(Mishnah): If the end of a row entered into barley, it's permitted as it's apparent that it's the end of his field.

If he is growing wheat and his neighbor is growing another species, he may plant a row of that species at the border of his field (thereby avoiding the required separation of a Beis Rova between the fields).

If he is growing wheat and his neighbor is also growing it, at the border he may make a row only of flax but not of another species.

(R. Shimon): Both flax and other species are permitted.

(R. Yosi): Even in the middle of his field, he may test the soil using a row of flax.

(Reish Lakish citing Chizkiyah): If the end of a row entered another species but then the row itself became destroyed, it is still permitted.

(R. Yochanan): The Mishnah only permitted it because the row was still in existence; otherwise it's forbidden.

(Chizkiyah): If a person has a field the size of a Beis Rova (50 by 50 Amos that is sown with barley) and any sized field next to it - that small field could act as the end of a row to plant in it any species.

(R. Yochanan): He may only plant the same species as in the large field.

(R. Zeira): R. Yochanan agrees that if it was 11/2 Beis Rova (each 1/2 Beis Rova can be viewed as a separate field and) he can plant half a Beis Rova on each side and the middle half a Beis Rova can be viewed like both. (He can plant section one with one species, section 3 with another species and the middle section is considered separate and cannot be planted with either species.)

Question (Baraisa against R. Yochanan): If a person had two fields, one sown with wheat and one with barley, it's permitted to place a furrow between them and plant one half of it wheat and one half barley, because the wheat looks like the end of the field of wheat and the barley looks like the end of the field of barley. What does R. Yochanan do with that Baraisa?

Answer: That Baraisa follows R. Eliezer ben Yaakov, who said that even a row of one wheat seed following after one wheat seed that enters into barley or vice-versa is permitted. But if there was just one wheat seed embedded in a field of barley or vice-versa, it is prohibited.