek 2 Mishnah 10

כל שהוא בתוך רובע – as for example, ravines filled with water, for even though they are not appropriate for sowing, they count for the measurement of a Bet Rova/a piece of ground of the capacity of one Rova of seed (i.e., one-quarter of a Kab) to be considered the distance between each and every species.

אכילת הגפן – what the vine seizes and consumes around it, which is in order that it functions, counts for the measurement of a Bet Rova to complete the ten cubits and two-and-one-half handbreadths length by ten cubits width, as we stated above (in Mishnah 9) which is the measurement of a Bet Rova that one must form a partition between each species. And similarly, a grave also counts and similarly a rock, but we are speaking of a rock that is not ten [handbreadths] for if it is ten [handbreadths], it is a partition and separates by itself.

תבואה בתבואה בית רובע -if his field was sown with wheat and he wishes to sow in it barley, he needs to distance it a Bet Rova (i.e., a piece of ground the capacity of one Rova/one-quarter Kab of seed), as long as there won’t be any wheat surrounding the barley from the four directions for there the distance does not benefit until would be open from one direction.

ירק בתבואה בית רובע – and especially when it is quadrangular that we require this, but if he wanted to make one row of vegetables within the field of grain, he makes the length of the row of ten cubits and one-half by a width of six handbreadths for they were more stringent with a quadrangular [plot] than with a row, and this is what we stated in the Jerusalem Talmud.

רבי אליעזר אומר ירק בתבואה ששה טפחים – for he holds that we are not stringent with a quadrangular [plot] more than that of a row. But the Halakha is not according to Rabbi Eliezer.