(Mishnah): If he put the Eruv on top of a reed or pole, if it was initially detached and later inserted into the ground, it is an Eruv.

Inference: This is because it was detached and inserted - but if it was not detached and inserted, it is not an Eruv.

Question: Does this argue with Shmuel? Rav Yehudah said in the name of Shmuel that the case is, the trunk is four [by four wide! Presumably, the reed is not four wide, and even so it is forbidden!]

Answer (R. Mana): [We forbid when it was not detached and inserted] when there is a board [four by four] on top of a reed [and the Eruv is on the board].