Question: (The Mishnah taught (Menachos 38(c)) that if he is growing wheat and his neighbor is also growing it, at the border he may make a row only of flax.) What's the case? If he intends it to test the field, even in the middle of the field would be permitted. If not, why only flax but no other species?

Answer: He intended to test it - R. Yosi wasn't concerned for Maaris HaAyin and the Rabbanan were concerned.

(R. Shmuel citing R. Zeira): (The Mishnah taught that R. Shimon permitted both flax and other species.) It's because R. Shimon holds that a person cannot prohibit another person's item.

(Rav): (If one's wheat is surrounded by barley, it prohibits; but if the barley belongs to another person, it does not prohibit, because) a person cannot surround (and thereby prohibit) something that's not his. (The only concern would be Maaris HaAyin, which doesn't apply when he is planting to test.)

(R. Yona): Rav follows R. Shimon who holds that one cannot prohibit that which is not his. However, the Rabbanan hold that for species other than flax, there is still a prohibition because of Maaris HaAyin.